NOMENCLATURE OF THE ALUNITE SUPERGROUP - RRuff

1323

The Canarlian MineralogistVo l . 37 . pp . 1323 -1341 t I 999 )

NOMENCLATURE OF THE ALUNITE SUPERGROUP

JOHN L. JAMBORS

Depat'tment of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T lZ4, Canadn

Aesrnlcr

The alunite supergroup consists of more than 40 mineral species with the general formula DG3(ZO4)2(OH,H2O)6, in which Dis occupiedby monovalent (e.g., K, Na, NHa, H3O), divalent (e.g., Ca, Ba, Pb), ortrivalent (e.g., Bi, REE) ions, G is typically A13*or Fe3+, and Zis 56*, Ass*, or Ps*. The current nomenclature classification is unusual in that, within the temary system defined bythe SOa, AsOa, and POa apices, compositions are divided into five fields rather than the three that are conventionally recom-mended for such systems by the Commission on New Minerals and Mineral Names (CNMMN) of the International MineralogicalAssociation. The current compositional boundaries are arbitrary, and the supergroup is examined to determine the repercussionsthat would ensue from adoption of a conventional ternary compositional system. As a result ofthe review, several inconsistencieshave been revealed; for example, beaverite and osarizawaite, which are commonly formulated as Pb(Cu,Fe)3(SO+)z(OH)e andPb(Cu,Al)dSOa)z(OH)0, respectively, not only have formula Fe > Cu and Al > Cu, but the amount of substitutional Cu also isvariable. Beaverite is therefore compositionally equivalent to Cu-bearing plumbojarosite. The CNMMN system also permits theintroduction of new mineral names if a supercell is present; within the alunite supergroup, the supercell is typically manifested bya doubling of the c axis to -34 A, and the effect is evident on X-ray powder pattems by the appearance of a diffraction line_or peakat 1 1 A. In addition to the supercell, however, several other departures from the standard trigonal cell with space group R3n havebeen observed To accommodate these structural variations, thereby minimizing the introduction of numerous potential newmineral names, the possibility of incorporating a suffix modifier is explored. To keep within CNMMN nomenclature protocols,potential solutions are offered, but none is proposed.

Keywortls'. alunite supergroup, nomenclature, alunite group, beudantite group, crandallite group, compositions, sffuctures.

)OMMAIRE

Le supergroupe de l'alunite comprend plus de quarante espbces min6rales rdpondant h la formule gdndraleDQ(TOD2(OH,H2O)6; le site D contient des ions monovalents (e.g., K, Na, NHa, H3O), divalents (e.g.,Ca, Ba, Pb), ou trivalents(e g , Bi, tenes rares), G contient en gdndral A13+ ou Fe3*, et lreprdsente 56+, As5+, ou Ps*. Le systbme de classification en vigueuractuellement est anomal: dans le contexte d'un systbme temaire d6fini par les p6les SO+, AsO+, et PO+, les compositions sontrdparties en cinq domaines plutOt que trois, comme le recommande la Commission sur les Nouveaux Mindraux et Noms deMindraux de I'Association Min6ralogique Internationale. La ddlimitation des divers champs est arbitraire. Les r6percussions del'adoption d'un systbme temaire de nomenclature de ce supergroupe sont ici passdes sous revue. Plusieurs cas de non concor-dance sont relevds; d titre d'exemple, la beaverite et 1'osarizawaite, auxquelles on attribue couramment les formulesPb(Cu,Fe)3(SOa)2(OH)6 et Pb(Cu,Al)3(SOa)2(OH)6, respectivement, ont une teneur en Fe et en Al supdrieure d celle du Cu, maiscette teneur en cuivre peut aussi Otre variable. La beaverite serait donc dquivalente en composition d une plumbojarosite cuprifBre.Le systdme en place actuellement permet I'introduction de nouveaux noms de mindraux si une supermaille est manifestde. Ausein du supergoupe de 1'alunite, la pr6sence d'une supermaille se voit par le d6doublage de la pdriode c jusqu'd environ 34 A, etpar la prdsence d'un pic ou d'une raie d 11 A dans un spectre de diffraction. En plus de la supermaille, toutefois, on observeplusieurs autres 6cafts par rapport h la maille trigonale standard dans le groupe spatial R3m. Afin d'accommoder ces variationsstructurales, et ainsi de minimiser l'introduction potentielle de piusieurs nouveaux noms de mindraux, il est possible d'ajouter unsuffixe au nom. Afin de satisfaire aux exigeances de la Commission, des solutions possibles au dilemne sont prdsentdes, maisaucune n'est proposde.

(Traduit par la Rddaction)

Mots-clds: supergroupe de l'alunite, nomenclature, groupe de l'alunite, groupe de la beudantite, groupe de la crandallite, compo-s1t10ns. structures.

E E-mail address: jlj @wimsey com

t324 THE CANADIAN MINERALOGIST

INrnooucrroN

The alunite supergroup consists of three mineralgroups that, combined, contain more than 40 mineralspecies with the general formila DGz(TO r,)z(OH,H2O)6,wherein D represents cations with a coordination num-ber greater or equal to 9, and G and T represent siteswith octahedral and tetrahedral coordination, respec-tively (Smith et al.1998). The supergroup consists ofthe alunite group, the beudantite group, and thecrandallite group (Mandarino 1999). In all three, the ?in_ (7Oa)2 is dominated by one or more of 56*, As5*, orP'*. The alunite group is characterized by (SOa)2-domi-nant minerals, whereas in the beudantite group, one ofthe two SOa groups is replaced by POa or AsOa. In thecrandallite group, the (7Oa)2 represents one or both ofPOa and AsO+. Thus the change from the alunite to thebeudantite group, and thence to the crandallite group,can be viewed as a progression from (SO+)2, to (SO4)(POa) or (SO+)(AsO+), and thence to either (POa)2 or(AsO+)2. This progression follows thatinThe System ofMineralogy (Palache et al. I95l), wherein the alunitegroup is classihed with the sulfates, the beudantite groupfalls within the category of "compound phosphates,etc." , andtheplumbogummite group is equivalent to thecurrent crandallite group. In the beudantite group, asmall departure from the 1:1 ratio for (?Oa):(7Oa) wasrecognized (Palache et al. l95l), and this variation isevident in all of the compositions listed therein. Forexample, the two compositions chosen for beudantitesensu strictohave SO3 contents of 12.30 and 74.82wt%a,whereas the appropriate value is 11.24 wt%o for(SOa):(AsOa) = 1:1. Under the current rules of nomen-clature for such a binary system, these compositions failto meet the "5OVo rule" in that they are sulfate-domr-nant rather than precisely at the 50:50 boundary thatseparates them from the arsenate-dominant mineral spe-cies. Compositions of some of the other minerals withinthe group, however, exceed the 50Vo reqtirement andfall within the PO+-dominant field.

After publication of The System of Mineralogy, itbecame apparent that the l:1 ratio of anions was not a

meaningful compositional boundary, and that mutualsubstitutions of SO+, PO+, and AsOa are extensivewithin the alunite supergroup. To rationalize the previ-ously indefinite boundaries between the SOa-dominantand other members of the family, Scott (1987) proposedthat the boundaries be set at 0.5 formula (SO+)z and l 5formula (SOa)z; these values arc 25Vo and 75Vo of totalTO4, as shown in Figure 1. The proposal was acceptedby the Commission on New Minerals and MineralNames (CNMMN). The effect of Scott's classificationis that each SO+-AsO+-POa "ternary" diagram incor-porates five compositional fields and mineral names,four of which are partly governed by the POa-AsOabinary dividing line (Fig. 1). Nov6k et al. (1994) pro-posed that the SOa-POa-AsOa held be divided into sixunits (Fig. 1), a system that requires the redefinition ofseveral minerals. Their system has not been submittedto the CNMMN for a vote, but its usage has been propa-gated in several publications (Nov6k & Jansa 1997,Novilk e/ a|.1997,1998, Sejkora et al.1998).

The nomenclature of the alunite supergroup is com-plex and promises to be increasingly so if the CNMMN-approved system is not modified to take account of thecrystal-structure variations that have been recognized tooccur within the supergroup. In the following discus-sion, the nomenclature is evaluated to explore not onlywhat happens in a ternary compositional system, butalso to explore the repercussions of designating the crys-tal-structure variations by suffixes. In the compositionalsystem herein, the ternary series is divided into threeequal fields (Fig. 1, left), which is in accord with cur-rent CNMMN recommendations for naming ternarysolid-solutions that are complete and without structuralorder of the ions defining the end members (Nickel1992).

Tun, AluNrrB Gnoup

The minerals of the alunite group are listed inTable l. Figure 2 shows that little substitution of SO+by POa and AsOa occurs for members in which D ismonovalent; where D is divalent, however, TOa substi-

As04 Poa AsO4

Ftc. l. Left diagram shows the nomenclature scheme for complete tema-ry solid-solutions according to recommendations by theCNMMN (Nickel 1992). Middle diagram is the system in current use for the alunite supergroup (Scott 1987), and the diagramon the right shows the composition fields proposed by Novdk et al (1994).

NOMENCLATURE OF THE ALUNITE SUPERGROUP 1325

aluiteKAls(S0rl(0rr)6

mtroaluiteNaAL(s01),(0l{)6

monioaluite(NlI4)Aq(SO1L(Orr)5

schlossmcherite(H'o,ca)Al'(so.)"(ol0"

oseiawaitePb(Al,Cu)3(SO4)r(OEII2O)6

minuiite*(NaCa,K),AUsoa)a(olo'

humgite*CaAl6(Sq)4(OI{)n

ualthisite+BaAL(SO)(oII)u

jrositeKFq(soa),(olD5

mlrojao$teN&Fe3(SOJdOH)6

moniojuosite(NHr)Fq(soXoH)6

hydronim jrcsiteGr.O)re(so.)doH)6

ilgentojmsiteAeFq(so.),(OH)"

doEllchtriterlFq(soa),(oH).

b@vqitePb(Fe, cu[SO,),(OH,H,O)"

plMbojilosit€*PbFe(SOJr(oII)o

TABLE I MINERALS OF IIIE ALIINITE GROTJP (CURRENI USAGE) tution may be substantial, and these minerals a"re dis-cussed separately. In the synthetic alunite-jarosite se-ries, Fe-for-Al solid solution is complete (Brophy et al.1962, Hartig et al. 1984); although natural memberswith intermediate compositions are known (Palache eral. 1951, van Tassel 1958), the degree of Fe-for-Alsubstitution in most occuffences is limited, and composi-tions are close to those of either the Fe or Al end-members.

Solid solution involving K+, Na+, and (H3O)+ is ex-tensive in both alunite andjarosite (Brophy et al. 1962,Parker 1962, Brophy & Sheridan 1965, Kubisz 1960,1970, Stoffregen & Cygan 1990,Liet al. 1992). Substi-tution involving K+ and Pb2* has been shown by Scott(1987) to be extensive in alunite, and by de Oliveira etal. (1996) and Roca et al. (1999) to be equally exten-sive in jarosite. The incorporation of NHa in ammonio-alunite (Altaner et al. 1988) and ammoniojarosite(Odum er al. 1982) seems to be mainly at the expenseof K+ and (H:O)*. Apparent deficiencies in K + Na +NH+ D-site occupancy are generally attributed to(Hso)*.

* Length of c unit-cell pdameter is double that of the other members of the group

F e > A l

A l > F e

unchanged:iarositenatrojarositehydronium jarositeammoniojarositeargentojarositedoral lcharite

AsOo

aluni tenatroalunite, minamiitesch lossmacherite

geo:

SO, SOo .ttonio"lrnit"

soo

K: jarositeNa: natrojarosite

HrO: hydronium jarosite

NHo: ammoniojarositeAg: argentojarositeTl: doral lcharite

Na: natroalunite, minamiite

HrO: schlossmacherite

NHo: ammonioa lun i te

Frc. 2. Minerals of the alunite supergroup with monovalent ions predominant as the D-site cation. Left diagram shows thecurrent system of nomenclature, and diagram on the right shows the effects of adopting a ternary system. Minerals with Fe >Al and with Al > Fe are, respectively, jarosite - alunite, natrojarosite - natroalunite, ammoniojarosite - ammonioalunite,hydronium jarosite - schlossmacherite. Argentojarosite and dorallcharite do not have Al > Fe analogues.

r326 THE CANADIAN MINERALOGIST

Natural argentojarosite is generally near the end-member composition, although in synthetic systems thesolid solution between Ag-Pb (t H3O) and Ag-K(+ H:O) has been shown to be complete (Ildefonse elal. 1986, Dutrizac & Jambor 1984). The Tl+ member,dorallcharite, is known only from one locality, and themean occupancy of the D site is (TlosrKole) (Bali6Zuntd et al. 1994). The remaining members of the alunitegroup, except minamiite and schlossmacherite, containa predominance of divalent D-site cations. As theseminerals present some difficulties in nomenclature, thespecies are discussed individually.

Minamiite

The ideal formula of minamiite is (Na,Ca,K)2A16(SO4)4(OH)12. Chemical analysis of the type specimengave (Nas a6K0 1eCa6 1atr6 r+):o s:Al: l1(SO4)2(OH)5 70on the basis of SOa - 2, and the ideal formula of thecrystal used for the X-ray structure study (Ossaka er al.1982) was determined to be (Na6 36K0 tCaoztloz)>ogzAI3(SO4)2(OH)6. The n in the formula represents va-cancies that compensate for the presence of the divalention (Ca2*) in the D position. The mineral was deter-mined to- be trigonal. space group R3m - a = 6.98 | . c =33.490 A. Thus, type minamiite is compositionallyequivalent to a Ca-rich natroalunite, but the c axis inminamiite is doubled because of oartial order of the Dcations. This results in a superstructure that yields acharacteristic powder-diffraction line (or peak) at about11 A (Okada et al.1987).

Although type minamiite is Na-dominant (Ossaka eral. 1982), the same authors subsequently synthesized theCa-dominant analogue, and they explicitly referred tothe end-member composition of minamiite as Cao sAl3(SO4)2(OH)6 (Ossaka et al. 1987, Okada et al. 1987).Nevertheless, although Ossaka et al. (1987) showed thatextensive substitution involving Na-K-Ca occurs rnnatural minamiite. none of the comoositions indicated apredominance of Ca as the D-site iation.

Most minerals of the alunite group have D occupiedby a monovalent ion, and for these minerals the substi-tution of SOa by POa or AsOa is exrremely limited(Fig. 2). Adoption of a ternary system SOa-POa-AsOafor these minerals therefore would not affect the nomen-clature of the existing species. The three divalent ele-ments predominating at the D site in the alunite-groupminerals are Ba. Ca. and Pb.

Huangite and wahhierite

The mineral with Ca predominant, ideally Caa 5Al3(SO4)2(OH)6, was named huangite, and that with Bapredominant, ideally Ba6 5AI3(SO4)2(OH)6, was namedwalthierite (Li et al. 1992). X-ray powder patterns ofboth minerals show an 11 A diffraction effect, indicaring that their c axis is doubled to 33-34 A. Acceptanceof huangite as a new mineral indicated that the CNMMN

regarded minamiite to be compositionally equivalent tocalcian natroalunite, but with the c axis doubled inminamiite. A plot by Matsubara et al. (1998) of Na + K(alunite-natroalunite) versus Ca (huangite) does notshow significant compositional gaps to be present in theseries. Because of small grain-size, however, X-ray datahave not been obtained for all compositions. Thus, ifordering of Ca is assumed to be the sole cause for thedoubling of c, the lower limit to trigger the change hasnot been established.

B eav erite and o s arizaw aite

Beaverite is typically assigned the formula PbCuFez(SO+)z(OH)e or Pb(Cuz*,Fer+,AD3(SO4)2(OH)6 @alacheet al. 1951, Gaines et al. 1997). Osaizawaite is the Al-dominant analogue of beaverite, i.e., Al > Fe (Taguchi1961). As all compositions of beaverite and osarizawaitehave Fe > Cu or Al > Cu, the respective formulas shouldbe written as Pb(Fe,Cu,Al)3(SO4)2(OH)6 and Pb(Al,Cu,Fe)3(SOa)2(OH)0. Most compositions of beaverite haveCu:(Fe,Al) near 33:67, but both higher and lower val-ues have been reported (van Tassel 1958, Yakhontovaet al. 1988, Breidenstein et a\.7992). As summarizedby Yakhontova et al. (1988), the values of the relevantratio range from 36:64 to 25:75.

As for beaverite, most analyses of the Al > Fe ana-logue, osarizawaite, indicate Cu:(Al,Fe) near 33:67.However, Paar et al. (1980) reported an occurrence ofosarizawaite for which the ratio is 25:75. [The mineraldescribed by Cortelezzi (1977) as osarizawaite seemsto be a Pb-Cu-rich alunite, requiring re-analysisl.

The composit ional variat ions in beaverite-osarizawaite indicate that the ratio of divalent (Cu2+ andZn2+1 to trivalent (Fe3+ and Al3*) ions need not bestrictly 33 :67 . In synthetic beaverite-plumbojarosite,substitution of Cu2* for Fe3* was found to be completeover the range Cu:Fe = 0:100 to Cu:Fe = 33:67 (Jambor& Dutrizac 1985). Up to 0.76 mol (Zn + CLr) has beenreported for Pb-rich alunite (Scott 1987). For corkite,which is also in the supergroup and is described fartherbelow, substitution of 0.42 to 0.64 mol Cu has been re-ported by de Bruiyn et al. (199O) and Tsvetanova(1995), respectively.

Giuseppetti & Tadini (1980) solved the structure ofosarizawaite of composit ion Pb(Alr 62Cu6 esFes 3sZn6s2)23ss(SO+)z(OH)o in space group R3nz, whichrequires that the Cu and Al-Fe be disordered. X-raypowder-dif fract ion patterns reported for naturalbeaverite-osarizawaite have so far conformed to a ba-sic cell of a = -7, c =-tZ A, but as Cu2* decreases, Pbcontent also must decrease to maintain charge balance.At some point, therefore, the unit cell must transform tothe doubled c of -34 A that is commonly accepted ascharacteristic of plumbojarosite. In synthetic plumbo-jarosite-beaverite, however, doubled cells appeared ran-domly along the series (Jambor & Dutrizac 1983).

NOMENCLATURE OF THE ALUNITE SUPERGROUP t327

In terms of composition, beaverite is a Cu-richplumbojarosite. All occurrences of natural plumbo-jarosite have so far been reported to have a doubled caxis, but in synthetic hydronium-b-earing plumbo-jarosite, the c may be either 17 or 34 A, apparently de-pending on whether Pb is ordered or disordered. Thedegree of order can be variable, and this is readily indi-cated by variations in the intensity ofthe 11 A diffrac-tion line or peak in the X-ray pattern.

Schlossmacherite

Schlossmacherite contains substantial amounts ofboth SO+ and AsO+; thus the mineral has been variablyassigned to the alunite group (Gaines et al. 1997) or thebeudantite group (Mandarino 1999; Table 2). Recalcu-lation of the only analytical data available (Schmetzeret al. 1980) gives [(H3O)6:zno zsCao 26Na6 67K6.05516 s1Ba6 sl l ;1 oo (Alz eaFe6 02Cu6 06)>3 02 [(SO+)r sr(AsO+)0.+ql(OH)s zo on the basis of TOa = 2 and afterreassignment of Cu from the D to the G position. Theformula conforms with that generalized by the authors(Schmetzer et al. 1980). The ratio of SOa:AsO+ is75.6:24.4, thus placing schlossmacherite in the SOa-AsOa-dominant field in Figure 2. The mineral is there-fore the Al-dominant analogue of hydronium jarosite.Cell dimensions ate a = 6.998, c = 16.67 A.

Tue CneNoallrrE GRoUP

The minerals of the crandallite group are listed inTable 3. Nine of the members have Ba. Sr. or Ca domi-

TABLE 2 MINERALS OF TIIE BEUDANTITE GROIJP (CIJRRENT USAGE)

b@dutiterbre3[(As,S)OJdOH,HTO)5

hidalgoitePbAl3(As,S)O4],(OH,n 0)6

kenmlitzite(sr,ce)Al.t(nq s)oJloH,Eo)6

gallobeudmtitePbGa3(As, S)O4]2(OH,H,O)6

corkitePbFe[(P,S)OJ,(OH,H,o)6

hinsdalitePbAl3t@,s)orl,(orlE 0)6

srubqgitesrAl,lG, s)O-l,(oEH'o)"

woodhouwiteCaAl3[(P, S)O.],(OH,II'o).

Either As or S my be predominaot in (A5,S)O4, ud eiths P or S may be

predonimt in (P,S)O,

nant in D (three members for each ofthese cations), fourmembers have Pb dominant, and the remainder has REE,Ca, or Th dominant. As in the alunite group, a primarydistinction rests on whether the proportion of Fe isgreater than that of Al or vice versa.

Ba predominance

The Ba-dominant member of the alunite group iswalthierite Bao sAI3(SO4)2(OH)0, and its relationship toother Ba-dominant members of the alunite supergroupis shown in Figure 3. AlthoughLiet al. (1992) obtaineda = 7.08, c = 1'7.18 A by electron-diffraction study ofwalthiedte, X-ray powder pattems of bulk samples showan ll A diffraction peak, requiring that the length of cbe doubled. The inconsistency was attributed to disor-der induced by the electron-diffraction beam.

TABLE 3. MINERALS OF THE CRANDALLITE GROI.]P (CTJRRENT USAGE)

A l > F e F e > A l

dusrertitegorceixiteBaAlj(Po1)eq.orD(oII)6

qmdallite

CaAl3[PO3(O1D(OI'rJ],(OID6goyuite

STALIPO3(On(OH)rrl,(OH)6plMbogwite

PbAl3@o4)'(oH,r{'o)6florencit€-(Ce)

CeAIr(POo)r(OHLflorscite-(La)

LaA[(PO1)b(OFD6

floretrcite-(Nd)NdAteo4)'(or{)6

waylmdite(Bi, Ca)AL@Or,SiO,),(OH)e

e'.lsttssite(ThPb)ALeO4,SiO4)'(oIr)6

rsnogor@ixiteBaAL(AsO{XASO3.OID(OlI)6

reocmdalliteCaAl3[AsO j(On(OID;)]r(OIt6

dmogoyeteSrAl[AsO.(O,"(OH)'J] {0II)6

philipsbomitePbAl3(AsO4XOH,IIP)6

trsofloretrcite-(Ce)CeAl(AsO'),(OtI)c

"dsoflorflcits(La)"*LaA13(As04),(OlD5

"usmoflorscite-(Nd)"*NdAl3(A504)2(0II)6

"ilsrcwylmdite"f

@i,Ca)Al (AsOo),(OH,IlO).

b@uite

kintorcite

BaFes(AsOJdOH)6

zakite

Srre(PO,),(OH,H,O)6wEnitite

pbpe@o.),(oEEo)6 PbFq(.csoaXOH,ILo)6

BiFeeoXoH).

Othes

springcr@kite

* not CNMMN-approved, but included here for completeness.

BaV3eOaL(OH,ILO)6

t328 THE CANADIAN MINERALOGIST

Poo

The main concem in the Ba-dominant group is sub-stitution atthe T site; for walthierite, such substitutionwas not detected (Li et al. 1992). Likewise, composi-tions of gorceixite are typically close to, or at, the POaend-member. For arsenogorceixite, however, the typematerial shows AsOa:POq = 74:26 (Walenta & Dunn1993). The AsOa (arsenogorceixite) and PO4 (gorceixite)end-members have been synthesized by Schwab et al.(1990, 1991).

"Weilerite" is included as a valid species in somemodern compilations (Clark 1993, Gaines er al. 1997),but the mineral was discredited by the CNMMN (IMA1968). As no quantitative chemical data had been pub-lished for the mineral (Fleischer 1962, 1967), the posr-tion on the AsO+-SO+ join is uncertain, and the mineralmay have been identical to the more recently describedarsenogorceixite. The removal of "weilerite" from thesystem is appropriate, thus leaving a gap betweengorceixite-arsenogorceixite and walthierite (Fig. 3).

Dussertite is the only known member with Fe > A1,thus theoretically allowing six additional names to beintroduced in the left diagram of Figure 3. In a ternarysystem, the potential six are reduced to two.

soo

> A l

dussertite

\

sorceixite arsenogorceixiter/

soo

Fe

AsOo POo

AI

FIc. 3. Minerals of the alunite supergroup with Ba as the predominant D-site cation. Left diagram shows the cunent nomencla-ture, and diagram on right shows the effects of adopting a temary system. "Weilerite" is not a valid species (see text).

Ca predominance

All Ca-dominant minerals in the alunite supergroupare also Al-dominant (Fig. a). Numerous compositionsof crandallite are near that of the POa end-member. butsubstitution of S for P is extensive, and compositionsextend well into the current field for woodhouseite(Stoffregen & Alpers 1987, Spcitl 1990, Li et al. 1992),including compositions with formula SO+ > PO+ (Wise1975). For arsenocrandallite (Walenta 1981), the typematerial contains substantial Si in the Z position:(Aso qqPo zsSio z6). The As:P ratio is 57:43, which is wellwithin the field for arsenocrandallite.

Adoption of a temary system for the Ca-Al-domi-nant minerals would pose the problem of whetherwoodhouseite or huangite is to be used to designate theSOa end-member. Type woodhouseite has POa:SOa =54:46 (Lemmon 1937), but it has long been acceptedthat either formula S or P can be predominant. The onlycomposition given by Palache et al. (1951) has P > S,but more modern analyses, by electron microprobe, ofwoodhouseite from the type locality have shown bothformula P > S and S > P (Wise 1975). Moreover. the

soo

soo

arsenogorceixite

walthieri te


AsO, POo

soo soo

Frc. 4. Minerals of the alunite supergroup with Ca as the predominant D-site cation. Left diagram shows the cu[ent nomencla-ture, and diagram on right shows the effects of adopting a ternary system.

current CNMMN-approved nomenclature extends thecomposition of woodhouseite well into the SOa-domi-nant field (Fig. a). Accordingly, woodhouseite has beenadopted to designate the field with S > P in the ternarysystem (Fig. 4).

Sr predominance

The minerals with Sr predominant in the D site areshown in Figure 5. The two minerals with Fe > A1 arebenauite and an associated unnamed mineral richer inSOa (Walenta et al. 1996). The empirical formula forbenauite is (Sre 67Ba6 16Pbs e7Ca6 61Ko ot):,0 sz (Fez goAlo o:):z s: [(PO+)r +s(SO+)o as(AsO4)0 04](OH,H2O)8 3,which was simplified by Walenta et al. (1996) asSrFe3(POa)2(OH,HzO)0. The proportion of atoms in theT site, however, is P:S:As = 74:24:2, which places themineral just beyond the boundary designated by the sim-plified formula, and into the field of the unnamed SOa-richer mineral (Fig. 5). The range in compositions (wt7o)reported by Walenta et al. (1996) is PzOs l'1 .98-19.93(mean 18.53 used to calculate the formula), As2O5 0.64-0.94 (mean 0.78), SO3 6.24-:7.37 (mean 6.79), and thus

the compositions probably straddle the aforementionedboundary. The formula of the unnamed mineral rs(Srz srBao oo)Fe: r:(PO+)r :s(SO+)o os(OH,H2O),, whichplaces the mineral well beyond the field of benauite. IfScott's (1987) nomenclature system is to be adhered to,benauite requires redefinition as ideally SrFe3[e,S)O4]2(OH,H2O)6. The field shown Bb occupied by benauite inFigure 5 would therefore remain vaguint.

For the Al-dominant group. (Fig. 5), goyazite andsvanbergite have long been usedto indicate the POa andPO+-SO+ minerals, respectively. For svanbergite, thecompositions listed in Palache et al. (1951) have for-mula P > S, but compositions with S > P also are known(Wise 1975); the CNMMN-approved system acceptsthat either S or P may predominate (Fig. 5). Typearsenogoyazite (Walenta & Dunn 1984) has AsOa:PO+= 64:36, but the pure end-member has also been de-scribed (Zhang et al. 1987). Kemmlitzite (Hak et al.1969) was regarded by Fleischer (1970) to be the arsen-ate analogue of svanbergite; analysis of type material,however, gave [(AsO+)o es@Oa)6 a2(SOa)s 3e(SiOa)s 1e]11 e3, which places kemmlitzite in the field of arseno-goyazite (Fig. 5). Re-examination of the holotype speci-

Soo

arsenocrandal I i tec randa l l i t e

woodhousei te

c randa l l i t e a r senoc randa l l i t e

1330 THE CANADIAN MINERALOGIST

AsOo PO,

soo

Fe

/ a"n^ui ta

\ covazite ^"tloo'o'^'n"/

soo

Poo

men of kemmlitzite has shown it to be zoned and inho-mogeneous (Novdk et al. 1994). Kemmlitzite predatesarsenogoyazite, and the latter was specifically definedas occupying the AsOa-dominant field because adop-tion of the current (Scott) system of nomenclature hadthe effect of entrenching kemmlitzite as representativeof SOa-rich compositions (Fig. 5). Such a SOa-rich com-position has been reported by Nov6k et al. (1997). lnview of the reported zoning and lack of homogeneity intype kemmlitzite, retention of the name arsenogoyazitewould seem, arguably, to be preferred. In a ternary sys-tem, therefore, the most appropriate names are consid-ered to be goyazite, arsenogoyazite, and svanbergite forthe Al-dominant members, and benauite for the Fe-dominant mineral with POa greater than AsOa or SOa(Fie. s).

Bi predominance

Minerals with Bi predominant in D are shown inFigure 6. Analyses of waylandite (Clark et al. 1986i) and

zairite (van Wambeke 1975) are within the designatedcompositional fields. "Arsenowaylandite" is an unoffi-cial name (Scharm et al.1994) that has not been sub-mitted to the CNMMN for approval. The anhydrouscomposition for ZOa = 2 vaies from (Bi68esr005Cao os)>o s7 (Al2 e6Fe6 10) [(AsOa)1 or(SO+)o zz@O+)o rr]to (Bi6 eeSrs 65)1 1 6a(A12 e6Fe6 ea) [(AsO+) r sr(SO+)o rs,.

Pb predominance

Figure 7 is illustrative of the profusion of mineralnames possible as the compositional fields within thealunite supergroup are frlled. For Fe-dominant miner-als, plumbojarosite, corkite, and beudantite have beenin use for many years, and corkite and beudantite oc-cupy the bulk ofthe field. Segnitite (Birch et al. 1992)and kintoreite (Pring et al. 1995) occupy the SOa-poorfields that opened only as a consequence of the adop-tion of the current CNMMN-approved system of no-menclature. For Al-dominant minerals, plumbogummiteand hinsdalite have been known for many years (Palache

AI

Ftc 5. Minerals of the alunite supergroup with Sr as the predominant D-site cation. Left diagram shows the current nomencla-ture; unnamed mineral is that of Walenta et al ( 1996), point "a" is the composition of type arsenogoyazite, and "k" is that fort)?e kemmlitzite. Right diagram shows the effects of adopting a ternary system.

soo

soo

arsenogoyazrte

soo

NOMENCLATURE OF THE ATUNITE SUPERGROUP r33r

AsOo POo

FIc. 6. Minerals of the alunite supergroup with Bi as the predominant D-site cation Left diagram shows current nomenclature,and that on the right shows the effects of adopting a temary system. "Arsenowaylandite" (Scharm et aI. 1994) is not anapproved name.

et al. l95l). Hidalgoite has the formula ratio SOa:AsOa= 59:47, and the mineral was specifically named as thearsenate analogue of hinsdalite (Smith er al. 1953).Philipsbornite (Walenta et al. 1982) is somewhatunusual in its high Cr content in TO+, for whichAsOa:CrOa:SO+ = 76:19 5. The small freld at the SO+apex (Figs. 7, 8) has been referred to as "plumboalunite"(Novrik et al. 1994) and "plumbian alunite" (Sejkora eral. 1998). However, if it is accepted that (Cu + Zn) sub-stitution is non-essential, then the Pb-Al sulfate in thisfield is osarizawaite.

Adoption of a ternary system for the Pb-dominantmembers would pose some difficulties. In the systemwith Fe predominant in G, corkite and beudantite havepriority, thereby requiring the abandonment of the re-cently named kintoreite and segnitite. In the system withAl predominant in G, plumbogummite and hinsdalitehave priority. Osarizawaite was named as the Al ana-logue ofbeaverite, but the requirement, as will be dis-cussed, is that neither be retained as a mineral name.Plumbogummite and hinsdalite thus occupy the PO+-

SO+ join as shown in Figure 7. The consequence is thatthe field for hidalgoite, which includes the compositionoftype hidalgoite, would be overlapped by the new fieldfor hinsdalite. Therefore, as was done for arseno-goyazite, philipsbornite would be retained to designatethe As-dominant member.

REE predominance

The REE-bearing minerals are characterized by apredominance of trivalent cations in D, and it has beensuggested (Scott 1987) that such minerals be classifiedas the florencite group. Accepted minerals within thegroup (Fig. 9) are florencite-(Ce), florencite-(La),fl orencite-(Nd), and arsenofl orencite-(Ce). Composi-tions corresponding to the La and Nd analogues ofarsenoflorencite-(Ce) have been reported by Scharm eral. (1991,1994), but the new names have not been sub-mitted to the CNMMN for approval. Adoption of a ter-nary system for the REE-dominant members would notaffect the current nomenclature.

SO,

SO,

"arsenowaylwaylandite


Poo

soo

rmbojarosite

/ kintor"it" segnitite

\ lumbogummite

ohilivsbornite

,/

arizawaite

soo

soo Po.

AI

Ftc. 7. Minerals of the alunite supergroup with Pb predominant as the D-site cation. Left diagram shows current nomenclature.The mineral at the A1-SO4 apex is osarizawaite if it is accepted that Cu is non-essential. Compositions are shown by Sejkoraet al. (1998) as extending into this field, which has been named 'plumboalunite" by Nov6k e/ a/. (1994) Diagram on rightshows the results of adopting a temary system.

Other minerals

Eylettersite is considefed to be a Th-dominant mem-ber of the crandallite group, with P as the main cation in70+ (Table 3). Hqryever, the calculated formulas alsoincorporate substantiai (H+O+) in ZOa, and show exces-sive Al (>3.5 mol) for G (Van Wambeke 1972). Such aG-cation excess is untenable for an alunite-type min-eral, and eylettersite is considered to need restudy.

Gallobeudantite was defined as having subequal val-ues of AsOa and SO+, and a predominance of Ga in G(Jambor et al. 1996). However, ZO4 compositions ex-tend into the fields encompassed by Ga analogues ofsegnitite, corkite, and kintoreite, thus permitting the in-troduction of three additional new names for the Ga-dominant minerals. The distribution of 7Oa values inthe Ga minerals is such that adoption of a ternary sys-tem would not reduce the number of potential newnames. Also present with gallobeudantite is the Ga ana-logue of arsenocrandallite.

Other analogues

Numerous members of the alunite supergroup havebeen synthesized. Among those not known as mineralsare the Rb* and Hg2+ analogues of jarosite; the Hg-dominant and Pb-dominant phases described byDutizac & Kaiman (1976) and Dutrizac et al. (1980)are indexed with c = 33 A, but no diffraction line at 11A was reported. Mumme & Scott (1966) also noted thatthe 11 A line is absent from their synthetic plumbo-jarosite.

Tananaev et al. (1961) prepared analogues with Na,K, Rb, H3O, and NHa in D, and Ga in G. Only smallamounts of various divalent metals other than Cu2+ andZnz* have been reported to be taken up by the alunitesupergroup. The In3+, V3*, and Cr3* analogues havebeen synthesized (Dutrizac 1982, Dt;trizac & Dinardo1983, T,engauer et al. 1994): up to 0.6 mol V3* and 0.18mol Crr* have been reported in natural gorceixite (Johanet al. 1995), and Vr+ predominates at the G site in

soo

soo

plumbojarosite

beudantite

plumbogummite phil ipsbornite

SO,

Frc. 8 Compositions of minerals within the Pb-dominantfield, as in Figure 7, illustrating the extensive range of ZOasolid solution Solid circles: abridged data from Sejkora eral. (1998); open squares are compositions from Rattray elal. (1996\.

springcreekite (Table 3 ; Kolitsch et al. 1999a). Substan-tial Sbs*-for-Fe3+ substitution in dussertite has beendocumented by Kolitsch et al. (1999b).

The vanadate (Tudo et al. 1973), chromate (Powerset al. 1976, Cudennec et al. 1980), and selenate(Dutrizac et al. 1981, Breitinger et al.1997) analogueshave been synthesized, and substantial Cr has been re-ported in the Z site in philipsbornite (Walenta et al.1982). Some analyses reveal Si, which has been as-signed either as SiO+ (as in kemmlitzite and eylettersite)or as amorphous SiO2. Other substitutions, such as Nb(Lottermoser 1990) and CO3, are possible but generallynot significant; analyses showing percentage quantitiesof CO2 (e.9., Frirtsch 1967) predate the widespread useofthe electron microprobe, and the high values reportedin a few older analyses may be the result of inclusionsof carbonate minerals.

Substitution of Cl for OH is rarely reported and,where detected, amounts are small. Accounts of F sub-stitution are more cofirmon, and up to 4.'l wtVo F hasbeen determined to be present in gorceixite (Taylor etal. 1984\.

NOMENCLATIJRE OF THE ALUNITE SUPERGROUP r333

Stnucrunel Asprcrs

The classification of the alunite supergroup dis-cussed to this point has been mainly chemical, and struc-tural aspects have been largely ignored. Numeroussingle-crystal X-ray structure studies of the supergrouphave been done, and the findings are summarized inTable 4. Most of these studies have revealed that theminerals have trigonal symmetry, with a x 7, c = 17 A,space group R3m, but several exceptions have beendocumented.

The structures of alunite, jarosite, and plumbo-

a -^ jarosite were first determined by Hendricks (1937), whon)\-/4 concluded that because alunite andjarosite show strong

pyroelectric properties, their space group must be R3mrather than R3m. In co_ntrast, plumbojarosite was deter-mined to belong to R3m, and the Pb atoms showed anordered arrangement that had the effect of doubling thec axis, i.e., the typical cell of a = 7, c = 17 A increasedl o a = 7 , c x 3 4 A .

In addition to the data given in Table 4, several stud-ies of synthetic analogues have been done, either bysingle-crystal or Rietveld methods. All of the studies,both on minerals and their synthetic equivalents, and onthose not known as minerals, confirm that the basic to-pology of the structure remains the same regardless ofchemical composition. Even where a monoclinic or tri-clinic system has been established, the structures arestrongly pseudotrigonal.

The basic structural motif of the supergroup consistsof TOa tetrahedra and variably distorted R-cation octa-hedra, the latter corner-shared to form sheets perpen-dicular to the c axis. Substitutions involving G thereforemainly affect the a dimension, and a increases as Fe-for-Al substitution increases. The ZO+ tetrahedra, whichare aligned along [001], occur as two crystallographi-cally independent sets within a layer; one set of TOapoints upward along c, and this set altemates with an-other pointing downward. The oxygen and hydroxylform an icosahedron, amidst which is the D cation. Forcompositions with identical T tn TOq, the length of c ismainly influenced by the size of the D cation.

Symmetry changes arise principally because of or-der-disorder relationships among the 7Oa tetrahedra, orbecause of order-disorder or distortion involving Dsites. To maintain the space grotp R3m, the SO+-PO+-AsO+ tetrahedra must be disordered; ordering, as hasbeen found in corkite and gallobeudantite, reduces thesymmetry to R3m.

A second principal effect is related to the presenceof a divalent cation in D. In such a case, the standardjarosite-type formula D*G3*3(SOa)2(OH)6 becomes either-7|+162+

t(soo)r(og)ul, (ordered) or, D2* 112G3* 3(Soa)2(OH)o (disordered). Thus in plumbojarosite, for example,the D-site cation is Pb2*, and to maintain electroneutral-ity, half of the D sites are vacant. Ordering of the Pband these vacancies produces a supercell, which is mani-fested as a doubling of the c axis to 33-34 A.


Soo

F e ) A l f e > A l

AsOo PO, AsOuarsenoflorencite {Ce) arsenof loren cite-(Ce)

_(Nd)*-(La)*

A l > F e A l ) F e

sou 500

Frc 9. Minerals of the alunite-jarosite supergroup with REE as the predominant D-site cations. "Arsenoflorencite-(La)*" un6"arsenoflorencite-(Nd;*" *" names (Scharm et al. 1991, 1994) that have not been submitted to the CNMMN for a vote.Diagram on right shows the effects of adopting a temiuy system.

A third principal effect is related to the charge of theZOa group. Where ZOa is POa or AsOa rather than SOa,an extra proton is required to maintain charge balance.In crandallite, Blount (1974) concluded that the formulatakes the form CaAl3 [PO 30 1 p(OH) 1 p]2(OH)6, therebyachieving compensation by having partial substitutionof hydroxyl for oxygen of the PO+ tetrahedron.Radoslovich (1982), however, determined that such aconfiguration would not be permitted in gorceixite be-cause of the larger size of Ba. He therefore concludedthat the formula of gorceixite is best represented byBaAl3eO4)(PO3.OHXOH)6.

It is possible, therefore, that order may exist withoutthe necessity of having the 7 site occupied by differentelements (As, P, S). Furthermore, the crystal-structuredetermination (R = 0.0195) of unnamed PbFe3GO4)2(OH,H2O)6 by J.T. Szymanski (Table 4) revealed thatdistortion at the Pb sites leads to two indeoendent Pbatoms in the structure. Thus, a ruperstru.ture is formedin this mineral without the necessiw of havine D-sitevacancres.

Szymafiski (1985) has pointed out additional aspectsrelevant to the crystal chemistry of the alunite super-group. Among these is the report by Loiacono et al.(1982) that the space group of their natural alunite isR3m. For hydronium jarosite - schlossmacherite,Szymafiski (1985) pointed out that "... the hydroniumion, whether it remains in its original H3O+ form orwhether it is reduced to water, cannot have a centre ofsymmetry and cannot be located at a centre of symme-try. Any hydronium-for-cation substitution will destroythe symmetry, and reduce the space group to R3m orlower. This is not to say that the hydrogen atoms of thehydronium ion or the water molecule cannot be statisti-cally distributed or dynamically disordered, and hencegive the structure the overall appearance of having acentre of symmetry."

As with hydronium, the NHa ion cannot be accom-modated within R3m (Arkhipenko et al. 1985, Sema etal. 1986). However, as discussed by Szymahski (1985),in the absence ofincontrovertible sfuctural data, the spacegroup of alunite minerals should be considered as R3m.

florencite-(Ce)

soo

Soo

NOMENCLATURE OF THE ALUNITE SUPERGROUP

TABLE 4 SINGLB-CRYSTAL X-RAY STRUCTURE STTIDIES OF MINERALS IN TIIE ALUMTE SUPERGROTJP

Compositio4 oments a , L c, A spaegroup Ref.

I 335

alunite

natroalsite

mimiite

JilOSrte

gorccixite

qmdallite

woodhouseite

goyuite

svmbergite

os@awilte

plmbojaosite

bfldatite

corkite

hin$datite

galobildantite

plmbogwite

kirio!eite

du$ertite

nnnnrBed

florencite-(Ce)

@mpo$uoD nor grvm(Nao,slq rJAls(SO),(OH)6(Nao *Ig rCa".rrtrnr)Alr(Sq)r(O$.not givq; syntheticnot giv€n; mtural

[email protected](OID6Cao"At*P,*; CaAlrlPO3(O@(OIDdlr(OID6not givm; CaAI,(PO*)(SO4XOID5not givm; SrAI3[PO3(O,"(OID,J]2(OI06rot givs; SrAL(POa)(SO4XOID6Pb(Al, .rCuo ,rFeo ,lq d(SO1)r(OH)5trot givd; structure fomula u belowPbo Jq *So *; Pb[Fe(sOJr(o]D51,Pboe(FqsCuo@zrboilSO).lAsOJ.'(0II)6dPb, *(Fe,"Alo*)(AsOJr odsO4te(OEHp)6Pb,.(FerorAlo rr)(AsO,)r B(SO)0'(OH,ILO)6Pbo *(Fer"Cuo *)(PO4)I.6(SOJ0 *(Asq)o ml(OH)6 qPbALleo,), 3r(so{Lcl(orr,H,o)5Pbl u(Gal fAli laFeo szno ilGei 6XAsO{)r &(SQI *(OIO"

"PbALI(PoJ, s(Aso)o,J(oI!H,o).PbFq(POJ',(AsOn)ur(SO4)03(OII,Hp)6Ba(Fd.' s4sb*o,b)3(Aso"),(ollHp).Pbo oFq n,@Oo), ,,(SOn)o ,(OI1I4O)' ,,(CeoalrorNdo,rSmo*Car*)A1r(PO,l(OlD.

6970 17 27 Rim 1

6 990 16 095 Bm 2

6 981 33 490 R3m 3

1 305 l7 l9O Rlm 4

7304 17268 Rim 5monoclinic 6

monoclinic* Cm 7

7 005 16192 Rlm 8

6 993 16 386 Bm 9

? 015 16 558 Rlm 10

6992 16567 Rim 5

70'15 17 I Rlm 1l

720 33 60 Rim 12

7 3055 33 675 Em 13

1 339 17 034 R1m 14

7 3l5l I7 0355 Rlm 15

triclinic** 15

7 ZaO 16 821 R3m 16

7 029 16789 Rim f'l

7 225 17 03 R3m 18

7 039 16761 Bm 17't 331,0 16 885 Bm

'19

7 410 l'l 484 Rlm 20

7 28A5 33 680 R3m 21

6912 16-261 Bm 22

* a 12 195, b 7 o4o, c ? 055 A 0 125 l0' S@ slro BlEnchdd (1989)* * q 7 3r9, b 7 309, c 17 032 A, a 90 004, p 90 022, y 119 97 4"Referenes: 7 Wngetdl.(1965),2 Okedaetal 0982), 3 O$ska"t4l. (1982), 4 Mf,chetti&Sabeli(1976), 5 Kato&M{m(1977),6 Radoslovich&Slade(1980), ? Radostovich(1982), E Blout(1974),9 Kato097l, 1977)' l0 Ksto(1971, 198?), 1 I Giuseppetti & Tadini (1980), 12 Hendricks (1937), who also gave reflrlts for aluite edjeositq 13 Szf-miski (1985), 14 Giuseppelti & Tadini (1989), 15 SzJmiski (1988), 16 Giuseppetti & Tadini (1987), 17 Kolitshetdl.(1999c), 78 lanboretal (1996), 19 Khdirueta/. (1997), 20 Kolitsch€tdl (1999b), 2l IMANo 93-039,22 Kato(1990)

Formula calculations

Various methods have been used to calculate theformulas of minerals in the alunite supergroup. Amongthe most common are normalization to 14 oxygen at-oms, or to TO4 = 2. Some authors (e.g., Scharm et al.1991, Novrik & Jansa 1997, Sejkora et al. 1998)haveused G3(IOa)2 = 5. Use of TOa - 2 is preferred herebecause nonstoichiometry in both D and G is common,particularly in synthetic samples. To paraphraseSzymafrski (1985), use of ZO4 = 2 has a sound struc-tural basis because "... it is inconceivable to visualize astablejarosite structure with vacancies in the [TOa] lay-ers as well."

A second difficulty in formula calculations is thathydronium cannot be determined directly. This problemis not significant for most of the minerals in the super-group, but it does emphasize the precarious status of thesole composition available for schlossmacherite, forwhich D is [(H:O)o :zno z8ca0 26].Ia0 07Ko s5Sr6 slBao s1l.

Significance of the supercell

Regardless of symmetry variations (Table 4), thetopology of the unit cell of all minerals in the familycan be related to a rhombohedral cell that has hexago-nal dimensions of a x 7, c N l7 A. The development ofa supercell with c = 2 X l7 A can arise because of or-dering ofD-site cations, regardless of whether D con-tains vacancies or is filled completely. Moreover, orderdoes not require the presence of a combination ofmonovalent and divalent cations in D, but can take placesolely with monovalent ions (Dutrizac & Jambor 1984).Many rapidly crystallized synthetic products show signsof an ordered structure, and it is inconceivable that someof the commonly more slowly crystallized natural min-erals would not be ordered. On the other hand, syntheticPb-H3O systems show various degrees of disorder, andsynthetic plumbojarosite without detectable diffractioneffects due to a superstructure is well known. At theopposite extreme, in their X-ray structure studies both


Hendricks (1937) and Szymairski (1985) used plumbo-jarosite from the Tintic Standard mine, Utah, for whichan exceptionally high degree of order was found; in thecase of the latter author at least, the material was se-lected specifically because X-ray powder patterns hadpreviously indicated an exceptionally high degree oforder to be present.

If minerals in the alunite supergroup are to be namedsimply on the presence or absence of the c-axis super-structure, the potential for the introduction of "trivial"names will increase enormously. I believe that detec-tion of the superstructure will become more commononce there is an increased awareness of the sisnificanceof the I I A powder-diffraction line or peak.

Recol,rlr,rewoauoNs oN Noprexcr-erunE

If the proliferation of mineral names in the alunitesupergroup is to be avoided, two properties or factorsmust be addressed: (a) crystallographic, and (b) chemi-cal. In some cases these factors are independent, as forexample minamiite and as-yet-unnamed mineral IMANo. 93-039, both of which are distinguished frompreviously named minerals solely on the presence ofstandard cell versus supercell relationships. The fund-amental topology of the alunite structure remains thesame regardless of space group or symmetry changes.The fundamental cell is rhombohedral, space groupR3m, with a x 7 and c = 17 A as expressed witli hex-agonal parameters. Slight distortion of this cell can leadto orthorhombic (Jambor & Dutrizac 1983), monoclinic(Radoslovich 1982), or triclinic (Szymairski 1988) poly-morphs, and patterns of order of various kinds can leadto a doubling of c.

C rystallo graphic aspects

Various systems have been used to accommodatestructural changes within a mineral group while main-taining a comprehensible nomenclature. For example,Pring et al. (1990) used the CNMMN-approved namebaumhauerite-2a to designate a silver-bearing mineralhaving a superstructure derived from a baunihauerite-

like structure. For minerals of the alunite supergroup, asimilarly simplified system of nomenclature systemcould be adopted such that: (a) no modifier accompanythe mineral name if the unit cell has not been deter-mined, or if only one structure type is known; (b) if thereis a need to distinguish between the conventional celland the supercell, the former should be designated lc,and the latter 2c, each followed by the standard(CNMMN-approved) abbreviation for the unircell type[1: hexagonal, rt: rhombohedral, M: monoclinic, A: tri-clinic (anorthic), erc.l. Note that it is important to retainthe c to avoid confusion with the nomenclature desig-nations for polytypes. Thus, for example, rhombohedralplumbojarosite lacking a superstructure would be namedplumbojarosite-lcR, and that with a superstructurewould be plumbojarosite-2cR. Monoclinic gorceixiteremains as gorceixite, but if a doubled c axis for themonoclinic cell were to be found, the nomenclaturewould distinguish gorceixite-7cM and gorceixite-2cM.It has not yet been proved that gorceixite with a con-ventional rhombohedral cell exists, but such a mineralwould simply be designated as gorceixite-lcR.

Adoption of such a system would involve the fol-lowing nomenclature changes or revisions in:

1. Minamiite, which is fundamentally different fromnatroalunite only in order-disorder relationships, isnatroalunite-2cR.

2. Monoclinic gorceixite, assuming that the rhom-bohedral form also exists, would not be entitled to atrivial name.

3. Triclinic crandallite, reported by Cowgill et al.(1963), ifverified, would be designated crandallite-lcA.

4. Triclinic beudantite reported by Szymafiski(1988) would be designated with the suffix lcA.

5. Unnamed rhombohedral mineral IMA No. 93-039 would likewise adopt the appropriate already es-tablished name, together with the suffix 2cR.

6. Beaverite is cuprian plumbojarosite without su-perstructure reflections. Beaverite is therefore cuprianplumbojarosite-1cR.

7. Orthorhombic jarosite with the doubled c andcomposit ion K6e6Fe2qo(SO+)z(OH)6 16 (Jambor &Dfiizac 1983) is jarosite-2co.

TABLE 5 SI]MMARY OF POTENTIALLY CHANGED NOMENCLATUREFOR A TERNARY COMPOSITIONAL SYSTEM

Previous nue Prwiously wmed

mitmiite

huugite

kef,nrlitzite

b@vqite

kintoreite

segilwe

hidalgoite

osrizawaite

triclinic qedallite

triclinic beudmtite

rhombohedral kiatoreite

orthorhombic juosite

crmd.llite 1cl

b@ddtite- lc,{

corkite2cR

jtosite-2co

nahoalunil€-2cI

woodhoureite-2cR

menogoyuite

cupriu plufi$ojdositelcR

@rkite

bodmtite

hinsdalite

hinsdalite

Compositional aspects

Two approaches are used, namely, (a) an evaluationin terms of the existing (Scott 1987) system of nomen-clature, and (b) an evaluation on the basis of a ternarycompositional system. Neither system takes into accountpossible miscibility gaps, which have not been provedto exist within the alunite supergroup, although such apossibility is highly likely. For example, Stoffregen &Cygan (1990) have argued that a miscibility gap mayexist on the simple alunite-natroalunite binary join ifthese minerals crystallize under equilibrium conditions.Under nonequilibrium conditions, however, a gap is notevident. Numerous proposals for miscibility gaps amongminerals other than the alunite supergroup can be foundin the literature, but in many cases the purported gapssimply reflect plots of existing chemical data, withoutgood evidence that the gaps cannot be breached. For thealunite supergroup, it is likely that it will be many yearsbefore miscibility gaps can be incontrovertibly demon-strated to exist. This aspect, therefore, is not taken intoconsideration in this review of nomenclature; neverthe-less, it is evident that, over the long term, the multicom-paftment system in current use for alunite nomenclaturewould fare less well in terms of avoiding complexitythan would a ternary system.

If a ternary system of nomenclature, combined withthe superstructure-symmetry notation, were adopted forthe alunite supergroup, the following would result:

1. Alunite group with monovalent ions in D: re-moval of minamiite, beaverite, and osarizawaite, andexpansion of the compositional fields as shown in Fig-ure 2 (right).

2. Ba predominant in D (Fig. 3): the compositionalfield available for a "weilerite"-t1pe mineral disappears.

3. Ca predominant in D (Fig. 4): huangite iswoodhouseite-2cR.

4. Sr predominant in D (Fig. 5): kemmlitzite is nolonger retained.

5. Bi predominant in D (Fig. 6): no change otherthan expansion of the compositional fields.

6. Pb predominant in D (Fig. 7): for Fe > A1, corkiteand beudantite have priority, so that kintoreite andsegnitite are no longer retained. Similarly, hinsdalite haspriority over hidalgoite, and the latter is no longer re-tained.

7. REE predominant in D (Fig. 8): no change otherthan expansion of the compositional fields.

The nomenclature changes are summarized rnTable 5.

CoNcrusroNs

NOMENCLATURE OF THE ALUNITE SUPERGROUP r337

(b) Osarizawaite: Cu:(Al,Fe) is not 1:2, and Al rspredominant.

(c) Minamiite is compositionally equivalent tonatroalunite. but has c ! 33 A. Mineral IMA No. 93-039 (Table 4) could be given a trivial name on identicalgrounds.

(d) Benauite: the compositional field does not coin-cide with that of the ideal formula.

(e) Eylettersite requires re-examination because theformula(s) exceed acceptable limits for minerals of thealunite supergroup.

(f) The possible triclinic analogue of crandallite(Cowgill et al. 1963, Blount 1974) requires re-exami-nation both with respect to composition and symmetry;no single-crystal X-ray study has been done, and theformula deviates significantly from that of the alunitesupergroup.

(g) Orpheite, a Pb-Al phosphate-sulfate, is vari-ously classified as in (Gaines et al. 1997) or out(Mandarino 1999) of the alunite supergroup. Fleischer(in Fleischer et al. 7976) concluded that orpheite ishinsdalite, but orpheite has retained species status.

AcrNowrsocrvsrvrs

The initial version of the manuscript was examinedby, among others, J.E. Dutrizac, J.D. Grice, U. Kolitsch,J.A. Mandarino, E.H. Nickel, and A.C. Roberts. I amgrateful for their comments and, in some cases, theirstrong encouragement to continue to pursue the nomen-clature issues. I am also thankful to W.D. Birch for hisappreciable input during the formative stages of thisreview, to P. Bayliss for incisive and useful referee com-ments, and to R.F. Martin both for editorial commentsand for accepting that this review of nomenclature mer-ited airing. Acknowledgement is not intended to implyagreement with the views that have been expressed.

RrpBnsNcBs

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ZneNc Ruso, Y,qNc Nr,lNznpN, YI SHUANGTTNc & DuCnoNcI-IlNc (1987): Arsenogoyazite discovered inXinjiang, China. Acta Mineral Sinica 7, 313-316 (in Chr-nese).

Received June I 1, 1999, reyised mnnuscript accepted Novem-ber 12,1999.

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NOMENCLATURE OF THE ALUNITE SUPERGROUP - RRuff