Phase Relations Among Selenides Sulfides Tellurides and Oxides I 96_Simon-Essene

Economic Geology Vol. 91, 1996, pp. 1183-1208

Phase Relations among Selenides, Sulfides, Tellurides, and Oxides' I. Thermodynamic Properties and Calculated Equilibria

GRIGORE SIMON AND ERIC J. ESSENE Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, Michigan 48109-1063

Abstract

Published thermodynamic properties of selenide minerals were used to construct fugacity-temperature diagrams to estimate relative stabilities of the minerals as a function of temperature. The phase relations among selenides, sulfides, tellurides, and oxides were investigated by constructing fugacity-fugacity diagrams at temperatures between 100 ø and 300øC. These diagrams are used to predict equilibrium mineral assemblages and their variation with temperature, the relative tendency of various metals to form selenides, and to estimate the prevailingfses() fss(, fTe() andfosc ) and their evolution during mineral deposition and/or reequilibration g • g)' g g '

from mineral assemblages recognized in the deposit. These diagrams are also used to explain the occurrence of selenide minerals in ore deposits and to predict the mineral assemblages of some economically important chemical elements including Au, Ag, and Cu. The stability fields of selenide minerals are generally more restricted than those of sulfide minerals but are larger than those of corresponding telluride minerals for similar values offset, fTe-, andfse2, ,. Exceptions include hessire (Ag.2Te) and calaverite (AuTe2), whose stability ß g)• )• •g•

fields are larger than •ose of naumannite (A•Se) and AuSe, respectively. Most oxide minerals, except cassiterite, magnetite, hematite, and uraninite are unstable relative to the corresponding selenide minerals in typical hydrothermal fluids. The larger the stability field of the selenide, the more common it is in ore deposits (e.g., clausthalite, PbSe). Some transition metal selenide minerals that have not been found in nature (e.g., NiaSe.2) are stable only at very low fs2(> and/or fo2(g / and are unlikely to occur in the earth's crust. Some compounds such as AuSe and those in t•e T1-Se binary system are likely to be found in natural assemblages. They have not been observed because of our failure to detect them or because ternary gold and thallium minerals, respectively, are formed instead. The electrum-naumannite assemblage may act as a sliding-scale indicator Offses(g), similar to the electrum-Ag2S system fOrfs2(g ).

Introduction

SELENIUM minerals have been reported in epithermal veins, skam deposits, sandstone- or unconformity-type uranium deposits, as well as in coal deposits. Selenide minerals are closely associated with sulfide, oxide, and telluride minerals and/or native elements in a variety of mineral assemblages. In order to understand the geochemical significance of these assemblages we have used thermodynamic properties to calculate relative stabilities of native elements, sulfide, selenide, telluride, and oxide minerals as a function of fugacity of S•/g/, Se•/g/, Te•2(a/, and O2/g/. A similar approach has been taken for metal sulfide and oxide mineral assemblages by Holland (1959, 1965) and tellurides by Afifi et al. (1988). Similar studies involving the transport and stability of aqueous tellurium species are also available (Jaireth, 1991; Zhang and Spry, 1994). Fugacity-fugacity diagrams may be criticized because the gaseous species probably do not contribute much to the deposition of ore minerals. Nevertheless, the fugacity of O2/g/, Ss/g/, Te•2(g/, and Se•2/g/are useful measures of relative stability of solid phases, because the relative stability fields of solid phases are independent of whether we use aqueous solutions or gases to write the equilibrium reactions for solid phases.

A systematic thermodynamic study of selenide stabilities relative to native elements, oxide, sulfide, and telluride minerals is important to quantify the fugacities of O2ig/, Ss/g/, Te•2/g), and Se•2(gl which govern the formation of metal compounds during ore deposition. This study supplements previous evaluations of reactions involving sulfide, oxide, and telluride minerals (Vaughan and Craig, 1978; Barton and Skinner, 1979; Afifi et al., 1988) and fills the gap on the relative stability of selenides

relative to native elements, oxide, sulfide, and telluride minerals. We have used fugacity-temperature and fugacity-fugacity diagrams to depict equilibrium mineral assemblages and to explain observed assemblages in ore deposits.

Thermodynamic Properties

The fugacity of selenium (fs•.2(g•) is an important variable used in this study to explain the stability of metal selenides relative to their corresponding native elements, oxides, sulfides, and tellurides. Thermodynamic properties for elements, oxide, sulfide, and telluride minerals were taken from compilations (Barin and Knacke, 1973; Mills, 1974; Barton and Skinner, 1979; Afifi et al., 1988; Barin, 1989, 1993; Robie and Hemingway, 1995). Wherever possible we include recent thermochemical properties, such as those for pyrrhotite (Fe•_xS; GrOnvold and StOlen, 1992), bornitc, and thaicopy- rite (Robie et al., 1994), stibnite (Seal et al., 1992), FeTe0.9 (Shukla et al., 1990), and coloradoitc (HgTe; Nasar and Shamsuddin, 1990a). The pure ideal Se•2 gas (Se•2(g)) at 1 bar and temperature of interest was adopted as a reference state in our thermodynamic calculations, because Se=(g) is the most abundant gaseous selenium species over geologically significant ranges in temperature (Fi•. 1). At a total pressure of gaseous selenium species of 10 -ø bars, other selenium gases become important at temperatures below 150øC (Fig. 1). The effect of the total pressure of the gaseous selenium species is an important factor in the speciation of selenium gases. As the total pressure of gaseous selenium species becomes lower, Se•2(g) becomes more important (Fig. 2). Based on the extrapo- lations of the speciation of selenium gases from 127øC (400K)

0361-0128/96/1865/1183-2655.00 1183

1184 SIMON AND ESSENE

-8 se2 (g)

(Profab= 10 '8 bars

Se(g)

-12

-16

Se 3 (g)

So4 (g)

/ Se 6 (g)

/Se 8 (g) \ Se s (g) , ? !

!

-2o ß i i i

400 600 800 1000 1200

T (K)

FIG. 1. Partial pressures of gaseous selenium molecules as a function of temperature at Ptota• = 10-s bars based on Mills (1974). Dashed line represents an extrapolation.

and a total pressure of selenium species of 10-7 and 10 -s (table 9, Mills, 1974) to lower values of the total pressure of selenium species at the same temperature, it is likely that at

•0

a pressure of gaseous selenium species lower than 10- bars, Ses(g) is the most important gaseous selenium species, even at temperatures lower than 150øC. We prefer to use the pure ideal Ses(g) at 1 bar and temperature as a reference state for selenium for the same reason that Ss(g) is used as a standard state for sulfur (Barton and Skinner, 1979). Adoption of a single state for selenium means that the standard equations for selenides do not have a slope change at the melting and boiling point of selenium. Adoption of pure ideal Ses(g) at 1 bar and temperature of interest as a reference state in our thermodynamic calculations is also consistent with the common usage of aliaromic molecules Ss(g), Os(g), and Tes(g) as indicators of sulfidation, oxidation, and telluridation states (Holland, 1959, 1965; Afifi et al., 1988).

All Gibbs free energies of formation used in this study for selenide, telluride, sulfide, and oxide minerals are those from elements in their stable form at the temperature of interest and pure ideal Ses(g), Tes(g), Ss(g), and Os•g) gases, respectively. The notation used in this paper for the Gibbs free energy of formation of binary selenide minerals from elements in their stable form at the temperature of interest and pure ideal Ses(g) is ArG,•. The Gibbs free energies of formation from elements in their stable form at the temperature of interest and pure ideal Ses(g) for binary selenide compounds were calculated from the standard Gibbs free energy of formation of the binary selenide from the elements in their most stable

form at 1 bar and the temperature of interest (A•;), below the vaporization of the melt at 1 bar, by adding the/•rG• of the Ses(g) condensation reaction. A more detailed discussion on this calculation is provided later in this paper. The same procedures were followed to compile the Gibbs free energy of formation from elements in their stable form at the temperature of interest and pure ideal S2(g ) and Tes(g) for sulfide and telluride minerals. These Gibbs free energies of formation (ARGO-) and standard Gibbs free energies of formation (A•) used in this and other similar studies (e.g., Barton and Skinner, 1979; Afifi et al., 1988) are not compatible for direct use with the "standard apparent" Gibbs free energy of formation tabulated in other studies. The Gibbs free energy of formation of binary selenium compounds from the elements in their stable form and pure ideal Ses(g) were calculated at 1 bar and 25 ø, 100 ø, 150 ø, 200 ø, 250 ø, 300øC and at phase-transition temperatures. A first- or second-order polynomial was fitted to the values of A•G• calculated at the above-mentioned temperatures using a least squares tech- nique as a function of temperature (Appendix I). The standard Gibbs free energies of formation (A•o•r) of the selenide, sulfide, telluride, and oxide minerals from elements in their most stable form at 1 bar and temperature of interest used to calculate the Gibbs free energy of formation of binary selenium, sulfur, tellurium, and oxygen compounds from the elements in their stable form and pure ideal Ses(g), Ss(g), Te2(g), and Os(g) (A•GS) are listed in Appendix II.

Enthalpies of formation (AfHø), heat contents (H,-H.•oc), S ø heat capacities (Cp), and molar entropies (•øc) for selenium

SELENIDE MINERALS PHASE RELATIONS: I. THERMODYNAMICS 1185

FIG. 2.

-10

-12

-14

(T = 500 K)

I I

Se7 8e8 (g)

$e (g)

•4

-8 -7 -6 -5

Log Ptotal (bars)

The molecular constitution of selenium vapor at 500 K as a function of Ptotal using the data of Mills (1974).

compounds were used or estimated using the methods outlined by Mills (1974) when there was no information on the standard Gibbs free energy of formation from elements in their stable state at i bar and the temperature of interest. Wherever possible we used the phase relations (e.g., invariant points) in the binary systems and the standard Gibbs free energy of formation of other species to estimate the thermodynamic properties for other compounds in the same system for which the standard Gibbs free energies of formation are not currently available. For example, the standard Gibbs free energy of formation (ZXiGSr) of krutaite (CuSes) was calculated at 332øC, the partial melting point of krutaite (Chakrabarti and Laughlin, 1981), based on the equilibrium reaction:

CuSes(s) = CuSe, + Se(1). krutaite ldockmannite

(1)

There are no thermodynamic properties to allow the calculation of reliable standard Gibbs free energies of formation for most ternary compounds in the systems considered. As a first approximation the standard Gibbs free energy of formation for ternary selenides can be calculated by analogy with ternary sulfides and sulfosalts as outlined by Craig and Barton (1973). In this approach the ternary compounds are arbitrarily stabilized by a small decrease in Gibbs free energy over mix- tures of binary equivalents, but it is this small change that makes all the difference in terms of phase equilibria (cf. the plague of small free energy differences of Fyfe et al., 1958).

Table 1 is a summary of transition, decomposition, and melting temperatures of geologically significant selenide minerals. Minerals that decompose or melt at temperatures less

than 400øC are useful to constrain the temperature of deposition or reequilibration during subsequent metamorphism of particular selenide deposits. Because of their relatively low temperature of decomposition or melting, selenide assemblages are particularly useful for this purpose. For example, the decomposition of athabascaite (CusSe4) (<100øC) and umangite (CusSes) (112øC), the partial melting point ofklock- mannitc (CuSe) (377øC) and krutaite (CuSes) (332øC), and the melting point of assemblages such as laphamite (AssSea) + selenium (122øC) or tellurobismuthite (BisTe3) + selentellurium (Se,Te) (340øC) might be used in geothermometry. A detailed discussion of the mineralogy, phase relations, and thermodynamic properties of the most important geologic systems is given below. Se, Se-O, and Se-O-H

Solid selenium is stable up to its melting point at 220øC at i bar (Kubaschewski and Alcock, 1977). It is found both as a primary mineral and as a weathering product in selenium- bearing deposits. The occurrence of primary native selenium indicates that fSe•(g), like Te .• can reach and thus f •,, saturation it is stable in some hydrothermal systems. Three phases are known in the Se-O system (SeOs, SeaOs, and SeO3), but only SeOs is reported as a mineral downeyitc.

Thermodynamic properties for gaseous species Ses(g>, HsSe(g>, SeO•, and SeOs•, together with those of native selenium and downeyitc, were calculated using data from Mills (1974). Although further experimental data are required, estimates of the thermodynamic properties of the aqueous species HsSe, HSe-, Se•-, HsSeO•, HSeO•',


TABLE 1. Invariant Points in Selenide Systems of Possible Use in Geothermometry

Temperature System Low-temperature assemblage High-temperature assemblage (øC) References

Se Crystal Melt 220 Ag-Se Naumannite (Ag•Se) High naumannite (Ag•Se) 133 -+ 1

High naumannite + selenium melt Melt 616 High naumannite (Ag•Se) Melt 897 q- 3

As-Se Laphamite (As2Se3) + Se Melt (As2oSeso) 180 Laphamite (As2Sea) Melt 375

AuSe AuSe + Se Melt (Auo.•Se999 218 AuSe Au + selenium melt (Auo28e99.s) 425 Au + selenium melt (AukS%s) Melt (Au7oSeao) 760

Bi-Se Nevskite (BiSe) Guanajuatite (Bi2Sea) + melt (Bis7.sSe4•s) 609

Cd-Se

Co-Se

Cu-Se

Fe-Se

Hg-Se

Mo-Se

Ni-Se

Pb-Se

Pd-Se S-Se

Sb-Se

Te-Se

Zn-Se

Ag-S-Se

Guanajuatite (Bi•Sea) + selenium melt

Guanajuatite (Bi•Sea) Laitakarite (Bi4Se3) Cadmoselite (CdSe) + selenium melt Cadmoselite (CdSe) Freboldite (CoSe) + bornhardtite? (Co38e4) Freboldite (CoSe) + Co Co•_xSe s.s.

Trogtalite (CoSe2) Co•-xSe (bornhardtite?) + selenium melt Bellidoite (aCu2Se) Berzelianite (•3Cu•_xSe) + Cu Berzelianite (•3CU•_xSe) + copper melt Berzelianite (•3Cu•_xSe) Umangite (CuaSe•)

Klockmannite (CuSe) High Idockmannite (•3CuSe) y klockmannite (yCuSe) Berzelianite (•3Cu2_xSe) + selenium melt Krutaite (CuSe•)

Athabascaite (CusSe4)

/3Fe•+xSe (achavahte?) Ferroselite (FeSez) Tiemannite (HgSe) + selenium melt

(Hg• 6Ses74) Tiemannite (HgSe) + Hg melt (Hg77.saSea2.•7) Tiemannite (HgSe) Drysdallite (MoSe2) Makinenite (NiSe) Wilkmanite (NiaSe4)

Penroseite (NiSe•) Sederholmite (Ni•_•Se) + selenium melt Sederholmite (Ni•_xSe) Clausthalite (PbSe) + selenium melt Clausthalite (PbSe) PalladeseYte (Pd•7Se•s) Rhombic aS

(S,Se) ltd s.s. (as.•) + (Se,S) ltd s.s. (y•)

Yss Antimonselite (Sb•Sea) + Sb Antimonselite (Sb•Ses) Selentellurium (Se, Te) Stilleite (ZnSe) Hexagonal ZnSe Naumannite ltd s.s. (Ag2Se•) Aguilarite ltd s.s. (AgaSeS• •.) Acanthite ltd s.s. (Ag•S•) Aguilarite (AgaSeS)

Melt 618

Melt 706

Decompose? 470 Melt 991

Melt 1,264 COl-xSe s.s. 540 Melt (Co•.,Se44 s) 910 Melt (Co465e54) 1,078 Co•_•Se (bornhardtite?) + selenium melt 938 Melt (CoaoSe7o) 952 Berzelianite (•3Cu•_xSe) 123 q- 15 Melt (Cu•s •Se• s) 1,063 Melt 1,100 Melt 1,130 Berzelianite (•3Cu•_•Se) + high 112

klockmannite (•3CuSe) High klockmannite (•3CuSe) 51 y klockmannite (yCuSe) 120 Berzelianite (•3Cuz_xSe) + selenium melt 377 Melt (Cu4215e•79) 523 y klockmannite (yCuSe) + selenium 332

melt

Umangite (CuaSe•) + klockmannite <100 (CuSe)

5Fe•_•Se + aFe 455 yFe•_xSe + selenium melt 585 Melt (Hgao.•Sea9s) 685

Melt (Hg•s 9Se3• •) 708 Melt 795 q- 5

Mo38e4 q- selenium gas 1,150 q- 50 NiaSe• + sederholmite (Ni•_xSe) 375 Sederholmite (Ni•_•Se) + penroseite 300-400

(NiSe•) Sederholmite (Ni•_•Se) + selenium melt 853 Melt (Nia2Se•s) 856 Melt (Ni4•.•Sesa.5) 959 Melt (Pb•4.•Se75.9) 678 Melt 1,079 PdSe• + melt 680 Monoclinic •38 95.5 (/3S,Se) ltd s.s. (Ss•Se•) (/3•) 75 Melt (Ss08e40) 105 (Se,S) ltd s.s. + melt 160 Melt (Sb•oSe•o) 541 Melt 590

Melt 221-449.6

Hexagonal ZnSe "High temperature" Melt 1,515 q- 20 Cubic Ag•(S,Se) s.s. 133 Cubic Ag•(S,Se) s.s. 122 Cubic Ag•(S,Se) s.s. 176 Melt 791 q- 3

Kubaschewski and Alcock (1977) $hunk (1969) Clark and Rapoport (1970) Hansen and Anderko (1958) Eliot (1965) Massalski et al. (1990a) Massalski et al. (1990a) Okamoto and Massalski (1986) Okamoto and Massalski (1986) Okamoto and Massalski (1986) Gather and Blachnik (1975) Chang and Ondik (1992) Elliot (1965) Gather and Blachnik (1975) Elliot (1965) Okamoto (1990c) $harma and Chang (1990a) $harma and Chang (1990a) Massalski et al. (1990b) Massalski et al. (1990b) Massalski et al. (1990b) Massalski et al. (1990b) Massalski et al. (1990b) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981)

Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981) Chakrabarti and Laughlin (1981)

Harris et al. (1970)

Okamoto (1990a) Troften and Kullenld (1961) Sharma and Chang (1990b)

$harma and Chang (1990b) Elliot (1965) Massalski et al. (1990c) Elliot (1965) Komarek and Wessely (1972)

Komarek and Wessely (1972) Komarek and Wessely (1972) Komarek and Wessely (1972) Lin et al. (1986) Lin et al. (1986) Okamoto (1990b) $harma and Chang (1990c) Sharma and Chang (1990c) $harma and Chang (1990c) Sharma and Chang (1990c) Massalski et al. (1990d) Massalski et al. (1990d) Sharma et al. (1990) Elliot (1965) Elliot (1965) Petnlk et al. (1974) Petnlk et al. (1974) Petnlk et al. (1974) Main et al. (1972)


T^BLE 1. (Cont.)

System Low-temperature assemblage High-temperature assemblage Temperature

(oc) References

Ag-Bi-Se

Au-Ag-Se

Au-Bi-Se

Au-Pb-Se

Au-Sb-Se Bi-Te-Se

Cu-Ag-Se

Cu-Sb-Se

Cu-As-Se

Cu-Fe-Se

Hg-S-Se

Pb-S-Se

Pb-Se-Te Pb-Zn-Se

S-Se-Te

Sb-Se-Te

T1-S-Se

Zn-S-Se

Pb-S-Zn-Se Pb-S-Cd-Se

Low bohdanowiczite (AgBiSe2) Medium bohdanowiczite (AgBiSe2) High bohdanowiczite (AgBiSe•) + guanajuatite

(Bi•Se3) High bohdanowiczite (AgBiSe•) Fischesserite (AgaAuSe2)

Naumannite (Ag•Se) + fischesserite (Ag•AuSe2)

High naumannite (Ag•Se) + high fischesserite (AgaAuSe2)

High fisehesserite (AgaAuSe2)

Maldonite (AusBi) + Bi + Bi3Se•

Guanajuatite (Bi•Se3) + AuSe + Se Maldonite (Au•Bi) + Bi3Ses

Nevskite (BiSe) + Au

AuSe + guanajuatite (Bi•Se3)

AuSe + clausthalite (PbSe) + Se AuSe + dausthalite (PbSe) Clausthalite (PbSe) + Au + melt (Se>•a) Au + clausthalite (PbSe)

Au + antimonselite (Sb2Se3) Bi•(Se, Te)3 complete s.s.

Tellurobismuthite (Bi•2Te3) + selentellurium (Se, Te)

Eucairite (CuAgSe) + naumannite (Ag•Se) Eucairite (CuAgSe) + bellidoite (CurSe) Eucairite (CuAgSe) Permingeatite (Cu3SbSe4) Mgriite (Cu•AsSe3) As•Cu4Se5 + berzelianite s.s. (/3Cus_xSe•s) Mgriite (Cu3AsSe3) Eskebornite (CuFeSe2) Low Hg(S,Se)Hd s.s. + Hg(Se,S)Hd s.s. Hg(S,Se) complete s.s. Galena (Pb(S,Se)) + clausthalite (Pb(Se,S)) Pb(S,Se) complete s.s. Clausthalite (Pb(Se,Te) + altaire (Pb(Te,Se)) Clausthalite (PbSe) + stilleite (ZnSe) (S,Se) s.s. (?) + selentellurium (Se,Te) Antimonselite (Sb•Se3) + Sb2Te3_xSex

(0 < x < 2) (T1S, high T1Se) complete s.s. Sphalerite (Zn(S,Se)) + stilleite (Zn(Se,S)) Clausthalite (PbSe) + sphalerite (ZnS) (PbSe,CdS) ltd s.s. + CdS

Medium bohdanowiczite (AgBiSe•) 120 High bohdanowiczite (AgBiSes) Varies 285-355 Melt 675

Melt 765

High fischesserite (AgaAuSe•) 267

High naumannite (Ag•Se) + fischesserite 133 (AgaAuSe=)

Complete s.s. 267

Melt 742

(730) Melt (Au•sBis4aSe0a) 210

Melt 216

Au + melt (Au30BiesSe•) 367

Guanajuatite (Bi2Se•) + melt 582 (AusBi49Se43)

Au + melt •424

Melt (Se>99 0 217 Au + melt (Se>ooo) •395 Melt (tu2133Pb•63aSe6•.3a) 530 Melt i (AuaPb•5a3Se367) + melt 2 955

(AuuPb41Se4s) Melt i 584 _ 2

Melt Varies (593-705) (585-700)

Melt •340

Melt 728 _+ 3 Melt 745 _+ 3 Melt 750 Melt •405

Berzelianite s.s. (/3Cu•_xSes,) + melt 500 Mgriite (Cu3AsSea) 430 Melt + berzelianite s.s. (/3Cu•_xSe,•) 500 Melt 577

Hg(S,Se) complete s.s. 350 Melt Varies (800-830) Pb(S,Se) complete s.s. <300 Melt Varies (1,079-1,108) Pb(Se,Te) complete s.s. ? Melt 1,010 Melt < 100 Melt 560

566

Melt Varies 240-340

Zn(S,Se) complete s.s. <300 Melt (Pb9Zn4•S4•Se9) 1,019 Melt 1,000

Petzow and Effenberg (1988) Petzow and Effenberg (1988) Petzow and Effenberg (1988)

Petzow and Effenberg (1988) Smit et al. (1970); Wiegers

(1976) Wiegers (1976)

Wiegers (1976)

Wiegers (1976) Smit et al. (1970) Gather and Blachnik (1975)

Chang and Ondik (1992) Chang and Ondik (1992) Gather and Blachnik (1975)

Chang and Ondik (1992) Gather and Blachnik (1975)

Chang and Ondik (1992) Gather and Blachnik (1975)

Chang and Ondik (1992) Gather and Prince (1990) Gather and Prince (1990) Gather and Prince (1990) Gather and Prince (1990)

Gather (1976) Bankina and Abrikosov (1964) Bankina and Abrikosov (1964) Dumas et al. (1987)

Schafer (1995) Schafer (1995) Schafer (1995) Scott and Kench (1973) Dembrovskii et al. (1971) Blachnik and Kurz (1984) Blachnik and Kurz (1984) McCormick et al. (1994) Asadov (1983) Asadov (1983) Wright et al. (1965) Simpson (1964) Steininger (1970) Oleinik et al. (1982) Losana (1923) Ivlieva and Abrikosov (1964) Andriamfi•aja et al. (1985) Itoga and Kannewurf (1971) Wright et al. (1965) Oleinik et al. (1982) Tomashik et al. (1981)

Abbreviations: ltd= limited, s.s. = solid solution

ScOa s- are also available between 25 ø and 300øC (e.g., D'yachkova and Khondakovskiy, 1968; Johnson et al., 1992).

Ag-Se

The only known compound in this system (Hansen and Anderko, 1958; Eliot, 1965; Shunk, 1969; Clark and Rapo-

port, 1970) is AgaSe (naumannite), which is stable in two structural modifications, with a transition temperature of 133 ø _ 1øC, up to its melting point at 897 ø ___ 3øC. The Gibbs free energy of formation from elements in their standard state and Sea(g) for Ag2Se was calculated from the com- pilation of standard Gibbs free energy of formation from


elements in their standard state of Barin (1989), who used the enthalpy of formation and entropy data of Wagman et al. (1982) and the heat capacity data of Kubasehewski and Aleoek (1977).

As-Se

The phase diagram (Massalski et al., 1990a) contains the compounds As4Sea, AsSe, and AssSea, although only AssSea is known as a mineral (laphamite; Dunn et al., 1986). Sulfur can substitute for selenium in natural laphamite up to 19 mole percent (Dunn et al., 1986). Zhukov et al. (1974) studied the As-S-Se system at temperatures higher than 100øC and found a complete solid solution between orpiment (AssSa) and laphamite (AssSea). The other arsenic sulfoselenide, jeromite, As(S,Se)s, has not been successfully synthesized. At temperatures higher than g50øC, a liquid field appears, (As, Se)i•/, as a result of the reaction AsSe with As (Massalski et al., 1990a).

Thermodynamic properties for laphamite were calculated from compilations of O'Hare et al. (1990). For AsSe we used the enthalpy and entropy of formation at 25øC (A•H•soc, S•.•oc) from Mills (1974) and estimated the heat capacity by analogy with AsS. Thermodynamic properties for were estimated at 300øC, using the thermodymanic properties of AsSe, and assuming an As/Se composition of 1/1 by analogy to thermodynamic properties of (As,S)• (Barton, 1969). A 1/1 composition for (As,Se)(•) at 300øC is in agreement with phase relations in the As-Se system at this temperature (Massalski et al., 1990a). The compound As4Sea was not considered because of the lack of the thermodynamic properties and its absence in geologic occurrences. Au-Se

AuSe is the only known binary compound in this system and it is stable between room temperature and its melting point at 375øC (Okamoto and Massalski, 1986). Although this compound has been identified in the binary system and in the ternary systems Au-Te-Se (Cranton and Heyding, 1968), Au-Bi-Se (Gather and Blaehnik, 1975), and Au-Pb-Se (Gather and Prince, 1990), it has not yet been identified as a mineral. Native gold and native selenium are not stable together at any temperature (Okamoto and Massalski, 1986). Immiseible liquids are encountered above 760øC (Okamoto and Massal- ski, 1986). Thermodynamic properties for AuSe were obtained from compilations of Mills (1974). Bi-Se and Bi-Se-S

The phase diagrams for these systems are still uncertain. Three compounds (BiaSes, Bi.2Sea, and BiSe(s.s.) phase) were identified by Gather and Blaehnik (1975) at temperatures above 100øC. Eleven additional compounds were identified by Okamoto (1990e), but their thermal stability fields are uncertain. Four bismuth selenide (_+ sulfide) minerals are known: guanajuatite (orthorhombie BisSea), paraguanajuatite (hexagonal Bis(Se,S)a), laitakarite (Bi4(Se,S)a, Vorma, 1959), and nevskite (Bi(Se,S), Neehelynstov et al., 1984). The binary equivalents of guanajuatite, laitakarite, and nevskite are stable from below 100øC to their melting points at 706 ø, 470ø(?), and 609øC, respectively.

The thermodynamic properties for bismuth compounds in

general are subject to uncertainty. Ideal compositions were assumed for the bismuth-selenium compounds. Compilations of the standard Gibbs free energy of formation of Mills (1974) and Barin (1989) for BisSe3 were used in our calculations because they are the only data available at 100 ø to 300øC. Thermodynamic properties for laitakarite and nevskite are currently not available in the temperature range of interest; therefore, these minerals were not included in our calculations.

If we consider the Bi/Se ratios and some possible reactions like:

4BiSe + Ses(g) = 2BisSe3 , (2) nevskite guanajuatite

the following minerals form with increasing fse2<g): bismuth- laitakarite-nevskite-guanajuatite. Co-Se

This system contains the phases Co9Ses, CoSes, and Co1_xSe (x = 0.0-0.30, stable at temperatures above 540øC; Massalski et al., 1990b). At temperatures below 540øC, Co1_xSe decomposes into Co•_xSe (x = 0.0-0.12) and Co1_xSe (x = 0.2-0.28), which correspond roughly to frebol- dire (CoSe) and bornhardtite (CoaSe4; Ramdohr and Schmitt, 1955). The compound CoSes also exists in nature as two minerals, hastitc (orthorhombic) and trogtalite (cubic) (Ram- dohr and Schmitt, 1955). The compound Co9Ses has not been reported as a mineral. Unfortunately, reliable thermodynamic properties for these compounds are not currently available. Cu-Se

The Cu-Se system is a useful system for geothermometry. The phase diagram in the Cu-Se system is based on the experimental work of Heyding (1966) and Chakrabarti and Laughlin (1981) and includes: CusSe, CuaSes, CuSe, CuSes, and the compounds of the Cus_xSe phase (Fig. 3). All these phases correspond to minerals that are found in nature: bellidoite (CusSe; De Montreuil, 1975), umangite (Cu3Ses), ldock- mannitc (CuSe), krutaite (CuSes; Johan et al., 1972), and berzelianite (Cus_xSe). Athabaseaite (CusSe4) has not been synthesized; it decomposes to umangite and kloekmannite at a temperature above 100øC (Harris et al., 1970). A phase transition at lg3øC separates bellidoite (CusSe) from berzelianite (Cus_xSe, a higher temperature phase). With increasing selenium concentration, however, berzelianite becomes stable together with bellidoite at temperatures below 123øC. Klockmannite (CuSe) is stable in three structural modifications from room temperature to its melting point at 377øC. Umangite (CuaSes) decomposes at 11gøC to berzelianite and klockmannite. Krutaite (CuSes) melts incongruently at 33gøC to klockmannite and selenium melt. The copper selenide tyr- rellite ((Cu,Co,Ni)aSe4; Bobinson and Brooker, 1952) was not synthesized in the Cu-Se system and may be stable only in the Cu-Co-Ni-Se system.

Copper selenide assemblages are very useful, not only in geothermometry but also to decide if a mineral assemblage represents an equilibrium association. If we consider reactions such as:

2CuSe + Ses(g) = 2CuSes, klockmannite krutaite

(3)


0

•4ooJ

1200-

8{3O

0 0

Weight Percent Selenium lo ao •o 4o •o ep 7o eo 90 •oo

332øC 2•.øC

($e)---

30 40 50 60 70 80 ' 9•'" 100 Atomic Percent Selenium Se

FIG. 3. Phase relations in the system Cu-Se (Chakrabarti and Laughlin, 1981).

the minerals that become stable with increasing the fugacity of selenium in the system are controlled by temperature: up to 112øC, copper-bellidoite-berzelianite(?)-umangite-klockmannite-krutaite-selenium; 112 ø to 123øC, copper-bellidoite- berzelianite (?)-klockmannite-krutaite-selenium; 123 ø to 220øC, copper-berzelianite-klockmannite-krutaite-selenium; 220 ø to 332øC, copper-berzelianite-klockmannite-krutaite; 332 ø to 377øC, copper-berzelianite-klockmannite; 377 ø to 1,063øC, copper-berzelianite.

Thermodynamic properties for Cu2Se were calculated from compilations of Mills (1974) and Barin (1989). The thermodynamic properties for CuaSe2, CuSe, and CuSe= were calculated using the enthalpy and entropy at 25øC (AfH•5oc, S•5oc) from Mills (1974) and the heat capacity were estimated based on the methods outlined by Mills (1974), using the thermodynamic properties of CuS and the invariant points in the Cu-Se system (Chakrabarti and Laughlin, 1981).

Fe-Se

The only Fe-Se compounds apparently stable at room temperature are FeSe0.96 (fiFel+xSe) and FeSea (Mills, 1974), the equivalents of achavalite(?) (FeSe?; Olsacher, 1939), and ferroselite (FeSea; Bur'yanova and Komkov 1955). The exis- tence of achavalite as a mineral species (Olsacher, 1939) is uncertain since the mineral has not been ratified by the Inter- national Mineralogical Association (IMA). Above 200øC the phase diagram for this system (Troften and Kullerud, 1961; Schuster et al., 1979; Of<amoto, 1990a) contains three compounds, fiFe•+xSe (x = 0.04-0.06), 6Fel_xSe (x = 0.00-0.28), and TFel-xSe (x = 0.18-0.37), together with FeSea and Fe7Ses. The fl phase decomposes above 460øC to native iron and the 6 phase. The stability field of the compound Fe7Ses

is not knmvn, but it may decompose at around 300øC to form the 6 and T phases (Okamoto, 1990a). Ferroselite is stable from at least 200øC up to around 680øC, where it melts ineongruently. The 6 and T phases are stable up to 1,075 ø and 720øC, respectively. Thermodynamic properties for iron selenides were calculated using compilations of Mills (1974). Thermodynamic properties for fiFe•+•Se, 6Fel_•Se, and TFel_•Se were estimated using the thermodynamic properties of FeSe0.96, FeSel.3a, and FeSe• •4 compounds from Mills (1974). The exact composition of the 6Fel_.,Se and TFel-•Se phases used in our calculations are based on the phase relations in the Fe-Se system at a temperature of interest (Troften and Kullerud, 1961; Schuster et al., 1979; Okamoto, 1990a).

Ni-Se

The phase diagram for this system (Eliot, 1965; Komarek and Wessely, 1972) contains five compounds: NiaSe2, fiNia+_xSe2 (x = 0.19-0.29), Ni6Se.5, Nix_fie (x = 0.0-0.25), and NiSea. Most of the mineralogical observations summa- rized in this section were made by Vuorelainen et al. (1964). The two end members of the Nix_fie phase correspond to the minerals makinenite CTNiSe") and wilkmanite (monoclinic NiaSe4) or trtistedtite (cubic (Ni,Co)3Se4). Trtistedtite is reported as a primary' mineral in which cobalt substitutes for nickel, whereas wilkmanite can occur both as a primary mineral and as an alteration product of sederholmite. The name sederholmite CfiNiSe") has been used to represent all compounds of the Nii_xSe with chemical compositions between miikinenite and wilkmanite. The compound NiSe• is knmvn in natural occurrences as penroseite ((Ni,Co,Cu)Sea) and kullerudite (NiSea). Penroseite shows a large chemical variation due to the substitution of Co, Cu, Pb, and Ag for Ni (Anthony


et al., 1990). Kullerudite was observed only as an alteration product of wilkmanite. The compound NiaSe2 is not known as a mineral. Thermodynamic properties for NiSe2 and the end members of the Nil_xSe solid solution, NiSel.05 and NiSel.• (the equivalents of m•ikinenite and wilkmanite, re- speetively), were calculated from compilations of Mills (1974) and Barin (1989). The thermodynamic properties for NiaSez are currently not available and were estimated using the thermodynamic properties for NiaSz compound following the methods outlined by Mills (1974).

Other binary systems Binary phase diagrams are available for the systems involv-

ing CdSe (eadmoselite; Bur'yanova et al., 1957), HgSe (tie- mannitc), PbSe (elausthalite), SbzSea (antimonselite), and ZnSe (stillcite). These compounds are stable from room temperature to their melting points at 991 ø , 795 ø , 1,079 ø , 590 ø , and 1,515øC, respectively. Other synthetic compounds are known in the binary systems Mo-Se and Pd-Se but only MoSe= (drysdallite; Ceeh et al., 1973) and Pd17Se15 (pallad- seYte; Davis et al., 1977) are found as minerals. Oosterbosehite ((Pd,Cu)7Ses; Johan et al., 1970) has not been synthesized (Okamoto, 1990b). Knowledge of the Se-Te system is limited to temperatures above 100øC (Sharma et al., 1990). Selentel- lurium (Se,Te) is the mineral name used for the compounds of the complete solid solution series that occurs between native selenium and native tellurium. Selentellurium is stable

from at least 100øC to its melting point, which varies with composition from 221 ø up to 450øC. Selenium can substitute for sulfur in a large number of sulfide minerals, e.g., ikunolite (Bi4(S,Se)3; Kato, 1959) and jeromite (As(S,Se)2; Lausen, 1928).

Thermodynamic properties for CdSe, HgSe, PbSe, Sb•Se,, and ZnSe were taken from compilations of Mills (1974), Barin (1989), and Nasar and Shamsuddin (1990b). Thermodynamic properties for MoSe• were estimated from molar enthalpy and entropy data at 25øC (AfHøm,•oc and Søm,•oc) of O'Hare et al. (1987) and an estimation of the heat capacity using the thermodynamic properties for MoS2 and following the methodology outlined by Miller (1974). Unit-cell volume data of DrSbek (1995) on MoS•-MoSe• solid solution show positive deviations from linearity, suggesting that large pressure variations will affect the activity-composition (a/X) relations.

Phase Relations in Three or More Component Systems About 20 ternary selenide minerals are known in nature,

and there are many other minerals in which selenium can substitute for sulfur in significant amounts. Unfortunately, there are no thermodynamic properties for most of these metal sulfide-selenide ternary compounds. As a first approximation the standard Gibbs free energy of formation of ternary selenides can be estimated by analogy with ternary sulfides and sulfosalts (Craig and Barton, 1973). The standard Gibbs free energy of formation for ternary selenides may be assumed to be only slightly more negative than the sum of the equivalent mixture of corresponding binary selenides. This approximation is supported by the coexistence of both ternary and their corresponding binary selenides in the same natural association, e.g., the association of eueairite (CuAgSe) with naumannite (Ag•Se) and ldoekmannite (CuSe) in assemblages

that seem to be in equilibrium (Stanley et al., 1990). Specific ternary systems are discussed below.

Au-Ag-Se

The only published data on this system are for the ternary compound Ag•AuSe• and the phase diagram for the Ag•Se- Ag•AuSe2 section (Wiegers, 1976). The phase Ag•AuSe• exists in two structural modifications. The low-temperature form (]3Ag•AuSe•) is stable below around 270øC (Smit et al., 1970; Wiegers, 1976) and corresponds to the mineral fischesserite (Johan et al., 1971), which is isostructural with petzite (Ag•AuTe•). The high-temperature form (aAg•AuSe•) is stable from 270 ø to 742øC (Wiegers, 1976). The pseudobinary section Ag2Se-Ag•AuSe• shows a continuous solid solution from aAg•Se to aAgaAuSe• and extends to higher values of Au than that in ideal Ag•AuSe• (Wiegers, 1976). Other minerals that include gold and selenium are pentzinite ((Ag,Cu)4- Au(S,Se)4; Bochek et al., 1984) and petrovskaite (AgAu(S,Se); Nesterenko et al., 1984).

Au-Bi-Se, Au-Pb-Se, and Au-Sb-Se

Although no ternary compounds have been reported in these systems, the understanding of these systems is important to predict the behavior of gold in selenium-bearing geologic systems. The Au-Bi-Se system is dominated by a large stability field of native gold (Prince et al., 1990). The Au-Bi•S% join divides the ternary system into two partial ternary systems Au-Bi-Bi2Sea with a ternary eutectic point at 217øC (L = Se + AuSe + Bi•Sea) and Au-Se-Bi•Sea with a ternary eutectic point at 240øC (L = Bi + Au•Bi + BiaSe•). Native gold coexists with all phases in this system (BiaSe2, BiSe, Bi•Se•, Au•Bi, AuSe) except native selenium. Native selenium can crystallize in this system only in equilibrium with guanajuatite (Bi•Sea) and gold selenide (AuSe).

The Au-Pb-Se system (Rouland et al., 1977) is divided into two partial systems, Au-PbSe-Se and Au-PbSe-Pb, by an invariant reaction at 955øC (Au + PbSe = L• + L•). In the partial ternary Au-PbSe-Se, the reaction AuSe + PbSe = Au + L occurs at 395øC and the ternary eutectic system AuSe + PbSe + Se = L occurs at 217øC. Phase relations in this

system show that native gold cannot be in equilibrium with selenium but will form AuSe. Information about the Au-Sb-

Se system is restricted to the Au-Sb2S% system (Gather, 1976). Native gold is stable with Sb•Sea up to around 590øC. In their review of the ternary gold alloys, Prince et al. (1990) suggested that AuSe is the phase stable with native selenium.

Experimental data show that native gold is stable with bismuth selenides, elausthalite, SbsSea, and probably with most of the other selenides over a large temperature range but never with native selenium (solid). A ternary compound, AuKSe, is known in the Au-K-Se system, but it has not been found in natural assemblages, probably because it requires both an extremely reducing environment and the absence of silica, which are unlikely conditions in terrestrial environments.

Ag-S-Se, Ag-Bi-Se, and Ag-Sb-Se One ternary compound has been identified in each of these

systems (Ag4SeS, AgBiSes, and AgSbSes), two of which are known as minerals: aguilarite (Ag4SeS) and bohdanowiezite


(AgBiSe•2; Banas and Ottemann, 1971). Phase relations in the Ag-S-Se system are based on the speculative phase diagram of Petruk et al. (1974), which suggests that aguilarite is stable from room temperature up to 122øC. At temperatures higher than 122øC, aguilarite (orthorhombic) is thought to form an Ag•2(S,Se) solid solution (cubic). At low temperatures there is a limited solid solution of Ag•2Se in Ag•2S up to around 12 mole percent and up to around 15 mole percent Ag•2S in Ag•2Se. Bohdanowiczite is stable in three structural modifications from room temperature up to its melting point at 760øC (Petzow and Effenberg, 1988).

Cu-Ag-Se, Cu-Sb-Se, Cu-As-Se, and Cu-Fe-Se

This group of systems is important because each system has at least one ternary compound that is present in nature as a mineral. Eueairite (CuAgSe) is the only ternary compound known in the Cu-Ag-Se system (Frueh et al., 1957; Sehafer, 1995). It is stable in two structural modifications from room temperature up to its melting point at 750øC. Eueairite coexists with all binary phases present in the system except krutaite (CuSe2) at 300øC (Sehafer, 1995). SLx compounds were identified in the Cu-As-Se system, but only one of them occurs as a mineral, mgriite (Cu3AsSe3; Dymkov et al., 1982), which is stable up to 500øC (Dembrowskii et al., 1971). Per- mingeatite (Cu3SbSe4) is the only known mineral in the Cu- Sb-Se system (Scott and Keneh, 1973). The compound CuSb- Se2, which has been identified experimentally, has not been found as a mineral.

Two ternary phases were identified in the Cu-Fe-Se system, CuFeSe•2 and (Cu,Fe)Se•2_x solid solution (Bernardini et al., 1983, 1987). Eskebornite (CuFeSe•2; Ramdohr, 1949) is stable up to 577øC (McCormick et al., 1994). The other ternary phase identified in this system ((Cu,Fe)Sez_,) is stable in two structural modifications up to its melting point at 660øC, where it melts ineongruently to •/Fe•_xSe and liquid (Bernar- dini et al., 1987), but it has not been identified so far in natural occurrences. Bernardini et al. (1987) suggested that this phase is present in nature as krutaite. This is unlikely because the amount of iron reported in krutaite, apparently in equilibrium with ferroselite (FeSe•2) and eskebornite, is only up to 0.6 wt percent (Johan et al., 1972). Pb-Bi-Se and Pb-Bi-Se-S

Although three compounds and a number of solid solutions are known in the Pb-Bi-Se system, none of them has been encountered as a mineral. Elagina (1961) reported three in- terme&ate phases (Pb3Bi4Se9, PbBi2Se4, PbBi4Se7) and up to 20 mole percent BL2Se3 in PbSe-type solid solution at 720øC. At 500øC, Godovikov et al. (1967) reported only PbBi•2Se4 and PbBi4Se7 and a limited range of solid solution, based on Pb•2BL2Se.•. Platynite, PbBi2(Se,S)3, is the only mineral reported in this system, but it has not been synthesized to date (Elagina, 1961; Godovikov et al., 1967). Phase relations in the PbS-PbSe-BL2S3-BL2Se3 system were examined at 500øC by Liu and Chang (1994). Heyrovskyite, Pb9Bi4S15, and the selenian analogue of heyrovskyite, Pb9Bi4Se•5, form a complete series of solid solutions, whereas lillianitc, Pb8Bi6S17, and the selenium analogue of lillianitc, PbsBi6Se•7, form two terminal solid solutions. Along the PbBi•2S4-PbBL2Se4 joint there is an extensive region of solid solution based on weibul-

lite, Pbl+xBi•2+x(S,Se)4+x+l.sy, which ranges from PbBL2Se4 to 90 mole percent PbBi•2S4 (Liu and Chang, 1994).

Other systems Complete solid solution has been found between PbS and

PbSe, HgS and HgSe, and ZnS and ZnSe at temperatures of <300øC (Wright et al., 1965; Asadov, 1983). A large number of T1-Se-bearing ternary compounds have been synthesized in ternary systems, but only sabatierite (Cu6T1Se4, Johan et al., 1978), crookesitc (Cu7(TI,Ag)Se4; Johan and Kvacek, 1971), and bukovite (TL2(Cu,Fe)4Se4; Johan and Kvacek, 1971) have been found as minerals. The stability of kitkaite (NiTeSe; Hiikli et al., 1965) is not known and there are no phase diagrams available for the Ni-Te-Se system. Other complex selenides (e.g., poubaite, PbBL2Se•2(Te,S)2, and petrovi- cite, PbHgCu•BiSes) have been reported but their stability is unknown. The quaternary systems are dominated by extensive or complete solid solution at higher temperatures. Phase diagrams are also available for other multicomponent systems, such as Cu-Ag-S-Se (Sch/ifer, 1995), but no quaternary phases have been identified in these systems.

Selenide Stability Diagrams Method of calculation

The Gibbs free energies of formation for binary selenides were referenced to the standard state of ideal and pure Se•2(g) and pure elements in their stable state at 1 bar and the temperature of interest. The use of the ideal Se2(g) as a standard state for selenium instead of selenium in its stable form

(solid, liquid) does not offer a computational advantage, but as Afifi et al. (1988) noted for tellurides, it is consistent with convention. The method of calculation of the Gibbs free en-

ergy of formation of selenides relative to ideal Se.2(g) as a standard state is similar to that outlined by Afifi et al. (1988). In order to use the Se•2(g) as a standard state, the ArG• of the reactions:

$e•2(g) = 25e(s) for T < 220øC, (4) and

$e•2lg) = 25e/1) for T > 220øC, (5)

has to be added to the reaction of formation of the binary selenide from the elements in their stable form at 1 bar and

the temperature of interest below the vaporization of the melt at 1 bar. For example, the formation of ferroselite (FeSe•2) can be described both by reactions from elements in their standard state:

Fe + 2Se(s> = FeSe•2 for T < 220øC, (6)

and

Fe + 2Se(•) = FeSe•2 for T > 220øC, (7)

or by reactions between pure elemental iron and Se•2(g)

Fe + Se•2(g) = FeSe•2. (8)

Equilibrium between selenide and sulfide, telluride, and oxide minerals can be represented as a function offse2lg/and fs2/g), fTetg / andfo2/g/, respectively, using reactions such as:


2PbS + Sealgt = 2PbSe + Sa(gt, (9) galena clausthalite

2PbTe + Sea(g) = 2PbSe + Tea(g), (10) altaitc clausthalite

2PbSe + Oalg) = 2PbO + Seacg). (11) clausthalite massicot

A general reaction can be written as: n 111

• %I + Z %I = 0, (12) 1 1

where n and m represent the number of solid and gaseous phases, respectively, and % represents the stoichiometric coefficient of the Jth phase in the reaction. The equilibrium constant (K,q) of reaction (12) can be obtained from the Gibbs free energy of the reaction at 1 bar and the temperature of interest (/krG•-) using the relation:

In/%q = -/XrG(øT>/RT (13) or

n n

log/%q = -/XrG•T)/2.303RT = • /Jj log O/j q- Z /Jj log f, (14) i 1

where % andf represent the activity of solid phases and the fugacity of gaseous species, respectively. For example the equilibrium constant for reaction (10) will be:

Keq = (fTe(g,) (fse2(g))-•(aPbSe)a(aPbTe) -a. (15) If we assume that the solid phases are pure, % equals

1. The equilibrium constant is a function of temperature. Therefore, using the thermodynamic properties from Appen- dix I the change of the equilibrium constant (/•q) with the temperature for reactions such as (8) can be calculated easily. This approach requires correction for solid solutions where % will be different than 1. As a first approximation a simple mixing model can be used to estimate the activities and fugacities if all phases in the reaction have been analyzed for their compositions.

Confining pressure may influence the equilibrium constant. The consequence of confining pressure on these reactions can be estimated using equation (13) and the following equation:

(0 log K•q/0P)T = -(0/XrG•-/0P)T/2.303RT = - AV/2.303RT, (16)

where AV, the change in volume for the solids, is generally small. Therefore, the effect of pressure on the equilibrium constant is not significant for pressures up to 1 kbar for most reactions, except for the condensation of selenium, and it was not considered in our calculations. The fugacity of Sea(g) in hydrothermal systems is controlled by the total concentration of selenium in the system, pH, Eh, and the partitioning of selenium among aqueous species and gaseous species in the hydrothermal fluid.

The speciation of selenium in hydrothermal fluids at temperatures between 100 ø and 300øC is poorly known. The thermodynamic properties for aqueous selenium species at 25øC shows significant discrepancies (Woods and Garrels, 1987). At higher temperatures there is a lack of reliable experimental

data, the majority of which were approximated (D'yachkova and Khodakovskiy, 1968).

Uncertainties in the thermodynamic properties of aqueous selenium species and the speeiation of selenium among aqueous species and between aqueous species and gaseous species prevents a quantitative evaluation of the selenium concentration in hydrothermal fluids. A•ueous selenium spedes can exist both as reduced (e.g., Se -, HSe-) and oxidized (e.g., HSeO:, HSeO;- forms, suggesting that pH and fo2 play an important role in controlling selenium behavior in solutions. Although it is difficult to determine the changes in aqueous s•eeies that control variations in fs ..... the effect of tem-

vPoe•'vla•res;l;dnifds•7 sfu•ia•,f•ii)'Ui:• d, (2•ig?•nxi•qu,2lsisb?2•a•n• can be evaluated constructing fugacity-temperature diagrams (fse•0:T) and isothermal fugacity-fugacity diagrams

Fugacity-temperature diagrams OCse•2<g) - T) Univariant reactions may be represented as a function of

the difference from fugacity of Sea(g) at saturation and the reciprocal of the absolute temperature, 10 • K -1 (Figs. 4, 5). The upper limit of the fugacity of Sea(g) in natural systems is the condensation of solid or liquid selenium. The lower limit, estimated from selenide-bearing natural assemblages, corresponds roughly to the Ni3Se:Nil_xSe univariant reaction. These two limits delineate a very large spectra offs .... in geologic systems, which is not very helpful in our understa,,,a g of the genesis and evolution of selenide-bearing deposits. Therefore, we are interested in better constraining the fSe•g> in the system during the mineral deposition. This problem may be solved by the use of continuous mineral reactions that are controlled byfse• , or by use of mineral associations kg;'

that are stable at spedfie •fSez•g/ values. Two important continuous indicators of fugaeity were pro-

posed by Barton and Toulmin (1964) forfse•.•, and a variant of this method using the reaction between si]•er dissolved in electrum and hessitc was proposed by Barton and Skinner (1979) and Afifi et al. (1988) for fTe•/g ) based on electrum tarnish method. The composition of electrum (Au-Ag solid solution) in equilibrium with Ag2S tarnish is an indicator of the fugaeity of sulfur, the higher the fugaeity of sulfur the more gold rich the electrum. By analogy with these two methods the reaction between silver dissolved in electrum and

naumannite can be used to determine fSe2/g): 4Ag + Sea/g)= 2AgaSe . (17)

electrum gas naumannite

Derivation of the fse•,g• equation is similar to that used by Barton and Toulmin (1964) for the reaction of electrum to argentitc:

logfse2•g) = [--11,150 + (5.745 -- 4 log X^g)T + 0.874 (1 -- X^g)a(5,650-1,600 (1 - X^g) - 1.375T)]/T (18)

for 298.15 K < T < 406.15 K and

1Ogfse•(g• = [--9986 + (2.873 -- 4 log X^g)T q- 0.874

(1 -- X^g)a(5,650- 1,600 (1 - X^g)- 1.375T)]/T (19)

SELENIDE MINERALS PHASE RELATIONS.. I. THERMOD)'NAMICS 1193

-10

-15

-20

-25

-30

100% møc 3ooø0

/

/

/

/

/

/

/

/

!

!

3.0 2'5103/T (14;) 2.0 1.5 Fie. 4. Diagram showing different reactions as a function of temperature (10•/T(K)) and the fugacity of selenium, log

fs½.2 <•. Dashed lines represent estimated univariant reactions. Solid circles (in all figures) represent the invariant points.

for 406.15 K < T < T(s). In equations 18 and 19 X^g is the atomic fraction of Ag in electrum, T is the temperature in Kelvin, and T(sl is the solidus temperature in the Au-Ag system. These equations include the equation for the chemical potential of Ag in electrum (White et al., 1957) and assume that the activity of AgeSe in naumannite is equal to 1. Equa- tions (18) and (19) ]nay be corrected for the substitution of gold in naumannite, assuming that this substitution is similar to that in argentitc and using the correction factors calculated by Barton and Toulmin (1964) and Barton (1980). In the range of 25 ø to 400øC this correction affects only electrum with an X^g less than 0.5. At higher temperatures the predicted corrections are higher, up to 1 log unit offs,,tg), and affect the whole range of electrum compositions (Barton and Toulmin, 1964). The correction factor calculated by Barton (1980) is with 0.3 logfs,, units higher at low X^g than predicted by Barton and •ouhnin (1964) and affects only the electrum with X^g less than 0.7. Although the mineralogy of the Au-Ag-Se system suggests that the crystal chemistry of this system is more complex than Au-Ag-S, particularly at higher temperatures, the additional correction for the substitution of gold in naumannite is probably lower than 0.5 log

units offse,,•g,. The composition of electrum (X^g) in equilibrium with naumannite, calculated by analogy with Barton and Toulmin (1964) and Barton (1980), is represented graphically in Figure 6 as a function of temperature. This system should be investigated experimentally to calibrate the a/X relations for Ag2Se-Ag2S solid solutions for more accurate applications.

Another way to infer thefse_o/• in the system during mineral deposition is to use all mineral associations (selenide, sulfide, telluride, and oxide minerals) present in the system. This can be achieved constructing fugaeity-temperature and fugaeity- fugaeity diagrams. The fugaeity-temperature-type diagrams for selenide, sulfide (Barton and Skinner, 1979), and telluride minerals (Afifi et al., 1988) have the advantage of representing most of the univariant reactions by linear equations and can be used to predict selenide assemblages at various temperatures. Two main types of fugaeity-temperature diagrams are most frequently used in the geologic literature, log of fugaeity-temperature and delta log fugaeity-temperature. The use of log lug^city-temperature diagrams (e.g., logfs•.,t •> - T) has an advantage that is readily comparable with fuga•fy-fugaeity diagrams. The use of delta log fugaeity-temperature diagrams (e.g., Alog fs•<g• - T) shows the differences between the


2SøC selenium 100øC i

condensation

-15

-2O

3.0 2.5 2.0 1.5 103/T (K)

F]c. 5. Diagram showing different reactions as a function of temperature (103FF(K)) and the difference between the fugacity of selenium in the reactions and its fugacity at saturation. Dashed lines represent estimated univariant reactions.

fugacity of selenium in a reaction and its fugacity at saturation and has the advantage of highlighting more clearly the univariant reactions, the invariant points, and the evolution of phase relations with temperature. The use of Alogfdiagrams is becoming widely adopted for phase equilibria involving Oalgt relative to common mineral buffers such as QFM (quartz-fayalite-magnetite) and HM (hematite-magnetite). The disadvantage is the need to have used internally consistent data sets which are not always available.

Fugacity-temperature diagrams also show the relative tendency of various metals to form selenide minerals. For example, the logfses<g> - T (Fig. 4) and/Xlogfse•<g> - T diagrams (Fig. 5) suggest that copper tends to react more easily than silver with selenium, in agreement with experiments in the Ag-Cu-Se system (Sch•ifer, 1995). Fugacity-temperature diagrams may also be used to predict incompatible mineral associations and upper and/or lower temperature limits for various selenide assemblages. For example, any equilibrium assemblages that include umangite are restricted to temperatures up to 112øC (Figs. 4, 5). In addition, Hg, Ag, and Bi cannot form equilibrium assemblages with umangite, klockmannite, and krutaite at any temperature up to 400øC (Figs. 4, 5). Native gold can form equilibrium assemblages with umangite,

ldockmannite, and berzelianite up to 112 ø, 300 ø, and <1,064øC, respectively, and it cannot coexist in equilibrium with krutaite at any temperature (Figs. 4, 5).

A combination of the log fs•2tgt - T diagram (Fig. 4) and the composition of electrum in equilibrium with naumannite as a function of temperature (Fig. 6) can be used to predict both the temperature and the fugacity of selenium if certain mineral associations are present in equilibrium. The equilibrium association of electrum and naumannite with any two copper selenides, nickel selenides, Hg and tiemannite, or Bi and guanajuatite, will fix both the temperature and the fugacity of selenium. For example a composition of electrum X^g = 0.1 in equilibrium with naumannite, berzelianite, and ldockmannite indicates a temperature of around 190 ø to 200øC and logfs•g• between -9 to -10.

The applicability of the fugacity-temperature diagrams in geologic systems is not always straightforward because some other species such as Oa/g), Sa/g), and Tea/g) might compete with Sea(g) in their tendency to form oxide, sulfide, and telluride minerals. In these situations log fs•(•> - T (Fig. 4) and Alog fs•> - T diagrams (Fig. 5) may be used to set the lower limi]: of the lo- fse , reouired to form selenide minerals _ •d_, 2(g; . _ _ .

in natural systems. The presence of' other gases (e.g., O.2lg>,


-10

-25

3.0 2'51031T (K) 2.0 1.5 FIG. 6. fse2• •-temperature diagram showing the calculated isopleths of X^g in electrum in equilibrium with naumannite

as a function o• temperature. Solid lines represent the empirically corrected isopleths by analogy with Barton and Toulmin (1964), whereas the dashed lines represent the thermodynamically calculated correction by analogy with Barton (1980).

S2tg>, Te2tg/) will place additional constraints on the actual fugacity of selenium required to form selenides in these systems. Therefore, in natural systems, the fugacity-temperature diagrams should be used together with fugacity-fugacity diagrams.

Fugacity-fugacity diagrams

Fugacity-fugacity diagrams may be used to predict the equilibrium assemblages for a given system, to estimate the prevailingfse2• •,fs2• > fTe<> andfo2< > during mineral deposition g• g• g

and/or reequigiibration, and to evaluate the evolution of fs• •, fs• >, ftys- and fo•< > with time in a deposit. This type of g g ' g

diagram can •so be used to predict the mineralogic state of elements such as Au, Ag, and Cu. Three types of fugacity- fugacity diagrams with applicability in geologic systems are constructed: Aez,g)-fSa•g>, fs•a•l-fT•ig 1, and fse2•gy-fOalg >. A survey of the geothermometric data for selenium-bearing deposits indicates that these deposits formed in a range of temperatures from less than 100 ø up to around 300øC (e.g., Schsnwandt, 1983; Stanley et al., 1990; So et al., 1995), the temperature range for which the present diagrams are constructed.

f s•gl-f s•2lg t diagrams

The fs•l ,-fs2l i diagrams show the stability fields of binary selenide a'•d s•lfide minerals and provide a useful tool in characterizing parts of the chemical evolution of a hydrothermal fluid. The fs•< t-fs•<,• phase diagrams were constructed at 300 ø and 100øC. •he •[ower limit of the fs•<•t in selenide- bearing systems is probably close to Ag-naumannite and naumannite-argentite binaries, because both native silver and argentire are uncommon minerals in selenide-rich ore deposits of unconformity and sandstone-hosted uranium and tele- thermal selenide veins of the "Tilkerode-Zorge-Lerbach" type (Ramdohr, 1980). In the epithermal volcanic-hosted selenide-bearing ore deposits, native silver and argentire seem to be in equilibrium, or close to equilibrium, with naumann- ire. This lower limit offs•<• is also supported by the absence of NiaSe• and the presence•of Se-rich nickel minerals in selenide-bearing deposits. The fs•<• can be buffered by reactions between iron silicates and iron sulfides, but the mineral assemblages present in the selenide-bearing hydrothermal deposits suggest that it was probably close to the pyrrhotite- pyrite solvus.


-10

-15

-2O

-20 -15 -'10 -5

log fS2 (g)

FIG. 7. fse_,,•-fs2•> diagram showing the relative stability of some selenides and their corresponding sulfides as a function offs 2<> and fs2• >at 300øC. Abbreviations (for Fig. 7-13): asp = arsenopyrite, bn = bornitc, bz = berzelianite, cp = e.g g

chalcopyrite, fa = fayalite, fs = ferroselite, hm = hematite, hs = hessitc, k] = klockmannite, kr = krutaite, 1o = ]filingitc, mt = magnetite, nm = naumannite, po = pyrrhotite, py = pyrite, rd = rickardite, qz = quartz, sz = sttitzite, um= umangite, vu = vulcanRe, •vs = weissite, T = gamma phase.

Consideration of binary and ternary selenium-bearing phases for which thermodynamic properties are not currently available may cause some of the reactions shown in Figures 7 to 13 to become metastable. For example, only the native bismuth-guanajuatite reaction was considered for bismuth selenides because of the lack of the thermodynamic properties for the other bismuth selenides. Therefore, the Bi-BL2Sea assemblages will become metastable if the other bismuth selenides, e.g., laitakarite and nevskite, are stable. The presence of ternary phases will further limit the stability of coex- isting binary selenides and sulfides and the extent of their stability fields. Most of the univariant reactions involving copper selenides and sulfides are metastable with respect to reactions involving chalcopyrite, bornitc, and iron selenides and sulfides (Figs. 8, 13). The presence ofchalcopyrite will reduce the stability fields of copper telluride minerals and, to a lesser extent, selenide minerals. The stability fields of copper selenide and sulfide minerals are even more restricted if eucairite

(CuAgSe) is considered, but this restriction is not enough to preclude the presence of copper selenides and sulfides in natural selenide assemblages. Likewise, the stability fields of

As and Fe selenide minerals are restricted when 161lingitc and arsenopyrite are considered (Fig. 7). The presence of ternary phases will influence the stability fields ofAu, Ag, Cu, Fe, Sb, As, Bi, and Ag binary selenide and sulfide minerals.

The situation becomes even more complicated if we consider the extensive or complete solid solution between Pb, Ag, Hg, Bi, and Zn selenide and sulfide minerals. The effect of solid solution on the univariant reactions may be estimated by estimating the activities of the compounds of interest. For example, the equilibrium constant K•q for the PbS-PbSe univariant reaction:

2PbS + Sea(g)= 2PbSe + S2(g> (20) galena clausthalite

is:

Lq = (a•'•,Se)'2(f s2•g>)/(a•'•,S)'2(f Se2•g)), (21) where the activity of PbS in galena (a•,bs) and of PbSe in clausthalite (a•,bSe)would be <1 for Pb(S,Se) solid solution. Solid solutions such as PbSe in Pb(S,Se) and ZnSe in Zn(S,Se) may be approximated as ideal solid solutions (Barton and


-10

-15

-2o

I I I I I I I I I I I I I I I I I I I I

_ CuSe•(•r)

CuSe (Id) CuSe {id) // AuSe //,

I•l-x• • Fel.;•S e / /

8' 8.•: 8-.•

, •/•[[ , , , , , , , , I ' [ , , I , , -20 -15 -10 -5

log rS2 (g)

F•a. 8. fse2( )-fs•( diagram showing the relative stability of additional selenides and their corresponding sulfides as a function offse2•;, anais .... at 300øC.

Skinner, 1979), such that activity (a) equals mole fraction (X). Such a correction applied to PbS-PbSe solid solution will displace the galena-clausthalite univariant reaction (Figsß 8, 13) by 1.2 log units to more positive values offs .... for avbse J' ztg• •' .U = 0.8 and avbs = 0.12, and to more negative values O•jse•<g> for av•se = 02 and avbs = 0.8 relative to the location of reaction (20) for av•s• = av•s = 0.5. Using this ideal solid solution model, the mole fraction of Se in any binary sulfoselenide fixes the fs•=/fs= ratio at a given temperature and pressure. Experimental calibration of a/X relations in the solid solutions should be undertaken to evaluate the mixing models that were employed.

There is a potential use of Pb(S,Se)-Zn(S,Se) and Pb(S,Se)- Agz(S,Se) pairs in geothermometry. For a reaction such as:

ZnS + PbSe = ZnSe + PbS,

the partition coefficient,

,'x s':•' xsg'•)/iXSs ph x•), (23) KD = t Se

is a function of temperature. At present, the use of these mineral pairs in geothermometry is precluded due to the lack of an experimentally determined partition eoeffleient (KD) for both pairs in the range of temperature of interest. Exchange

experiments could provide a calibration and yield mixing models for S-Se exchange in the solids.

An examination of the s• - s dia rams temperatures ß f 2(g)• 2(g) gl, at between 100 ø to 300øC shows larger stability fields for sulfide relative to selenide minerals. This observation together with the higher crustal abundance of sulfur relative to selenium suggests that it is more likely to find sulfides than selenides in ore deposits. The same diagrams also suggest that selenides such as dausthalite (PbSe), guanajuatite (Bi2Se3), naumannite (AgaSe), tiemannite (HgSe), and nickel selenides, which are stable at relatively 10wfs•,,), are more likely to be found in ore deposits together with gsulflde minerals than other selenide minerals such as stillcite (ZnSe), ferroselite (FeSe2), and krutaite (CuSe2), which require much higher fse2. ,. These diagrams are particularly useful in predicting possibl•'mineral associations and the mineralogie form in which one chemical element of interest (e.g., Au, Ag, and Cu) is likely to occur. For example thefs•z• 5fs,,• • diagrams at temperatures between ' tg ' kg . . 100 ø to 300øC show that native gold lS stable w•th almost all selenides and sulfides in the normal range offs•z•l and except native selenium and probably krutaite whidh are only stable at very high fSe2(g), dose to selenium, saturation. This observation is in agreement with the experimental data available in the binary and ternary systems that include Au and


-10

-15

-2O

-40 -35 -30 -25

log fc• (g)

FIG. 9. fse2tg:fO2( • diagram showing the relative stability of some selenides and their corresponding oxides as a function offse2•g• and fo• at õ00øC.

Se (Okamoto and Massalski, 1986). The reported association of native gold with native selenium in natural occurrences (e.g., So et al., 1995) is a roetastable assemblage. At 300øC, native silver coexists with Cu2Se (berzelianite) and CuAgSe (eucairite) in the metal-rich part of the system (Sch•ifer, 1995), which is consistent with thermodynamic calculations at low values of fse.2/gl (Fig. 8). Furthermore, the fugacity diagrams at 300øC suggest that increasing fse•g•, klockmannite-naumannite, and krutaite-naumannite associations become stable, consistent with experiments at the same temperature (Sch•ifer, 1995).

The fsL•gl-fs2•g I diagrams also provide explanations for the rarity or absence of some selenides from natural assemblages. The compound NisSe2 has not been encountered in natural assemblages, probably because fs•l is never sufficiently di- minished to reach the stability field of the compound in the presence of Se-bearing fluids (Fig. 8). Although the occurrence of this mineral is not excluded in nature, it is predicted to be very rare and restricted to very 1Owfs2ig I conditions. For the same reason, the most frequent iron selenide found in Se-bearing deposits is ferroselite (FeSe•), whereas achavalite (FeSe?) has been encountered only in one uncertain occurrence at Sierra de Cacheuta, Argentina (Olsacher, 1939), and the other Fe-Se compounds are unknown in natural assem-

blages. On the other hand, the fs• l-fs•l, diagrams show that some compounds such as AuSe, TI•Se, •d T1Se, which have not been found in natural assemblages, are likely to occur. The absence of AuSe and T1 selenides in mineral assemblages could be due to our inabili• to identify them or because ternary minerals form instead. Fischesserite (Ag•AuSe•), petrovskaite AgAu(S,Se), and pentzinite ((Ag,Cu)4Au(S,Se)4) can account for the absence of AuSe in geologic systems. Likewise, bukovite, TI•(Cu,Fe)4Se4 (Johan et al., 1971), crookesitc, Cuv(T1,Ag)Se4, and sabatierite, Cu•T1Se4, can account for the absence of binary thallium selenides. So et al. (1995) reported up to 18 wt percent Se (equivalent of up to 35.15 at. percent Se) in gold grains, with a fineness (1,000'Au/(Au + Ag), in wt %) of 817 to 980, from the Prasolovskoye Te- and Se-bearing gold deposit. This may re resent an intergrowth between gold and an Au selenide p , because no solid solution was reported in the Au-Se system (Okamoto and Massalski, 1986). The effect of Ag in solid solution in Au may affect the solubility of Se in electrum, but the amount of Ag encountered in gold in this deposit probably cannot account for such a high content of Se (35 at. % Se) in solid solution in electrum. The presence of the •old- silver selenides fischesserite (Ag•AuSe•) and petrovsl•aite (AgAu(S,Se)) intergrown with Au is not sup orted by the P observed high Se/Ag ratio in the Se-rich gold. Although a


, I t I ] I I , I I [ I ] [ I I [ I i

,,.,.

-lO

-15

I•.x•e (A•,•)ttq.

-20

-20 -15 -10 -5

log fTe2

FIo. 10. /Se•z( )-/Te•zl ) diagram showing the relative stability of some selenides and their corresponding tellurides as . g o a function offse2•g• an:•[fT•2rg> at 300 C.

metastable or disequilibrium association of selenium and gold cannot be excluded, this composition may represent a very fine intergrowth between Au and AuSe.

The diagrams also show the equilibrium selenide and sulfide assemblages. For example, naumannite can occur in equilibrium with pyrite, galena, stibnite, bismuthinite, chalcopyrite, bornitc, cinnabar, and nickel sulfides. Nickel selenides are stable only with chalcopyrite, bornitc, and pyrite or pyrrhotite, and unstable with argentitc, galena, bismuthinite, stibnite, carlinitc, and cinnabar. Mineral associations such as umangite + chalcopyrite or klockmannite + pyrite or krutaite + pyrite can be used to constrain and /S2(g ) conditions. For example, umangite + chalcopyrite represents an equilibrium assemblage only at logfs•,•,, between -14.3 to -14.7 and log fs.•, • between -22•.5 t•;g•-22.8 at 100øC (Fig. 13). The only •;•her Fe-bearing binary sulfide or selenide that can be in equilibrium with this association is ferroselite and not achavalite, pyrite, or pyrrhotite. These /Se2(g)-/S2(g ) diagrams are useful if tl% minerarh' s semblage con- sists of sulfide and selenide minerals. If oxide and/or tellu-

ride minerals are also present, application of other fugacity- fugacity diagral]•lS (e.g.,/Se2(g)-/O2(g ) and/Se2(g)-/Te(g)) may provide additional information.

fs•ig)-fo•i• diagrams

This type of diagram was constructed at 300øC (Fig. 9). The expected upper limit offo•<, is that in the atmosphere, a condition not attained during s•lenide deposition. The normal range of fs•, and fo2•. in selenide-bearing deposits is defined by the re•t•tion fay•J'ite + Oa/g/= quartz + magnetite and galena + Oa(g) = anglesite together with the previously mentioned constraints for fs•- (Fig. 9). The fs•, ,-fo•, diagrams offer the same type o•nformation as fs•igg•,-fs•iggl diagrams. Probably the most important observation in these diagrams refers to the large stability field of SnO• (cassiterite) when compared to SnSe and SnSe•, which explains why tin selenide minerals, that otherwise have a large stability field in the fs•igl-fs.•ig I diagrams are not present in natural assemblages. A similar observation may be made for uraninite. Iron and some manganese oxides (e.g., magnetite, hematite, manganosite, and hausmannite) represent another group of oxide minerals that are commonly observed in ore deposits. On the other hand, downeyitc (SeO•) requires extremely high fs•g• and fo•<g>. Therefore, it is unlikely to be found as a primary mineral. The majority of oxides of the other elements are unstable with respect to their corresponding selenides and


-10

-15

-2O

(•r)

-20 -15 -10 -5

•og fTe2

FIc. 11. fs•<g)-f•'•(• diagram showing the relative stability of additional selenides and their corresponding tellurides as a function offse2(a) ancl fTe.,(g, at 300øC.

sulfides (Fig. 9). Most selenides, except Sn selenides, are stable with uraninite and hematite over a large range of fse•tg> andfo•lg/, which is in agreement with the common association of these minerals in the sandstone- and unconformity- type uranium deposits.

/Se2(g )-iTc(g) diagrams The stability of metal selenide and telluride minerals is

shown in Figures 10 and 11 as a function offse2i•> andfTei•) at 300øC. The lower limit offTe2/g/ in telluride-bea•'ing deposits can be considered the native silver-hessite reaction, because native silver is not a common mineral in these deposits. Exam- ination of the fs•< .-fT• > diagrams at 300øC suggests that the fs•.•(g> required to s•'•abi[•ze the binary metal selenides, except naumannite, tiemannite, and AuSe, is lower than the f•<g> required to stabilize the equivalent tellurides. Therefore, thermodynamically, it is anticipated that selenides may be more widespread than tellurides in ore deposits. This predic- tion does not necessarily suggest that selenide-bearing deposits would be more abundant than telluride-bearing deposits but only that the binary metal selenides are more stable than their correspohding tellurides at the same value Offse.atg/and fTe(g)' The actual presence of selenide or telluride minerals

will depend on the relative values Offsea( > andfTe( > which will be related, among other factors, to theirgconcentrgation in the hydrothermal fluid. In nature, tellurium minerals are apparently more common than selenium minerals, probably because selenium is a highly chalcophile element in comparison to tellurium. Therefore, tellurium will form individual minerals, whereas selenium more readily substitutes in sulfide minerals. Partly for this reason, selenide minerals occur more frequently in oxidizing environments and not in sulfide-rich deposits. Copper selenide minerals, shown to have a larger stability field than corresponding copper telluride minerals in the/Se2{g)-/Te(g ) diagrams, are found more frequently in ore deposits tlian copper telluride minerals, as are stillcite (ZnSe), cadmoselite (CdSe), and drysdallite (MoSes), whereas the corresponding telluride minerals remain unknown.

Thefsea< >-fTe< > diagrams suggest that clausthalite (PbSe) is stable withgthe •najority of common tellurides, including coloradoitc (HgTe), hessitc (AgaTe), sttitzite (Ags_xTea), tellurantimony (Sb•T%), tellurobismuthite (Bi•Tea), and probably most of the ternary phases including petzite (Ag3AuTez), syl- vanitc ((Au,Ag)zTe4), and krennerite ((Au,Ag)Tez) over a large range Offsea< i and fTe< >. The associations naumannite- calaverite and tiem•tnnite-sttigtzite have a very restricted stabil-


-lO

-15

-20

-25

-30

-3o -25 -2o -15 -1 o

log rs2 (g)

FIe. 12. fse2• )-fs2• • diagram showing the relative stability of some selenides and their corresponding sulfides as a function offs .... andfs2•/gat 1•50øC. e g

• field m terms of se andfTe(g) (Fig. 11) and can be used to estimate the fugaeities of Se2(g) and Te2(g) during the deposition of selenide and telluride minerals.

Discussion

Although fugaeity-temperature and fugaeity-fugaeity diagrams do not entirely reflect the complexity of the geologic systems, they still can be used to evaluate of the genesis of Te- and Se-bearing ore deposits. The Te- and Se-bearing mineralization at Prasolovskoye, Kunashir Island, described by So et al. (1995), was chosen to show the possible application of fugaeity-fugaeity diagrams to geologic systems. Most of the associations observed by So et al. (1995) in this deposit are probably disequilibrium associations, such as the stage III gold-silver-type telluride (III-C) association consisting of naumannite, goldfieldire, altaite, joseite, Se joseire, native selenium, Se-Te alloys, native tellurium, sylvanire, and krennerite. At temperatures between 100 ø and 300øC naumann- ire is not stable together with altaire, and native selenium cannot be stable with native tellurium because these two

minerals show complete solid solution at temperatures higher than 100øC (Sharma et al., 1990). The (Se,Te) alloys (selentellurium) might have been precipitated from hydrothermal fluids or could represent a partial reequilibration

of the earlier native elements, native tellurium, or native selenium, with late hydrothermal solutions rich in selenium or tellurium, respectively. A more detailed discussion and some applications of the fugaeity-temperature and fugaeity- fugaeity diagrams to other selenide-bearing ore deposits is in preparation.

A thermodynamic study on a limited portion of the selenide, sulfide, and telluride system by Dr•tbek (1995) requires considerable revision. DrSbek failed to obtain a stability field of some compounds that had been experimentally proven to be stable in the range of temperature of interest (e.g., fiFel+,Se), miscalculated the stability field of others (e.g., MoS2), and reached conclusions that are not entirely supported by his calculated phase diagrams. The fse.,<g)-fs.,•g•, fTe(g)-fS2(g), and fs•.,l,-temperature diagrams constructed by DrSbel• (1995) are [Sased on thermodynamic properties com- piled by Mills (1974). Thefs•.,•ctemperature diagram suggests that FeSe0.96• (or fiFel+xSe) is metastable when compared to FeSe between 250 ø to 450øC, an observation that is in disagreement with the experimental data in the Fe-Se system (Okamoto, 1990a). The Mo-MoS2 univariant reaction is shifted by more than 6 log units to higher values Offs2lg) at 300øC compared to calculations using thermodynamic properties from Mills (1974), Barin (1989, 1993), and Robie and


-10

-15

-2O

-25

-30 -25 -20 -15 -10

•og rs2

FIC. 13. fse,,•g•-fs•<g• diagram showing the relative stability of additional selenides and their corresponding sulfides as a function offs • and fs• at 100øC. e g g

Hemingway (1995). Dribek concluded that molybdenite may form equilibrium assemblages with Ag, Pb, Fe, Hg, Bi, and Au tellurides. This conclusion is not entirely supported by the/S2(g)-/Te(g ) diagram presented, because there is no overlap between the stability field of Fe tellurides and molybdenite. However, this conclusion is entirely in agreement with our calculations.

Conclusions

1. Fugacity of Se2(g), S2(g), O2(g), and temperature, and to a lesser extent fre(./, are the most important variables that control selenide m•neral assemblages for any given metallic composition of the system.

2. The assemblage electrum-naumannite may be used as a sliding-scale indicator offse•(•.

3. Most of the oxide minerals, except those of Sn and Fe, are unstable relative to corresponding sulfide, selenide, and/ or telluride minerals over the normal range Offo2(gl encountered in ore deposits. Except for hessitc (Ag½Te) and calaver- itc (AuTe•), telluride minerals are not stable relative to the corresponding selenide minerals at the same values of the fugacity of Se•(g) and Te•(g).

4. Fugacity-temperature diagrams may be used to predict

the decomposition and/or melting temperatures of different compounds for which experimental data are not available. They may also be used to show the relative tendency of a metallic element to form selenium compounds.

5. Fugacity-fugacity diagrams constructed in this paper may explain the occurrence or absence of particular selenide compounds as minerals. Common selenides such as clausthai- itc (PbSe), naumannite (Ag2Se), and tiemannite (HgSe) have a relatively large stability field in the fugacity-fugacity diagrams over the normal range of fs•2<g>, fS=(g>, fo2(e f•:%>, and temperature in ore deposits. Some rare selenii]es such as stillcite (ZnSe) have a narrow stability field over the normal range offs•=(g>, fS=(g>, fo=(g>, fr%>, and temperature in ore deposits. Some other selenide compounds such as NiaSe•, /3Fe•+xSe, and Sn selenide minerals require uncommonly low values offs,21g) orfo,2/g) and therefore are predicted to be very rare in natural assemblages.

6. Fugacity-fugacity diagrams may be used to characterize observed mineral assemblages in terms of equilibrium associations and to predict associated and antipathetic minerals.

7. If partial information on the mineral associations and temperature of mineral deposition is available into an ore deposit, fugacity-fugacity diagrams may be used to predict


the mineralogic form of any chemical element of interest. Likewise, the temperature of mineral deposition may be pre-

dicted usingr•hentfS .... -temverature diagram if particular minerals are p se i•g•the •ame association. A more detailed discussion and examples are provided into a following paper.

Acknowledgments Support for this study was received by G.S. from the Insti-

tute of International Education and the International Insti-

tute at University of Michigan through a Fulbright grant and fellowship, respectively. This study benefited greatly from the close collaboration with S.E. Kesler, whose support and critical reviews of the early versions of this manuscript are gratefully acknowledged. The authors would like to thank W.C. Kelly, K. Kojonen, J.R. O'Neil, R.R. Seal, P.G. Spry, D.P. Stenger, G. Udubasa, and E.H.P. van Hees for insightful discussions and reviews. This manuscript has also benefited from the comments and critical reviews of Economic Geology referees.

April 15, October 14, 1996 REFERENCES

Afifi, A.M., Kelly, W.C., and Essene, E,J., 1988, Phase relations among tellurides, sulfides, and oxides: I.Thermochemical data and calculated equilibria: Economic Geology, v. 83, p. 377-394.

Andriamihaja, A., Ibanez, A., Jumas, j.-c., Olivier-Fourcade, J., and Phil- ippot, E., 1985, Evolution structurale de la solution solide Sb.2Te3-xSex (x = 0-2) dans le syst•me Sb2Te3-Sb.2Sea: Revue de Chemie Minerale, v. 22, p. 357-368.

Anthony, J.w., Bideaux, R.A., Bladh, K.W., and Nichols, M.C., 1990, Hand- book of mineralogy: Tucson, Arizona, Mineral Data Publishing, 588 p.

Asadov, M.M., 1983, The HgS + T1.2Se = HgSe + T1.2S reciprocal system: Russian Journal of Inorganic Chemistry, v. 23, p. 1025-1028.

Banas, M., and Ottemann, J., 1971, Supplementary data on bohdanowiczite, a natural silver-bismuth selenide: Mineralogia Polonica, v. 2, p. 37-44.

Bankina, V.F., and Abrikosov, N.K., 1964, The Bi.2Sea-Bi.2Te3 system: Russian Journal of Inorganic Chemistry, v. 9, p. 509-513.

Barin, I., 1989, Thermochemical data of pure substances (1st ed.): Weinheim, Germany, VCH Verlagsgesellschaft mbH, 1739 p.

--1993, Thermochemical data of pure substances (2nd ed.): Weinheim, Germany, VCH Verlagsgesellschaft mbH, 1874 p.

Barin, I., and Knacke, O., 1973, Thermochemical properties of inorganic substances: Berlin, Springer-Verlag, 921 p.

Barton, M.D., 1980, The Ag-Au-S system: ECONOMIC GEOI•OC¾, v. 75, p. 303-317.

Barton, P.B., Jr., 1969, Supplementary study of the system Fe-As-S: Geo- chimica et Cosmochimica Acta, v. 33, p. 841-857.

Barton, P.B., Jr., and Skinner, B.J., 1979, Sulfide mineral stabilities, in Barnes, H.L., ed., Geochemistry of hydrothermal ore deposits: New York, Wiley Interscience, p. 278-403.

Barton, P.B., Jr., and Toulmin, P., 1964, The electrum-tarnish method for determination of the fugacity of sulfur in the laboratory sulfide systems: Geochimica et Cosmochimica Acta, v. 28, p. 619-640.

Bernardini, G.P., Corsini, F., Lattanzi, P.F., and Mazzetti, G., 1983, The Cu-Fe-Se system: Summary of phase relations; comparison with natural occurrences and implications for the Cu-Fe-S-Se system: Neues Jahrbuch fiir Mineralogle, Monatshefte, v. 7, p. 323-336.

Bernardini, G.P., Corsini, F., Fei, A. and Mazzetti, G., 1987, The CuSe.2- FeSe•2 section of the Cu-Fe-Se system and the stability field of the (Cu,Fe)Se•2 phase: Neues Jahrbuch fiir Mineralogle, Monatshefte, v. 2, p. 82-96.

Blachnik, R., and Kurz, G., 1984, Compounds in the system As2Sea-Cu.2Se: Journal of Solid State Chemistry, v. 55, p. 218-224.

Bochek, L.I., Sandomirskaya, S.M., Chuvikina, N.G., and Khvorostov, V.P., 1984, A new selenium-containing sulfide of silver, gold and copper-pentzinite (Ag,Cu)4Au(S,Se)4: Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, v. 113, p. 356-360 (in Russian) and (1985) American Miner- alogist, v. 70, p. 875-876.

Bur'yanova, E.Z., and Komkov, A.I., 1955, A new mineral-ferroselite: Acade- mii Nauk SSSR Doklady, v. 105, p. 812-813 (in Russian) and (1956) American Mineralogist, v. 41, p. 671.

Bur'yanova, E.Z., Kovalev, G.A., and Komkov, A.I., 1957, The new mineral cadmoselite: Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, v. 86, p. 626-628 (in Russian) and American Mineralogist, v. 43, p. 623.

Cech, F., Rieder, M., and Vrana, S., 1973, Drysdallite, MoSe.2, a new mineral: Neues Jahrbuch fiir Mineralorie, Monatshefte, v. 10, p. 433-442.

Chakrabarti, D.J., and Laughlin, D.E., 1981, The Cu-Se (copper-selenium) system: Bulletin of Alloy Phase Diagrams, v. 2, p. 305-315.

Chang, L.L.Y., and Ondik, M.O., 1992, Se-Au-Bi system, in Stringfellow, G.B., ed., Phase equilibria diagrams, phase diagrams for ceramists: Colum- bus, Ohio, American Ceramic Society, v. 9, p. 331-333.

Clark, J.B., and Rapoport, E., 1970, Effect of pressure on solid-solid transi- tions in some silver and cuprous chalcogenides: Journal of Physics and Chemistry of Solids, v. 31, p. 247-254.

Craig, J.R., and Barton, P.B., Jr., 1973, Thermochemical approximations for sulfosalts: ECONOMIC GEOLOGY, v. 68, p. 493-506.

Cranton, G.E., and Heyding, R.D., 1968, The gold/selenium system and some gold seleno-tellurides: Canadian Journal of Chemistry, v. 46, p. 2632-2640.

Davis, R.J., Clark, A.M., and Criddle, A.J., 1977, Palladselte, a new mineral from Itabira, Minas Gerais, Brazil: Mineralogical Magazine, v. 41, p. 123.

Dembrovskii, S.A., Kirilenko, V.V., and Khvorostenko, A.S., 1971, Phase equilibria and glass formation in the system As.2Se•-Cu.2Se and As.2Se-SnSe (PbSe): Inorganic Materials, v. 7, p. 60-65.

De Montreuil, L.A., 1975, Bellidoite, a new copper selenide: ECONOMIC GEOLOGY, v. 70, p. 384--387.

Dr5bek, M., 1995, The Mo-Se-S and the Mo-Te-S systems: Neues Jahrbuch ftir Mineralogie, Abhancllungen, v. 169, p. 255-263.

Dumas, J.F., Brun, G., Liautard, B., Tedenac, J.C., and Maurin M., 1987, New contribution in the study of the Bi.2Te3-Bi2Sea system: Thermochim- ica Acta, v. 122, p. 135-141.

Dunn, P.J., Peacor, D.R., Criddle, A.J., and Finkelman, R.B., 1986, Lapha- mite, an arsenic selenide analogue of orpiment, from burning anthracite deposits in Pennsylvania: Mineralogical Magazine, v. 50, p. 279-282.

D'yachkova, I.B., and Khodakovskiy, V.I., 1968, Thermodynamic equilibria in the systems S-H.20, Se-H.20 and Te-H.20 in the 25-300øC range and their geochemical interpretation: Geochemistry International, v. 5, p. 1108-1125.

Dymkov, Y.M., Loseva, T.I., Zav'yalov, E.N., Ryzhov, B.I., and Bochek, L.I., 1982, Mgriite, (Cu,Fe)aAsSe3, a new mineral: Zapiski Vsesoynznogo Mineralogicheskogo Obshchestva, v. 111, p. 215-219 (in Russian) and (1983) American Mineralogist., v. 68, p. 280-281.

Elagina, E.I., 1961, The system PbSe-Bi.2Se•: Voprosy Metallogenii Fiziki Poluprovedikov, Akademiya Nauk SSSR, Moskow, p. 153-158 (in Rus- sian).

Eliot, R.P., 1965, Constitution of binary alloys (1st supplement): New York, McGraw-Hill, 877 p.

Frueh, A.J., Jr., Czamanske, G.K., and Knight, C.H., 1957, The crystallogra- phy of eucairite, AgCuSe: Zeitschrift for Kristallographie, Kristallgeome- trie, Kristallphysik, Kristallchemie, v. 108, p. 389-396.

Fyfe, W.S., Turner, F.J., and Verhoogen, J, 1958, Metamorphic reactions and metamorphic facies: Geological Society of America Memoir, v. 73, 259 p.

Gather, B., 1976, The Au-Sb-Se system: Unpublished Ph.D. dissertation, Clausthai, Germany, Technischen Universitat Clausthai, 211 p.

Gather, B., and Blachnik, R., 1975, Ternare chalkogenhaltige systeme. II Das system gold-wismut-selen: Zeitschrift fiir Metallkunde, v. 66, p. 356- 359.

Gather, B., and Prince, A., 1990, Au-Pb-Se system, in Prince, A., Raynor, G.V., and Evans, D.S., eds., Phase diagrams of ternary gold alloys: London, Brookfield, Vermont, Institute of Metals, p. 338-348,

Godovikov, A.A., Nenasheva, S.N., and Leibson, R.M., 1967, Lead selenium- bismuth selenium system: Trudy Institut Geologii i Geofiziki, Akademiya Nauk SSSR, Sibirskoe Otdelenie, v. 5, p. 34-54 (in Russian).

Gr0nvold, F., and Stolen, S., 1992, Thermodynamics of iron sulfides II. Heat capacity and thermodynamic properties of FeS and of Fe0 s75S at temperatures from 298.15 K to 1000 K, of Fe0.9sS from 298.15 K to 800 K, and of Fe0.s9S from 298.15 K to about 650 K. Thermodynamics of formation: Journal of Chemical Thermodynamics, v. 24, p. 913-936.

H'aldi, T.A., Vuorelainen, Y., and Sahama, T.G., 1965, Kitkaite (NiTeSe), a

19,04 SIMON AND ESSENE

new mineral from Kuusamo, northeast Finland: American Mineralogist, v. 50, p. 581-586.

Hansen, M., and Anderko, K., 1958, Constitution of binary alloys: New York, McGraw-Hill, 1305 p.

Harris, D.C., Cabri, L.J., and Kaiman, S., 1970, Athabascaite: A new copper selenide from Martin Lake, Saskatchewan: Canadian Mineralogist, v. 10, p. 207-215.

Heyding, R.D., 1966, The copper/selenium system: Canadian Journal of Chemistry, v. 44, p. 1233-1236.

Holland, H.D., 1959, Some application of thermochemical data to problems of ore deposits. I. Stability relations among oxides, sulfides, sulfates and carbonates of ore and gangue minerals: ECONOMIC GEOLOGY, v. 54, p. 184-233.

--1965, Some application of thermochemical data to problems of ore deposits. II. Mineral assemblages and the composition of ore-forming fluids: ECONOMIC GEOLOGY, v. 60, p. 1101-1166.

Itoga, R.S., and Kannewurf, C.R., 1971, Electrical resistivity, Hall effect and optical absorption in T1S, T1S0.5Se0.5 and T1Se: Journal of Physics and Chemistry of Solids, v. 32, p. 1099-1110.

Ivlieva, VA., and Abrikosov, N.K., 1964, Pseudobinary sections Sb2S3-Sb2Se3 and Sb2Se3-SbzTea: Akademii Nauk SSSR Doklady, v. 159, p. 1326-1329.

Jaireth, S., 1991, Hydrothermal geochemistry of Te, AgaTe and AuTez in epithermal precious metal deposits: North Queensland, James Cook Uni- versity, Geology Department, Economic Geology Research Unit Contri- bution 37, 22 p.

Johan, Z., and Kvacek, M., 1971, La bukovite, Cua+xTlzFeSe4_x, une nouvelle esp•ce minrrale: Soci•t• Fran•aise de Mineralogle et Christallographie Bulletin, v. 94, p. 529-533.

Johan, Z., Picot, P., Pierrot, R., and Verbeek, T., 1970, L'oosterboschite (Pd,Cu)7Se5, une nouvelle esp•ce minrrale, et la trogtalite cupro-palladi- f•re de Musonoi (Katanga):Soci•t• Fran•aise de Mineralogie et Christallo- graphic Bulletin, v. 93, p. 476-481.

Johan, Z., Picot, P., Pierrot, R., and Kvacek, M., 1971, La fischesserite, Ag3AuSez, premier seleniurs d'or, isotype de la petzite: Soci•t• Fran•aise de Mineralogle et Christallographie Bulletin, v. 94, p. 381-384.

--1972, La krutaite, CuSe2, un nouveau mineral du groupe de la pyrite: Socirt6 Fran•aise de Mineralogie et Christallographie Bulletin, v. 95, p. 475-481.

Johan, Z., Kvacek, M., and Picot, P., 1978, La sabatierite, un nouveau s61•ni- ure de cuivre et de thallium: Bulletin de Mineralogle, v. 101, p. 557-560.

Johnson, j.w., Oelkers, E.H., and Helgeson, H.C., 1992, SUPCRT92: A software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bars and 0 ø to 1000øC: Computers and Geosciences, v. 18, p. 899-947.

Kato, A., 1959, Ikunolite, a new bismuth mineral from the Ikuno mine, Japan: Mineralogical Journal (Japan), v. 2, p. 397-407.

Komarek, K.L., and Wessely, K., 1972, Obergangsmetall-Chalkogensysteme, 2 Mitt.: Die systeme nickel-selen und kobalt-nickel-selen: Monatshefte Chemic, v. 103, p. 923-933.

Kubaschewski, O., and Alcock, C.B., 1977, Metallurgical thermochemistry (5th ed.): London, Pergamon Press, 449 p.

Lausen, C., 1928, Hydrous sulphates formed under fumarolic conditions at the United Verde mine: American Mineralogist, v. 13, p. 227-229.

Lin, J.C., Sharma, R.C., and Chang, Y.A., 1986, Pb-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (lst ed.): Materials Park, Ohio, ASM International, p. 1842-1845.

Liu, H., and Chang, L.L.Y., 1994, Lead and bismuth calcogenide systems: American Mineralogist, v. 79, p. 1159-1166.

Losana, L., 1923, Confronto tra curve di dilatazione e curve termiche: Gaz- zeta Chimica Iraliana, v. 53, p. 393-410.

Main, J.V., Rodgers, K.A., Kobe, H.W., and Woods, C.P., 1972, Aguilarite from the Camoola reef, Maratoto Valley, New Zealand: Mineralogical Magazine, v. 38, p. 961-964.

Mallika, C., and Sreedharan, O.M., 1988, Standard molar Gibbs energy of formation of MoTez from e.m.f. measurements: Journal of Chemical Thermodynamics, v. 20, p. 769-775.

Massalski, T.B., Okamoto, H., Subramanian, P.R., and Kacprzak, L., 1990a, As-Se Phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (lst ed.): Materials Park, Ohio, ASM International, p. 318.

--1990b, Co-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 1237.

--1990c, Mo-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase

diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 2665. --1990d, Sb-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase

diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 3302. McCormick, S., McCormick, A., Morefield, G., and McCormick, M., 1994,

TAPP 2.2: A database of thermochemical and physical properties: Hamil- ton, Ohio, E.S. Microware.

Mills, K.C., 1974, Thermodynamic data for inorganic sulfides, selenides, and tellurides: London, Butterworths, 845 p.

Nasar, A., and Shamsuddin, M., 1990a, Thermodynamic investigations of HgTe: Journal of the Less-Common Metals, v. 161, p. 87-92.

1990b, Thermodynamic properties of cadmium selenide: Journal of the Less-Common Metals, v. 158, p. 131-135.

Nechelynstov, G.N., Christyakova, N.I., and Zav'yalov, E.N., 1984, Nevskite, a new bismuth selenide: Zapiski Vsesoynznogo Mineralogicheskogo Obsh- chestva, v. 113, p. 351-355 (in Russian) and (1985) American Mineralogist, v. 70, p. 875.

Nesterenko, G.V., Kuznetsova, A.I., Pal'chik, N.A., and Lavrent'ev, Y.G., 1984, Petrovskaite, AuAg(S,Se), a new selenium-containing sulfide of gold and silver: Zapiski Vsesoynznogo Mineralogicheskogo Obshchestva, v. 113, p. 602-607 (in Russian) and (1985) American Mineralogist, v. 70, p. 1331.

O'Hare, P.A.G., 1993, Calorimetric measurements of the specific energies of reaction of arsenic and of selenium with fluorine. Standard molar enthal-

pies of formation Af H• at the temperature 298.15 K of AsFs, SeFt, As2Se•, As4S4, and As•S3. Thermodynamic properties of AsF5 and SeF6 in the ideal-gas state. Critical assessment of/XfH•ø,(AsF3, 1), and the dissociation enthapies of As-F bonds: Journal of Chemical Thermodynamics, v. 25, p. 391-402.

O'Hare, P.A.G., Tasker, I.R., and Tarascon, J.M., 1987, A fluorine-combus- tion calorimetric study of two molybdenum selenides: MoSee and Mo•Se•: Journal of Chemical Thermodynamics, v. 19, p. 61-68.

O'Hare, P.A.G., Lewis, B.M., Susman, S., and Volin, K.J., 1990, Standard molar enthalpies of formation and transition at the temperature 298.15K and other thermodynamic properties of the crystalline and vitreous forms of sesquiselenide (AszSea). Dissociation enthalpies of As-Se bonds: Journal of Chemical Thermodynamics, v. 22, p. 1191-1206.

Okamoto, H., 1990a, Fe-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 1770.

--1990b, Pd-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 2665.

--1990c, Bi-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 790.

Ohmoto, H., and Massalski, T.B., 1986, Au-Se phase Diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 309-312.

Oleinik, G.S., Mizetskii, P.A., and Nizkova, A.I., 1982, Nature of the interac- tion between lead and zinc chalcogenides: Inorganic Materials, v. 18, p. 734- 735.

Olsacher, J., 1939, Achavalita, seleniuro de hierro, nueva especie mineral: Universidad Nacional de Cordoba (Argentina), Facultad de Ciencias, Bo- lefin, v. 3-4, p. 73-78.

Petruk, W., Owens, D.R., Stewart, J.M., and Murray, E.J., 1974, Observa- tions on acanthite, aguilarite and naumannite: Canadian Mineralogist, v. 12, p. 365-369.

Petzow, G., and Effenberg, G., 1988, Silver-selenium-tellurium, in Petzow, G., and Effenberg, G., eds., Ternary alloys: Weinheim, Germany, Vch Verlagsgesellschaft, p. 567-568.

Prince, A., Raynor, G.V., and Evans, D.S., 1990, Phase diagrams of ternary gold alloys: London, Brookfield, Vermont, Institute of Metals, 505 p.

Ramdohr, P., 1949, Neue Erzmineralien: Fortschritte der Mineralogle, v. 28, p. 69-70.

1980, The ore minerals and their intergrowths (2nd ed.): Berlin, Perga- mon Press, 1207 p.

Ramdohr, P., and Schmitt, M., 1955, Vier neue naturaliche kobaltselenide von Steinbruch Trogtal bei Lautenthal im Harz: Neues Jal•rbuch far Min- eralogle, Monatshefte, v. 6, p. 133-142.

Robie, R.A., and Hemingway, B.S., 1995, Thermodynamic properties of minerals and related substances at 298.15 K and i bar (105 pascals) pressure and at higher temperatures: U.S. Geological Survey Bulletin 2131, 461 p.

Robie, R.A., Seal, R.R., II, and Hemingway, B.S., 1994, Heat capacity and entropy ofbornite (Cu5FeS4) between 6 and 760 K and the thermodynamic

SELENIDE MINERALS PHASE RELATIONS: I. THERMODYNAMICS 19,05

properties of phases in the system Cu-Fe-S: Canadian Mineralogist, v. 32, p. 945-956.

Robinson, S.C., and Brooker, E.J., 1952, A cobalt-nickel-copper selenide from the Goldfields district, Saskatchewan: American Mineralogist, v. 37, p. 542-544.

Rouland, J.C., Legendre, B., and Souleau, C., 1977, The ternary system gold-lead-selenium: Journal of Chemical Research, Part S, Synopses, v. 10, p. 2754-2784.

Sch/ifer, E., 1995, Experimental investigations in the Cu-Ag-S-Se system: Neues Jahrbuch fiir Mineralogie, Abhandlungen, v. 169, p. 301-304.

Schuster, W., Milker, H., and Komarek, K.L, 1979, Transition metal-chalco- gen systems, VII: The iron-selenium phase diagram: Monatshefte fiir Chemic, v. 110, p. 1153-1170.

Sch0nwandt, H.K., 1983, Interpretation of ore microstructures from a sele- neous Cu-mineralization in South Greenland: Neues Jahrbuch fiir Miner- alogie, Abhandlungen, v. 146, p. 302-332.

Scott, W., and Kench, J.R., 1973, Phase diagram and properties of CuaSbSe4 and other Aa(I)B(V)C4(VI) compounds: Materials Research Bulletin, v. 8, p. 1257-1268.

Seal, R.R., II, Robie, R.A., Barton, P.B., Jr., and Hemingway, B.S., 1992, Superambient heat capacities of synthetic stibnite, berthierite, and chal- costibnite: Revised thermodynamic properties and implications for phase equilibria: ECONOMIC GEOLOGY, v. 87, p. 1911-1918.

Sharma, R.C., and Chang, Y.A., 1990a, Cd-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 1024.

--1990b, Hg-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 2165.

--1990c, S-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 3279.

Sharma, R.C., Li, D.T., and Chang, Y.A., 1990, Te-Se phase diagram, in Massalski, T.B., ed., Binary alloy phase diagram (2nd ed.): Materials Park, Ohio, ASM International, p. 3279.

Shukla, N.K., Prasad, R., Roy, K.N., and Sood, D.D., 1990, Standard molar enthalpies of formation at the temperature 298.15-K of iron telluride (FeTe0.9) and of nickel telluride (Ni0.s9sTe040.•): Journal of Chemical Ther- modynamics, v. 22, p. 899-903.

Shunk, F.A., 1969, Constitution of binary alloys (2nd supplement): New York, McGraw-Hill, 720 p.

Simpson, D.R., 1964, The binary system PbS-PbSe: ECONOMIC GEOLOGY, v. 59, p. 150-153.

Smit, T.J.M., Venema, E., Wiersma, J., and Baiwir, M., 1970, Phase transi-

tions in silver gold chalcogenides: Journal of Solid State Chemistry, v. 2, p. 309-312.

So, C-S., Dunchenko, V.Y., Yun, S-T., Park, M-E., Choi, S-G., and Shelton, K.L., 1995, Te- and Se-bearing epithermal Au-Ag mineralization, Praso- lovskoye, Kunashir Island, Kuril Island arc: ECONOMIC GEOLOGY, v. 90, p. 105-117

Stanley, C.J., Griddle, A.J., and Lloyd, D., 1990, Precious and base metal selenide mineralization at Hope's Nose, Torquay, Devon: Mineralogical Magazine, v. 54, p. 485-493.

Steininger, J., 1970, Phase diagram of the PbTe-PbSe pseudobinary system: Metallurgical Transactions, v. 1, p. 2939-2941.

Tomashik, Z.F., Oleinik, G.S., Tomashik, V.N., and Nizkova, A.I., 1981, Phase-equilibria in the PbSe-CdS system: Inorganic Materials, v. 17, p. 1610-1612.

Troften, P., and Kullernd, G., 1961 The Fe-Se system: Carnegie Institution of Washington, Papers from the Geophysical Laboratory, no. 1363, p. 176.

Vaughan, D.J., and Craig, J.R., 1978, Mineral chemistry of metal sulfides: Cambridge, Cambridge University Press, 493 p.

Vorma, A., 1959, Laitakarite, a new Bi-Se mineral in Orij•irvi: Geologi, v. 3, p. 11 (in Finnish) and (1959) American Mineralogist, v. 44, p. 908.

Vuorelainen, Y.A., Huhma, A., and H'aldi, A., 1964, Sederholmite, wilkman- itc, kullerudite, m'akinenite, and trtistedtite, five new nickel selenide minerals: Geological Society of Finland Bulletin, v. 36, p. 113-125.

Wagman, D.D., Evans, W.H., Parker, V.B., Schumm, R.H., Halow, I., Bailey, S.M., Churney, K.L., and Nuttall, R.L., 1982, The NBS tables of chemical thermodynamic properties: National Bureau of Standards (NBS), Wash- ington, D.C., Journal of Physical Chemistry Reference Data, 11, suppl.2.

White, J.L., Orr, R.L., and Hultgren, R., 1957, The thermodynamic properties of silver gold alloys: Acta Metallurgica, v. 5, p. 747-760.

Wiegers, G.A., 1976, Electonic and ionic conduction of solid solutions Ags-x- AuxSe (0 < X < 0.5): Journal of Less-Common Metals, v. 48, p. 269- 283.

Woods, T.L., and Garrels, R.M., 1987, Thermodynamic values at low temperature for natural inorganic materials: An uncritical summary: New York, Oxford University Press, 242 p.

Wright, H.D., Barnard, W.M., and Halbig, J.B., 1965, Solid solutions in the systems ZnS-ZnSe and PbS-PbSe at 300øC and above: American Mineralo- gist, v. 50, p. 1802-1815.

Zhang, X., and Spry, P.G., 1994, Calculated stability of aqueous tellurium species, calaverite and hessitc at elevated temperatures: ECONOMIC GE- OLOGY, v. 89, p. 1152-1166.

Zhukov, E.G., Dzhapparidze, O.I., Dembrovskii, S.A., and Popova, N.P., 1974, The system As2S3-As2Se3: Inorganic Materials,


APPENDIX I

Gibbs Free Energies of Formation of Binary Selenium Compounds • (kJ)

Temperature range System Reaction ArC, St (kJ) for T (K) Accuracy • (øC)

Se Se2(g) = 2Se(s) -140.742 + 0.175 T - 2.565' 10 -s T 2 A 25-220 Se2(g) = 2Se(•) -129.288 + 0.151 T - 2.300' 10 -s T 2 A 220-400

Ag-Se 4Ag q- Se•(g) = 2Ag•Se -213.485 + 0.110 T A 25-133 -191.193 + 0.055 T A 133-400

As-Se 2As q- Se2(g) = 2AsSe -213.890 + 0.152 T + 2.022' 10 -s T 2 B 25-264 2As + Se2(g) = 2(As,Se)o I -154.281 + 0.061 T B 250-350 4/3As + Se2(g) = 2/3As2Se3 -268.655 + 0.684 T - 1.208' 10 -3 T • + 8.424.10 -7 T a A 25-375

Au-Se 2Au q- Se2(g) = 2AuSe -167.303 + 0.197 T - 1.625' 10 s T 2 A 25-400 Bi-Se 4/3Bi q- Se2(g) = 2/3Bi2Se3 -232.714 + 0.162 T A 25-400 Cd-Se 2Cd + Se2(g) -- 2CdSe -427.232 + 0.179 T A 25-400 Cu-Se 4Cu + Se2(g) = 2Cu2Se -266.987 + 0.111 T A 25-123

-250.214 + 0.069 T A 123-400

3Cu + Seg(g) = Cu3Se2 -253.068 + 0.155 T B 25-112 2Cu + Se•2(g) = 2CuSe -222.294 + 0.155 T B 25-50

-223.453 + 0.162 T - 1.233' 10 -s T 2 B 50-377

Cu q- Se2(g) = CuSe2 -186.211 + 0.175 T - 2.148' 10 -s T 2 B 25-332 Fe-Se 2.083Fe + Se2(g) = 2/3Fe•.o4Se -277.265 + 0.159 T - 1.409' 10 -s T 2 A 25-400

/3Feio4Se + Se2(g) = 6Fe•_xSe -110.925 - 7.430' 10-3T B 250-350 1.5Fe + Se2(g) = 1.5FeSe•.333 -232.358 -0.105 T A 25-34

-234.379 + 0.104 T + 3.408- 10-* T • A 34-400

1.754Fe + Se•(g) = 1.754FeSe• •4 -264.895 + 0.204 T - 1.016' 10 -4 T 2 A 25-178 -279.215 + 0.239 T - 1.116' 10 -4 T • A 178-400

Fe + Se2(g) = FeSe2 -240.369 + 0.175 T + 4.800' 10 -6 T 2 A 25-400 Hg-Se 2Hg + Se•2(g) = 2HgSe -223.673 + 0.189 T A 25-400 Mn-Se 2Mn + Se2(g) = 2MnSe -480.612 + 0.124 T A 25-400 Mo-Se Mo + Se•2(g) = MoSe2 -374.430 + 0.191 T B 25-400 Ni-Se 3Ni + Se•2(g) = Ni3Se2 -327.822 + 0.231 T - 7.525' 10 -s T 2 B 25-400

1.905Ni + Se2(g) = 1.905NiSex.os -281.272 + 0.161 T - 7.758' 10 -6 T 2 A 25-400 2.667NiSe•.2s + Se2(g) = 2.667NiSe2 -206.890 + 0.183 T - 4.986' 10 -6 T 2 A 25-400 Ni + Se•(g) = NiSe2 -247.637 + 0.175 T - 9.406' 10 -6 T 2 A 25-400

Pb-Se 2Pb + Se2(g) = 2PbSe -337.317 + 0.166 T A 25-400 Sb-Se 4/3Sb + Se2(g) = 2/3Sb•Se3 -221.900 + 0.159 T A 25-400 Se-O Se•2(g) q- 202(g) = 2SEO2 -592.613 + 0.545 T - 3.902' 10 -s T 2 A 25-329 Sn-Se 2Sn q- Se2(g) = 2SnSe -315.185 + 0.166 T A 25-231

-327.718 q- 0.191 T A 231-400

Sn q- Se•(g) = SnSe• -247.438 + 0.147 T + 3.994' 10 -s T • A 25-400 T1-Se 4T1 + Se2(g) = 2T12Se -321.549 + 0.130 T + 2.228' 10 -s T 2 A 25-400

2T1 + Se•2<g) = 2T1Se -223.940 - 0.065 T + 2.770-10 -s T • A 25-192 -315.441 + 0.354 T - 2.032- 10 -4 W 2 A 192-400

Zn-Se 2Zn q- Se•2(g) = 2ZnSe -454.553 + 0.182 T A 25-400

• The Gibbs free energy of formation of binary selenide compounds from elements in their stable state at 1 bar and T, and pure, ideal Se2(g) at the same conditions, (ARGO), calculated using the standard Gibbs free energies of formation of the selenide compound from elements in their most stable state at 1 bar and T, (A•G•), and the condensation reaction of Se2(g), is expressed as a linear or polynomial function of temperature (K)

2 The estimated accuracy is referenced to formation of one mole of the compound from Se2<g); the uncertainty is shown by "A" where within 5 kJ and "B" where within 10 kJ


APPENDIX II

Standard Gibbs Free Energy of Formation from Elements • (kJ/mole)

100øc 150øc 200øc 250øc 300øc

Component Mineral species (373.15K) (423.15K) (473.15K) (523.15K) (573.15K) References

S.2lg• 67.63 59.99 52.66 45.64 38.94 Se,2(gl 78.94 71.19 63.58 56.59 50.31 Te2lgl 100.86 93.10 85.46 77.93 70.52 O2lg I 0 0 0 0 0 ADS Argentire -42.50 -43.71 -44.99 -46.44 -47.86 ADS e N aumannite - 46.69 - 48.78 - 50.69 - 52.81 - 54.64 AgaTe Hessite -43.08 -44.15 -46.13 -48.10 -50.07 Ag•.64Te Sttitzite -37.17 -38.01 -39.35 -40.65 -41.79 AsS Realgar -34.97 -34.35 -33.52 -32.58 -31.53 (As,S)•i• -31.78 As,2Sa Orpiment -86.99 -86.25 -84.94 -83.52 -82.14

AsSe -37.71 -37.38 -36.93 -36.13 -34.37

(As,Se)l•l -34.66 As,2 Sea Laphamite - 88.47 - 90.24 - 93.99 - 96.18 - 96.58 As.2Tea -39.96 -40.63 -41.11 -41.43 -41.58 AuSe -8.57 -7.84 -7.08 -6.06 -4.72 AuTe2 Calaverite - 16.83 - 16.57 - 16.30 - 16.02 - 15.71 Bi.2S,3 Bismuthinite -139.48 -138.46 -136.93 -134.68 -135.16 Bi.2S% Guanjuatite -139.92 -139.73 -139.41 -137.66 -134.26 Bi,2T% Tellurobismuthite - 76.96 - 76.83 - 76.66 - 75.89 - 74.44 Bi.2Oa Bismite -473.48 -460.27 -445.09 -433.71 -419.69 CdS Greenockite -142.23 -141.08 -139.75 -138.33 -136.81 CdSe Cadmoselite -140.71 -140.10 -139.44 -138.47 -137.17

CdTe -98.65 -98.22 -97.80 -97.36 -96.91

CdO Monteponite -221.88 -216.96 -212.06 -207.18 -202.30 Cu•S Chalcocite -87.85 -89.15 -90.45 -91.77 -93.12 CuS Covellite -53.50 -53.37 -53.07 -52.70 -52.24 Cu.2S e B erzelianite - 73.24 - 74.86 - 76.88 - 78.64 - 80.10 CuSe Klockmannite -42.82 -42.88 -42.91 -42.66 -42.07

CuaSe,2 Umangite - 116.25 CuSe.2 Krutaite -44.94 -44.77 -44.64 -43.90 -42.63 Cu.2Te Weissite -39.84 -40.05 -40.37 -40.69 -40.96 Cu4Tea Rickardite - 95.38 - 95.73 - 95.89 - 95.90 - 95.73 CuTe Vulcanite - 25.03 - 25.38 - 25.67 - 25.91 - 26.09 CuO Tenorite -121.36 -116.77 -112.23 -107.72 -103.25 CuO2 Cuprite -142.14 -138.32 -134.52 -130.73 -126.96 Ires Troilite - 102.97 - 103.07 - 103.33 - 103.67 - 104.11 Fe0.98S Pyrrhotite - 103.17 - 103.36 - 103.65 - 103.97 - 104.32 Fe0.9oS Pyrrhotite - 100.12 - 100.30 - 100.42 - 100.58 - 100.79 Fe0sgS Pyrrhotite -99.75 -99.98 -99.98 -100.10 -100.26 Feo.s75S Pyrrhotite - 99.24 - 99.40 - 99.45 - 99.55 - 99.70 FeS2 Pyrite -157.03 -154.60 -151.80 -148.84 -145.69 Fe Seo •6 • Achavalite ? - 67.52 - 67.72 - 68.00 - 67.69 - 67.30 FeSe• 14 -70.78 -71.61 -72.58 -72.91 -73.15 FeSe• aaa -74.46 -75.29 -76.01 -75.68 -75.45 Fe S e• Ferroselite - 95.24 - 93.98 - 92.74 - 90.51 - 87.88 FeTe09 -29.17 -29.59 -30.00 -30.40 -30.78

FeTe., Frohbergite -62.60 -61.24 -59.84 -58.40 -56.90 Fe,2Oa Hematite -723.93 -710.42 -697.07 -683.81 -670.66

FeaO4 Magnetite -987.08 -970.17 -953.47 -936.98 -920.73 HgS Cinnabar -43.73 -42.23 -40.56 -38.82 -36.99 HgSe Tiemannite -37.05 -36.17 -35.26 -34.08 -32.58 H gTe Coloradoite - 27.17 - 26.56 - 25.95 - 25.34 - 24.72

H gO Montroydite - 50.45 - 45.09 - 39.78 - 34.50 - 29.27 MnS Alabandite -219.41 -219.91 -220.24 -220.48 -220.63 MnS2 Hauerite -225.11 -224.82 -224.30 -223.57 -222.68 MnSe -177.69 -178.46 -179.17 -179.59 -179.65 MnTe -113.01 -112.58 -113.99 -115.06 -115.73 MnTe,2 -130.73 -131.38 -132.00 -132.56 -133.07 MnO Manganosite -357.32 -353.63 -349.96 -346.31 -342.67 MnaO4 Hausmannite - 1,256.86 - 1,239.90 - 1,223.05 - 1,206.24 - 1,189.49

Barin (1989) Barin (1989) Barin (1989)

Barin (1989) Barin (1989) Mills (1974); Barin (1989) Afifi et al. (1988) Craig and Barton (1973) Barton (1969) AfH•soc, O'Hare (1993);

S ø, Cp, Barin (1989) AfH.•soc, S ø, Mills (1974); Cp est. Est.

O'Hare et al. (1990) Afifi et al. (1988) Mills (1974); Barin (1989) Mills (1974); Afifi et al. (1988) Barin (1989) Mills (1974); Barin (1989) Barin (1989) Barin (1989) Mills (1974); Barin (1989) Barin (1989); Nasar and Shamsuddin

(1990b) Barin (1989) Barin (1993) Barin (1989) Barin (1989) Mills (1974); Barin (1989) A•H•oc, S ø, Mills (1974); Cp est. A•H•0o S ø, Mills (1974); Cp est. A•H•oc, S ø, Mills (1974); Cp est. Afifi et al. (1988) Afifi et al. (1988) Afifi et al. (1988) Robie and Hemingway (1995) Robie and Hemingway (1995) Gr0nvold and Stolen (1992) Gr0nvold and Stolen (1992) Gr0nvold and Stolen (1992) Gr0nvold and Stolen (1992) Robie and Hemingway (1995) Robie and Hemingway (1995) Mills (1974) Mills (1974) Mills (1974) Mills (1974) AfH.•5oc, Shulda et al. (1990) S ø, Cp, Mills (1974) Mills (1974); Afifi et al. (1988) Robie and Hemingway (1995) Robie and Hemingway (1995) Mills (1974); Barin (1989) Mills (1974); Barin (1989) Mills (1974); Nasar and Shamsuddin

(1990a) Mills (1974); Barin (1989) Barin (1989) Mills (1974); Barin (1989) Barin (1989) Mills (1974); Barin (1989) Mills (1974); Afifi et al. (1988) Robie and Hemingway (1995) Robie and Hemingway (1995)


APPENDIX II (Cont.)

100øC 150øC 200øC 250øC 300øC

Component Mineral species (373.15K) (423.15K) (473.15K) (523.15K) (573.15K) References

Mn•203 Bixbyite -862.80 -849.97 -837.17 -824.42 -811.72 Robie and Hemingway (1995) MnO2 Pyrolusite -451.24 -442.04 -432.84 -423.71 -414.66 Robie and Hemingway (1995) MoSs Molybdenite -260.36 -258.39 -255.99 -253.47 -250.81 Robie and Hemingway (1995) MoSe.2 Drysdallite -224.55 -222.59 -220.26 -217.78 -215.13 A•H•5oc, O'Hare et al. (1987); Cp est. MoTe2 -97.84 -95.64 -93.45 -91.25 -89.05 Mallika and Sreedharan (1988) Ni3S•2 Heazlewoodite -203.44 -202.49 -201.65 -200.51 -199.05 Barton and Skinner (1979) NiS Millerite -89.36 -88.98 -88.42 -87.86 -87.30 Barton and Skinner (1979)

Ni3S4 Polydymite -310.41 -306.06 -301.67 -297.29 -292.91 Barton and Skinner (1979) NiS.2 Vaesite -122.75 -121.10 -119.45 -117.80 -116.15 Barton and Skinner (1979) Ni•Sez -173.55 -172.69 -171.84 -170.98 -170.13 Est. NiSe•0• M•ikinenite -75.24 -75.24 -75.20 -74.84 -74.12 Mills (1974); Barin (1989) NiSe• •43 Sederholmite -79.37 -79.30 -79.21 -78.83 -78.09 Mills (1974); Barin (1989) NiSe•2.• Sederholmite -82.06 -81.88 -81.65 -81.07 -80.09 Mills (1974); Barin (1989) NiSe.2 Kullerudite? -104.70 -104.06 -103.35 -102.06 -100.11 Mills (1974); Barin (1989) NiTe• -57.21 -57.27 -57.25 -57.18 -57.04 Afifi et al. (1988) NiTez Mellonire -82.64 -82.29 -81.82 -81.24 -80.53 Afifi et al. (1988) NiO Bunsenire -204.12 -199.49 -194.94 -190.53 -186.02 Robie and Hemingway (1995) PbS Galena -96.51 -96.03 -95.37 -94.64 -93.82 Barin (1989) PbSe Clausthalite -98.27 -97.98 -97.63 -96.99 -96.01 Mills (1974); Barin (1989) PbTe Altaire -67.00 -66.75 -66.47 -66.16 -65.82 Mills (1974); Afifi et al. (1988) PbO Litharge -181.38 -176.43 -171.53 -166.68 -161.88 Robie and Hemingway (1995) PbSO4 Anglesite -786.07 -767.88 -749.48 -731.03 -712.53 Robie and Hemingway (1995) Sb•2S3 Stibnite -149.21 -148.37 -147.02 -145.47 -143.67 Seal et al. (1992); Robie and

Hemingway (1995) Sb•2Se3 Antimonselite -125.44 -125.02 -124.48 -123.07 -120.65 Mills (1974); Barin (1989) SbzTe3 Tellurantimony -59.06 -59.40 -59.73 -60.31 -60.33 Mills (1974); Afifi et al. (1988) Sb203 Valentinite -605.75 -592.10 -578.53 -565.02 -551.58 Barin (1989) SeO2 Downeyire -157.98 -149.01 -140.07 -130.92 -121.51 Barin (1989) SiO2 Quartz -842.59 -833.46 -824.31 -815.19 -806.09 Robie and Hemingway (1995) SnS Herzenbergite -105.53 -103.67 -104.28 -103.19 -101.66 Mills (1974); Barin (1989) Sn•2S3 Ottemannite -250.62 -248.28 -245.39 -241.67 -236.99 Mills (1974); Barin (1989) SnSz Vaesite -142.99 -141.15 -138.94 -136.27 -133.06 Mills (1974); Barin (1989) SnSe -87.21 -86.94 -86.62 -85.70 -84.07 Mills (1974); Barin (1989) SnSe2 -108.14 -106.82 -105.41 -103.10 -99.75 Mills (1974); Barin (1989) SnTe -61.64 -61.56 -61.46 -61.03 -60.20 Mills (1974); Afifi et al. (1988) SnOz Cassiterite -500.26 -489.93 -479.63 -469.05 -458.15 Robie and Hemingway (1995) TI•2S Carlinire -94.43 -94.23 -93.89 -93.38 -92.67 Barin (1989) TlzSe -95.28 -95.42 -95.94 -95.26 -94.52 Barin (1989) T1Se -65.30 -65.28 -64.85 -64.55 -64.40 Barin (1989) TI.2Te -78.88 -78.65 -78.39 -77.98 -77.39 Mills (1974); Barin (1989) Tl•20 -137.17 -133.05 -129.01 -124.90 -120.71 Barin (1989) Tl•203 -291.31 -277.67 -264.16 -250.64 -237.09 Barin (1989) ZnS Sphalerite -198.43 -197.44 -196.24 -195.00 -193.70 Robie and Hemingway (1995) ZnSe Stilleite -153.92 -153.21 -152.47 -151.45 -150.09 Mills (1974); Barin (1989) ZnTe -114.26 -113.56 -112.85 -112.12 -111.37 Mills (1974); Nasar and Shamsuddin

(1990b) ZnO Zineite -312.96 -307.96 -302.99 -298.04 -293.10 Barin (1989) CuFeS•2 Chaleopyrite -195.03 -194.72 -194.12 -193.41 -192.56 Robie et al. (1994); Robie and

Hemingway (1995) CusFeS4 Bornite -400.29 -403.60 -406.39 -409.31 -412.36 Robie et al. (1994); Robie and

Hemingway (1995) FeAs• LOllingite -54.39 -55.87 -57.36 -58.84 -60.33 Barton and Skinner (1979) FeAsS Arsenopyrite -110.19 -110.60 -110.85 -110.95 -110.89 Barton and Skinner (1979) F%SiO4 Fayalite -1,354.29 -1,337.83 -1,321.48 -1,305.20 -1,289.00 Robie and Hemingway (1995)

Standard Gibbs free energy of formation from elements in their most stable state at 1 bar and 373.15, 423.15, 473.15, 523.15, and 573.15 K

Documents

Phase Relations Among Selenides Sulfides Tellurides and Oxides I 96_Simon-Essene