Geochemical alteration of pyrochlore group minerals: Pyrochlore

American Mineralogist, Volume 80, pages 732-743, 1995

Geochemical alteration of pyrochlore group minerals: Pyrochlore subgroup

GnnconY R. LuvrpxrNAdvanced Materials Program, Australian Nuclear Science and Technology Organization, Menai 2234, New South Wales, Australia

Rooxny C. EwrNcDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

ABsrRAcr

Primary alteration of uranpyrochlore from granitic pegmatites is characterized by thesubstitutions AEYfI + ACaYO, ANaYF - ACaYO, and ANaYOH + ACaYO. Alteration oc-curred at -450-650 oC and 24 kbar with fluid-phase compositions characterized byrelatively low d*"*, high aso,, , and high pH. In contrast, primary alteration of pyrochlorefrom nepheline syenites and carbonatites follows a different trend represented by the sub-stitutions ANaYF + AIY! and ACaYO + ADYE. In carbonatites, primary alteration ofpyrochlore probably took place during and after replacement of diopside + forsterite *calcite by tremolite + dolomite + ankerite at -300-550 oC and G-2 kbar under conditionsof relatively low 4"., low 4*,*, low d*,*, low pH, and elevated activities of Fe and Sr.Microscopic observations suggest that some altered pyrochlores are transitional betweenprimary and secondary alteration. Alteration paths for these specimens scatter around thetrend ANaYF + AEYE. Alteration probably occurred at 200-350 'C in the presence of afluid phase similar in composition to the fluid present during primary alteration but withelevated activities of Ba and REEs. Mineral reactions in the system Na-Ca-Fe-Nb-O-Hindicate that replacement of pyrochlore by fersmite and columbite occurred at similarconditions with fluid conpositions having relatively low 4^,*, moderate 4-:*, and mod-erate to high 4..'*. Secondary alteration (<150 "C) is characterized by the substitutionsANaYF + AEYE, ACaYO - AEYE, and ACaxO ; AExfl together with moderate to extremehydration (10-15 wto/o HrO or 2-3 molecules per formula unit). Minor variations in theamounts of Mg, Al, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result ofsecondary alteration. Major cation exchange for K, Sr, and Ba is a feature of samples fromlaterite horizons overlying carbonatites. In most cases fJ, Th, and B-site cations remainrelatively constant. Radiogenic Pb is typically lost via long-term difrrsion, but in somegrains of uranpyrochlore 25-90o/o of the Pb is lost as a result of alteration.

IxrnonucnoN

The pyrochlore group consists of a chemically diversesuite of minerals having the general formulaA2-^B,X6 *Y,-,.pHrO, where A : Na, Ca, Mn, Fe2+,Sr, Sb, Cs, Ba, rare earth elements (REEs : Sc, Y, lan-thanides), Pb, Bi, Th, and U; B : Nb, Ta, Ti, Al, Fe3+,Zr,Srt^, and W; X: O, OH; and Y: O, OH, and F. Thestructure type is cubic (space group Fd3m, Z : 8), has aunit cell dimension of approximately 10.3-10.6 A, andtolerates vacancies at the A, X, and Y sites (m : 0-1.7 ,w: 0-0.7, n:0-l). Defect pyrochlore may be stabilizedby the incorporation of HrO molecules @ : 0-2) andOH groups, with total HrO contents of 10-15 wto/o(Lumpkin, 1989). Hogarth (1977) defined three majorsubgroups of the pyrochlore group on the basis of themajor B-site cations: microlite (Nb + Ta > 2Ti, Ta >Nb), pyrochlore (Nb + Ta > 2Ti, Nb > Ta), and betafite(2Ti > Nb + Ta). Individual species in each subgroup

0003-004v95/0708-0732$02.00

are defined by the A-site cation population (see Hogarth,1977 , Table l).

Microlite is mainly restricted to moderately to highlyfractionated rare-element granitic pegmatites; betafite isfound in geochemically more primitive granitic pegma-tites and in some carbonatites (eernf and Ercit, 1989).Members of the pyrochlore subgroup predominantly oc-cur in three major host rock categories: carbonatites,nepheline syenites, and granitic pegmatites. As a result,the general chemistry of the pyrochlore subgroup is ex-tremely diverse in relation to the microlite and betafitesubgroups (e.g., Hogarth, 196l; Perrault, 1968; Krivo-koneva and Sidorenko, l97l; Petruk and Owens, 1975).Because ofthe variety ofhost rocks encountered as wellas the complex compositions of members of the pyro-chlore subgroup, chemical effects of alteration are ex-pected to be complicated (Lumpkin and Ewing, 1985).

Geochemical alteration of pyrochlore in carbonatitesto a hydrated, defect pyrochlore enriched in Ba, Sr, or K

732

LUMPKIN AND EWING: GEOCHEMISTRY OF THE PYROCHLORE MINERALS

Trac 1, Localities, sources, host rocks, and associated minerals for 18 pyrochlore samples

733

No. Locality and host rock type Alteration oattem and mineral association ilh

084

086

090

177

181

187

191

197

214

216

289

290

294

298

Brevik, Norway; nepheline sye-nite

Lueshe, Lake Kivu, Zaire; car-bonatite

Hybla, Ontario, Canada; graniticpegmatite

Brevik, Norway; nepheline sye-nite

Stoken, Langesundfiord, Nor-way; nepheline syenite

Aln6. Sweden: carbonatite

Gastineau River. Ontario. Cana-da; carbonatite

Panda Hill, Mbeya, Tanzania;carbonatite

Woodcox mine, Hybla, Ontario;granitic pegmatite

MacDonald mine, Hybla, Onta-io; granitic pegmatite

MacDonald mine, Hybla, Ontar-io; granitic pegmatite

Panda Hill, Mbeya, Tanzania;carbonatite

Leushe, Lake Kivu, Zaire; car-bonatite

Fredricksvarn, NoMay; nephe-line syenite

Lueshe, Lake Kivu, Zaire; car-bonatite

Lueshe, Lake Kive, Zaire; car-bonatite

Jacupiranga, Sao Paulo, Brazil;carbonatite

Araxe, Minas Gerais, Brazil:carbonatite

3 mm octahedron with secondary alteration along frac-tures, Pl + Ap + Zrn

3 mm heavily fractured gray crystal from laterite, com-plete secondary alteration

>1 cm mass, transitional alteration, Otz + Cal + Pl

0.2-1 mm crystals with transitional alteration overprint-ed by secondary alteration along fractures, Kts +A m + B t + F l

0.1-1 mm crystals with primary alteration overprintedby transitional alteration, Kfs + Pyx + Ap

1-2 mm crystals with primary alteration overprinted bytransitional alteration, Cal + Ap + Phl + Ol + Srp

0.2-2 mm crystals, heavily fractured, smaller grains ex-hibit complete secondary alteration, Cal + Bt + Ap

3 mm heavily fractured ciystal from laterite, secondaryalteration plus unaltered relicts

>1 cm mass with transitional alteration and fracturecontrolled secondary alteration, Kfs + Pl + Qtz

>2 cm mass with primary alteration overprinted byfracture controlled secondary alteration, Cal + Zrn +Qtz

O.2-2 cm crystals with transitional alteration alongcrystal margins, Cal + Qtz + Zrn

5 mm single crystal, zoned, unaltered, Cal + Am + Bt+ A p + M a g + Z r n

4 mm greenish€ray single crystal trom laterite, heavilyfractured, complete secondary alteration

0.1-1 mm crystals with secondary alteration alongcrystal rims, Kfs + Bt + Am + Ap + Zrn + Ttn +Mag + 116

5 mm single crystal, unaltered, Cal + Pyx + Ap + Pl

4 mm greenish{ray single crystal from laterite, heavilyfractured, complete secondary alteration

1-2 mm crystals with primary alteration of crystal rims,baddeleyite inclusions, Cal + Ap + Mag + Ol + Srp+ Phl + l lm

0.1-1 mm tan crystals from laterite, microfractured,turbid secondary alteration, plus relicts ot unalteredpyrochlore phase

o.2

0.0

0.3-0.4

0.0

0.0

0.0

0.8-0.9

0.0

0.0

0.8-1.0

0.8-0.9

0.5-0.8

0.9-1.0

0.2-0.3

USNM 820541

usNM 117122

usNM 94802

AMNH C67239

AMNH 24334

AMNH 24310

AMNH 39924

AMNH 25430

HU 1024503

HU 118064

UNM

usNM 120178

USNM 117122

USNM R5033

USNM 114776

USNM 114776

UNM

A.N. Mariano

299A

2998

325

1C24

Note.' ,/ro determined by XRD ranges from 0.0 for completely metamict samples to 1.0 for highly crystalline samples. Am : amphibole; Ap : apatite;Bt:biotite; Cal :calcite; Fl :fluorite; llm:ilmenite; Kfs:potassiumfeldspar; Mag:magnetite; Ol :olivine; Phl =phlogopite; Pl :plagioclase;Pyx : pyroxene; Qtz : quartz; Srp : serpentine; Ttn : titanite; Zrn: arcon.

has been documented in several prior studies (e.g., Jiigeret al., 1959; van der Veen, 1963; Harris, 1965; VanWambeke, 1971, 1978). The general consensus is thatalteration occurred under low-temperature hydrothermalconditions or as a direct result of weathering of the hostrock. A range of alteration products of pyr<lchlore havealso been noted in the literature, including columbite,fersmite, lueshite, bastnaesite, and several iron titaniumoxides (James and McKie, 1958; van der Veen, 1960,1963; Skorobogatova, 1963; Van Wambeke, 1970; Hein-rich, 1980). Chemical reactions involving these mineralsand pyrochlore indicate that Fe, Ca, Na, and REEs maybe important constituents of the fluid phase during alter-ation, and, in specific cases, the B-site cations Nb, Ti,and Zr may have been mobile (see Gier6, I 990, for recentevidence on the mobility of Ti, Zr, and REEs in hydro-thermal fluids).

Apart from the available inforrration in the literatureon pyrochlores from carbonatites and their overlying lat-erites, the chemical effects of alteration in members ofthe pyrochlore subgroup from granitic pegmatites and

nepheline syenites are virtually unknown. The goal of thisinvestigation is to establish a general chemical frameworkfor the interaction ofminerals ofthe pyrochlore subgroupwith high-temperature fluids that evolve during host rockemplacement and with later, lower temperature hydro-thermal fluids and ground waters. Furthermore, the re-sults of this work provide additional information on thelong-term performance of crystalline phases with the py-rochlore structure in ceramic nuclear waste forms, in-cluding Synroc, tailored ceramics, and lanthanum zirco-nium oxide (e.g., Ringwood et al., 1988; Harker, 1988;Hayakawa and Kamizono, 1993).

Sancr,r DEscRrprroN

A detailed description of each sample is given in TableI using the previously adopted criteria for recognition ofalteration (Lumpkin and Ewing, 1992; see also VanWambeke, 1970; Ewing, 1975). In general, primary al-teration is characterized by large-scale intracrystalline dif-fusion with limited fracture control (e.g., samples l8l,187,216, and,325; see Fig. la). In carbonatites, primary

t J l LUMPKIN AND EWING: GEOCHEMISTRY OF THE PYROCHLORE MINERALS

Fig. I . Optical micrographs showing alteration effects in minerals of the pyrochlore subgroup. (a) Pale green primary alterationsurrounding unaltered yellow-orange core, sample 187, Alnij carbonatite, field of view 1.0 x 0.8 mm. (b) Colorless transitionalalteration surrounding unaltered yellow-brown areas, sample 289, MacDonald pegmatite, field of view 1.0 x 0.8 mm. (c) Colorlesssecondary alteration localized along microfractures, sample 214, Woodcox pegmatite, field of view 0.5 x 0.4 mm (d) Secondaryalteration localized along microfractures, sample 084, nepheline syenite, Norway, field of view 1.0 x 0.8 mm.

735

alteration of pyrochlore may be associated with the re-placement of the diopside + olivine + calcite assemblageby tremolite + dolomite. In contrast, secondary alter-ation is strongly fracture controlled (e.g., samples 084 and274 see Fig. lc and ld) and is often associated with thealteration of feldspars and micas to clay minerals, chlo-rite, and iron oxides. Some heavily fractured crystals fromlaterite zones overlying carbonatites may be completelyaltered, having a turbid appearance in thin section (sam-ples 086, 294,2998, 197, and lC24).

On the basis of established textural criteria for recog-nition of alteration, several specimens were found to ex-hibit a style of alteration between the extremes definedas primary and secondary flable l). Defined as transi-tional alteration, this pattern is characterized by a com-bination of intracrystalline diffusion and fracture control(e.g., samples 090, 177, l8l, 187, and 289; see Fig. lb).Transitional alteration of pyrochlore from carbonatitesmay be associated with the alteration of olivine, pyrox-ene, amphibole, and mica to serpentine, sodium amphi-bole, talc, and chlorite.

NrosruM oxrDE MTNERAL REAcrroNs

Idealized phase relations between pyrochlore and as-sociated niobium oxide minerals can be depicted in thesystem Na-Ca-Fe-Nb-O-H by replacing Mn and Ta inthe reactions listed in Table 2 of Lumpkin and Ewing(1992) with Fe and Nb, respectively. This system is rep-resented in carbonatites by the common association ofpyrochlore, ferrocolumbite, fersmite, and lueshite or na-troniobite (e.9., James and McKie, 1958; van der Veen,1960, 1963; Heinrich, 1980). In Figure 2 we have con-structed schematic phase relations based on the idealizedcompositions of pyrochlore (NaCaNbrOu r), ferrocolum-bite (FeNbrOu), fersmite (CaNbrO.), and lueshite (Na-MOr) together with the hypothetical phases Nb2Os,NarNboO,,, CaNboO,,, and CarNbrOr. Mineral phasesknown to exist in this system are shown by circles, where-as the observed replacement reactions are shown as ar-rows.

Figure 2a shows that pyrochlore may coexist withlueshite under conditions of relatively high a*"* duringcarbonatite formation. Although stable three-phase as-semblages have not been reported in this system, severalreplacement reactions appear to be quite common. Pyro-chlore can be replaced by lueshite, suggesting that 4""*can be maintained at moderate to high levels during thelate magmatic to hydrothermal stages of host rock em-placement. Pyrochlore is also known as a replacementproduct of natroniobite and fersmite, requiring a decreasein the a^^,/a6u:* rotio during subsequent magmatic-hy-drothermal evolution.

The phase relations depicted in Figure 2b represent asection at moderate d.",* through a three-dimensionalNa+-Ca2+-Fe2+ activity diagram (analogous to Figure 2of Lumpkin and Ewing, 1992). Figure 2b shows that fer-rocolumbite can coexist with pyrochlore or fersmite un-der conditions of moderate dF",+, moderata a1a,*, and low

+(!z(U

o,o

II

N"zM+Orr

-> ' log ^"a2+

Fig.2. Qualitative activity diagrams for the system Na-Ca-Fe-Nb-O-H derived from known mineral parageneses. (a) Na-Ca relations at low Fe2+ activity. (b) Na-Ca relations at moderateFe2+ activity. Diagrams based on data in Table 2 and Fig. 2 ofLumpkin and Ewing (1992). Circles represent coexisting phases,arrows denote replacement reactions.

to moderate 4",-. Stable three-phase assemblages havenot been documented; however, ferrocolumbite is knownto replace pyrochlore and fersmite or vice versa, consis-tent with variable activity ratios during the evolution ofthe carbonatite magma-fluid system.

Cnnurcll. EFT'Ecrs oF ALTERATToN

Experimental procedures used in this investigation areidentical to those described by Lumpkin and Ewing (1992)and need not be repeated here. Electron microprobe anal-yses and structural formulas normalized to 2.000 B-site


+(5z(U

t',o

II

736 LUMPKIN AND EWING: GEOCHEMISTRY OF THE PYROCHLORE MINERALS

TreLe 2. Representative electron microprobe analyses of unaltered and altered areas of six pyrochlore samples

1 8 1 1 8 1 p 1 8 1 t 187 187c 187t 325 325o 214 214s 299A 2998s

Nbros 60.9TarO, 0.00Tio, 3.71ZrO" 2.69Tho, 1.45UO, 0.00At2o3 0.00YrO. 0.25LnrO. 4.5MgO 0.05CaO 17.2MnO 0.04FeO 0.93SrO 0j7BaO 0.04Pbo 0-27NarO 5.12K,O 0.00F 3.3Sum 100.52O=F -1 .39

Total 99.13

Nb 1.740Ta 0.000Ti 0.177Zt 0.083Ar 0.000Th 0.021u 0.000Y 0.008Ln 0.104Mg 0.005Ca 1 .167Mn 0.002Fe 0.049Sr 0.006Ba 0.001Pb 0.005Na 0.629K 0.000> A 1.977o 6.304F 0.651> (x + Y) 6.955

60.90.003.901.641.690.000.000.266.20.03

10.40.232.461.410.640.313.320.002.3

95.74-0,9794.77

s4.9 60.6 61.80.06 0.06 0,055.07 3.63 3.902.52 3.24 2.624.09 1.08 1.700.05 0 .10 0 .130.o2 0.00 0.010.42 0.24 0.229.9 3.3 3.80.11 0.01 0.00

10.5 16.7 12.60.13 0.09 0.374.00 1.14 2.430.18 0.34 0.730.16 0.00 0.090.23 0.24 0.270.22 5.22 4.390.00 0.00 0.040.36 3.8 3.3

92.92 99.79 98.45-0 .15 -1 .60 -1 .3992.77 98.19 97.06

1 .761 1 660 1.727 1.7360.000 0.001 0.001 0.0010.188 0.255 0j72 0.1820.051 0.082 0.100 0.0800.000 0.002 0.000 0.0010.025 0.062 0.016 0.0240.001 0.001 0.001 0.0020.009 0.015 0.008 0.0070.145 0.241 0.077 0.0860.003 0.011 0.001 0.0000.713 0.753 1 j28 0.8390.012 0.007 0.005 0.0200.132 0.224 0.060 0.1260.052 0.007 0.012 0.0260.016 0.004 0.000 0.0020.00s 0.004 0.004 0.0050.412 0.029 0.638 0.5290.000 0.000 0.000 0.0031 .524 1.358 1.951 1.6706.065 6.326 6.172 6.0250.472 0.076 0.766 0.6396.537 6.402 6.938 6.664

Structu?al tormulas based on > B - 2,00

55.1 47.8 43.9o.32 10.5 10.15.09 0.54 0.442.69 1.35 1.363.77 1.49 1.240.11 22.1 21.60.02 0.08 0.160.39 0.00 0.007 6 0.08 0.220.07 0.29 0.62

10.s 8.43 6.600.22 0.00 0.814.55 1.15 0.260.28 0.00 3.970.00 0.02 't.370.15 0.35 0.000.23 1.57 0.640.04 0.04 0.120.00 1 .2 1 .4

91 .13 96.99 94.81-0.00 -0.50 -0.5991.13 96.49 94.22

1.652 1 .686 1.6720.006 0.223 0.2290.254 0.032 0.0280.087 0.052 0.0550.002 0.007 0.0160.057 0.026 0.0240.002 0.384 0.4060.014 0.000 0.0000.185 0.002 0.0070.007 0.034 0.0780.746 0.705 0.5960.012 0.000 0.0580.253 0.075 0.0180.01 1 0.000 0.1930.000 0.001 0.0450.003 0.007 0.0000.030 0.238 0.1040.003 0.004 0.0131.321 1.477 1 .5426.290 6.569 6.6710.000 0.299 0.3786.290 6.868 7.U9

34.5 35.0 68.0 75.09.05 8.99 0.03 0.14

12.3 11.1 3.18 4.400.00 0.00 0.01 0.341.13 0.89 0.07 0.16

19.7 18.2 0.00 0.420.07 0.45 0.01 0.160.32 0.94 0.15 0.190.49 1.3 0.47 0.260.07 0.17 0.00 0.00

12.3 6.96 15.3 0.370.32 0.22 0.O2 0.001.91 2.48 0.01 0.54o.17 0.62 0.67 4.030.17 0.40 0.05 1.081 .76 0.81 0.16 0.182.14 0.01 7.73 0.000.05 0.07 0.01 1 .151.5 0.25 5.4 0.11

97.95 88.86 101.27 88.53-0.63 -0.11 -2.27 -0.0597.32 88.75 99.00 88.48

1 .139 1 .1 65 1 .854 1 .8030.180 0.1 80 0.001 0.0020.676 0.615 0.144 0.1770.000 0.000 0.000 0.0000.006 0.039 0.001 0.0100.019 0.015 0.001 0.0020.320 0.299 0.000 0.0050.012 0.037 0.005 0.0050.013 0.034 0.010 0.0050.008 0.019 0.000 0.0000.962 0.549 0.989 0.0210.020 0.014 0.001 0.0000.1't7 0.153 0.001 0.0240.007 0.026 0.023 0.1240.005 0.012 0.001 0.0230.035 0.016 0.003 0.0030.303 0.001 0.904 0.0000.005 0.007 0.001 0.0781 .825 1 .181 1.939 0.2906.502 6.150 5.907 5.1510.356 0.058 1 .031 0.0196.858 6.208 6.938 5.170

Note: a lower-case letter at the end of the sample number indicates the type of alteration: p : primary alteratioR, t: transitional alteration, s :secondary alteration. B-site cations are Nb, Ta, Ti, Zr, and Al. All Fe assumed to be FeP* and allocated to the A-site. Si, Sb, Sn, Cs, W, and Bi aretypically near or below detection limits and are not reported. Ln : tanthanide.

cations for unaltered and altered areas ofeach sample aregiven in Tables 2 and 3.t Eighteen individual oxides plusthe sum of nine lanthanide oxides (LnrO., : La, Ce, Pr,Nd, Sm, Gd, Dy, Er, Yb) are listed together with F foreach analysis; the oxides of W, Si, Sn, Sb, Bi, and Cs aretypically below detection limits in this group of samplesand are not reported. The degree of hydration has beenestimated by the difference from l00o/o for each analysisand has a precision of approximately +2 wto/o HrO. Thevalidity of these estimates has been confirmed by ther-mogravimetric analysis and infrared spectroscopy for se-lected samples (Lumpkin, 1989).

' Table 3 may be ordered as Document AM-95-590 from rheBusiness Office, Mineralogical Society of America, ll30 Sev-enteenth Street NW, Suite 330, Washington, DC 20036, U.S.A.Please remit $5.00 in advance for the microfrche.

Primary alteration

Primary alteration was observed in two uranpyrochloresamples. In sample 325, from the Jacupiranga carbona-tite, alteration is characterized by exchange of Ca, Na,and F for Sr, O, and minor Mn, Fe, and Ba. Small amountsof Al, REEs, Mg, Pb, and K were also detected by electronmicroprobe analysis. Of these elements, REEs increasedand Pb decreased as a result of alteration. Structural for-mulas of unaltered and altered material indicate that onlyminor changes in the number of A-site and Y-site vacan-cies occurred during alteration. Specimen 216 is from theMacDonald pegmatite, Hybla, Ontario, Canada. Struc-tural formulas of unaltered and altered areas indicate ma-jor increases in Ca and O plus minor increases in Sr cou-pled with decreased Na, AE, and YLI (where tr indicates avacancy). On average, the amount of F decreased only

A-site vacancies

LUMPKIN AND EWING: GEOCHEMISTRY OF THE PYROCHLORE MINERALS 737

X+Y anion vacancies

A2* A*

Fig. 3. Triangular plots (atomic percent) of divalent A-sitecations (mainly Ca plus some Fe, Sr, and Ba), monovalent A-sitecations (mainly Na plus minor K), and A-site vacancies in un-altered and altered pyrochlore samples. Alteration vectors areshown as arrows pointing toward the altered composition.

slightly, which suggests that increased O contents werecompensated in part by loss of OH groups at the Y site.

Pyrochlore from carbonatites and nepheline syenitesexhibits an alteration pattern that differs from the ex-amples described above mainly in the role of vacancies.Structural formulas of specimens 181 and 187 indicateloss of Ca, Na, and F coupled with increased Mn, Fe, Sr,and both A-site and Y-site vacancies. There are also mi-nor to moderate increases in the inferred amount of HrOin the altered areas of both specimens. The overall alter-ation pattern resembles secondary alteration observed inmembers ofthe microlite subgroup (Lumpkin and Ewing,1992).lt is clear, however, that alteration ofthese pyro-chlores proceeded to a limited extent in terms of the ac-tual number of cations lost (0.3-0.5 A-site cations performula unit).

Primary alteration patterns are shown graphically inFigures 3-5. In Figure 3, the altered compositions plottoward the A2+- or A-site vacancy corner. Similar trendsare shown for the anions in Figure 4, where the alteredcompositions plot closer to the O - 5 or X * Y anionvacancy corner. The U and Th contents tend to remainrelatively constant in ?nost specimens (Fig. 5). Formalsubstitution schemes compatible with these trends areAEY! + ACaYO, oNa"F - ACaYO, ANaYF + A!YE, andACaYO + AEYE. [For all substitutions the format is un-altered - altered, where the arrow indicates "replacedby" or "goes to." The equivalent exchange operator no-tation after Burt (1989) is altered(unaltered)-,.1 Incor-poration of Sr during primary alteration of uranpyro-chlore from Jacupiranga is consistent with a substitutionof the form ANaYF + ASTYO. The substitution ANaYOH- ACaYO can be inferred in cases where there are onlyminor changes in the F content. There is little evidencefor the simple substitution YF + YOH, although at lowlevels this substitution may be obscured by the chemicalcomplexity of the system.

Of the minor elements, Mn, Fe, and REEs are com-monly present in unaltered pyrochlore and may increase,remain relatively constant, or, less commonly, decrease

o - 5 F

Fig. 4. Triangular plots (atomic percent) of X-site and Y-siteanions and X + Y anion vacancies in unaltered and alteredpyrochlore samples. The O content is plotted as O - 5 to givethe same scale as Fig. 3.

as a result ofalteration. The large cations Sr, Ba, and Kgenerally occur at low levels in unaltered material andserve as better chemical indicators of alteration. Figure 6shows that peak levels of approximately 0.2 Sr, 0.05 Ba,and 0.02 K atoms per formula unit occur at A-site va-cancy levels of 0.2-0.6 per formula unit as a result ofprimary alteration.

A number of chemical reactions may be postulated toaccount for the primary alteration patterns observed inpyrochlore from granitic pegmatites, nepheline syenites,and carbonatites:

H rO+Ca2++(CaNb ,O . ) ^ "

: (CarNbrOr)o". + 2H* (l)

HrO + Ca2+ + (NaCaNbrOuF)o""

: (CarNbrOr)e." + Na+ + HF + H+

Caz+ * (NaCaNbrOuOH)*.

: (CarNbrOr)o.. + Na* + H*

0.5HrO + Ca2+ + (NaCaNbrOu')o""

: (CarNbrOr)p"" + Na+ + H+

(2a)

(2b)

(2c)

A4* A2*

Fig. 5. Triangular plots (atomic percent) of tetravalent A-sitecations (Th and tI), divalent A-site cations (mainly Ca plus someFe, Sr, and Ba), and A-site vacancies in unaltered and alteredpyrochlore samples.

A-site vacancies


=o-

U)

0 . 4 5

0 . 3 5=E o.zs(Udl

0 . 1 5

0 . 0 5

0 . 0 5

0

0 . 5 1 1 . 5A-site vacancies (plu)

Fig. 6. Plots of large A-site cations (atoms per formula unit)vs. A-site vacancies in unaltered and altered pyrochlores. (a) Sr.(b) Ba. (c) K. Note how each element tends to peak at differentvacancy levels as a result of secondary alteration.

HrO + Sr2+ * (NaCaNbrOuF)*"

: (CaSrNbrOr)o"" + Na* + HF + H+ (3)

H+ + (NaCaNb2O6Do""

: (CaNbrOu)*" + Na+ * HF (4)

3H* + (NaCaNbrOuF)o""

: (NbrOr.HrO)o* * Na+ * Ca2+ * HF. (5)

To account for the presence of F, O, or OH at the Y site,Reaction 2 is given in three forms. Reactions l-3 suggest

that alteration occurs under relatively basic conditions.Reactions 4 and 5 model the alteration observed in sam-ples l8l and 187 and are consistent with relatively acidicconditions during alteration. Estimated increases of 2-5wto/o HrO (0.4- I .0 molecules per formula unit) are typicalfor most specimens.

Transitional alteration

Sample 187, from the carbonatite of AlnO, Sweden,provides a good example of transitional alteration ob-served in members of the pyrochlore subgroup. In com-parison with unaltered material, structural formulas ofaltered areas indicate major loss of Na and F, loss of someCa, and major increases in REEs and Fe. Hydration, cou-pled with increasing of A-site and Y-site vacancies, alsooccurred. This pattern of alteration is identical to thatobserved in sample l8l from a nepheline syenite hostrock in the Langesundfiord district of southern Norway.A similar style of alteration was observed in uranpyro-chlore from granitic pegmatites in the Bancroft district,Ontario, Canada (e.g., samples 090 and 289), but minorchanges in REEs and Fe were detected in these samples.

As shown in Figures 3 and 4, altered compositions gen-erally involve an increase in A-site vacancies and X + Yanion vacancies at the expense of A* cations (mainly Na)and F, which is consistent with a substitution of the formANaYF + AIYE. Some Ca loss occurs but is normallycompensated in part by cation exchange for Sr, Ba, andREEs. The relative amounts of U and Th remain ap-proximately constant (Fig. 5). Overall transitional alter-ation produced maximum amounts of approximately 0.2Sr, 0.08 Ba, and 0.04 K atoms per formula unit at A-sitevacancy levels of 0.7-l.l per formula unit (Fig. 6).

The major chemical changes associated with the tran-sitional alteration of pyrochlore are summarized in Re-actions 6-9. Although these reactions are similar to Re-action 4, they include structural HrO in the alterationproduct to account for moderate to major increases in thedegree ofhydration:

H+ + HrO + (NaCaNbrOuF)*"

: (CaNbrOu.HrO)o* + Na+ + HF (6)

H+ + HrO * Sr2* * (NaCaNbrOuF)o*

: (SrNbrOu.HrO)*" * Na+ + Ca2+ * HF (7)

l.sHro * Ce3+ + (NaCaNbrOuDo*

: (CeNbrOur.HrO)o* + Na+ + Ca'z+ + HF (8)

H* + HrO + Ba2+ + (NaCaNbrO.F)^"

: (BaNbrOu.HrO)o* + Na+ + Ca2+ + HF. (9)

The salient feature of transitional alteration is loss of Naand F combined with some cation exchange for Sr, Ba,REEs, and Fe to produce a hydrated pyrochlore nearABrO6.HrO in stoichiometry. HrO contents estimatedfrom analytical totals suggest that increases of 4-12 wto/o

0 . 5 5

0.45

0 . 3 5

0 . 2 5

0 . 1 5

0.05

a uc

oo

n

t r t r 8 ^o o o

o o ^ o o q]tr_ -D^ oo/#@A^gdS €c&ao

0 . 2

0 . 1 55

-o- 0.1)<

b

o unalteredt r p r im aryA t rans i t iona lo secondary

c

%bDoo


HrO (0.8-2.4 molecules per formula unit) occurred dur-ing this stage ofalteration.

Secondary alteration

Chemical changes accompanying alteration are exem-plified by sample 084 from Brevik, Norway. Unalteredareas of the sample contain major amounts of Ca, Na,and F. In contrast, altered areas are characterized by re-duced Ca, near complete loss of Na and F, increasedamounts of Mn, Fe, and REEs, and major increase in theinferred amount of HrO. The corresponding structuralformulas also indicate major increases in AE and Y!. Thisstyle of late, fracture-controlled alteration can be foundin pyrochlores from all three ofthe major host rock cat-egories, commonly cutting across earlier primary or tran-sitional alteration.

Major cation exchange effects are normally limited topyrochlores found in lateritic environments. Excellent ex-amples include bariopyrochlore lC24 from Arax6, MinasGerais, Brazil, and kalipyrochlore 086 and strontiopyro-chlore samples 294 and 2998 from Lueshe, Lake Kivu,Zaire. The structure formulas of these samples exhibitextreme numbers of A-site, Y-site, and X-site vacanciescharacteristic of secondary alteration. Efects of second-ary alteration on members of the pyrochlore subgroupare summarized in Figures 3-5. Altered compositions plotsignifrcantly toward the A-site vacancy corner of Figure3 and the X + Y anion vacancy corner ofFigure 4. Therelative amounts of U and Th in these samples remainessentially unchanged (Fig. 5). Maximum numbers of va-cancies are 1.7 A!, 1.0 YE, and 0.8 xD per formula unit.As shown in Figure 6, maximum amounts of 0.55 Sr,0.45 Ba, and 0.18 K atoms per formula unit occur atA-site vacancy levels of 0.9-1.7 per formula unit.

Secondary alteration trends are consistent with the sub-stitution schemes ACaYO + AEY!, ANaYF + AEYE, andACaxO + AEXE. In consideration ofcation exchange forSr, Ba, and K, the alteration process can be approximatedby reactions of the form:

3H* + H,O + (NaCaNbrOuF)o""

: OrbrO5'2H'O)^. * Na+ * Ca2+ + HF

2H+ + l.5HrO * 0.5Ba'* + (NaCaNbrOuDo*

(10)

: (BaorNbrOr j .2H2O)o"" + Na+ + Ca2+ + HF (l l )

2H+ + l.5HrO + O.SSr'z+ + (NaCaNbrOuD^

of the Fe and REEs incorporated by pyrochlore duringsecondary alteration. Silicate minerals are the likelysources of Ba, Sr, REEs, and other elements incorporatedby pyrochlores during late stage alteration of nephelinesyenite pegnatites. Reactions l0-13 involve hydration ofthe pyrochlore phase and should proceed to the right un-der conditions of low T, pH, clya+, (l11ps and c*'r. Esti-mated increases in HrO content as a result of secondaryalteration range from approximately 8 to 14 wto/o (1.5-2.8 molecules per formula unit) for most specimens.

Behavior of REEs

Pyrochlore from carbonatites and nepheline syenites isstrongly enriched in the light lanthanides La through Sm,with Ce typically accounting for 65-75Vo of the total REEcontent. Ianthanides heavier than Sm were rarely de-tected above the 0.1 wt0/o level and Y contents were low,typically 0.0-0.4 wto/o YrOr. Uranpyrochlore from gla-

nitic pegmatites has lower total lanthanide contents (Ceis commonly the only lanthanide significantly abovebackground) and slightly higher Y contents, typically 0. l-0.6 wto/o YrOr. For unaltered pyrochlore, we found Y/Ceatomic ratios of 0.16 + 0.04 (range 0.10-0.23), 0'27 +

0.13 (range 0.I l-0.56), and 1.4 + 0.5 (range 0.8-3.5) forsamples from nepheline syenites, carbonatites, and gra-nitic pegmatites, respectively. This trend is generally con-sistent with the results ofFleischer (1965) and Fleischerand Altschuler (1969), suggesting that Y/Ce ratios orchondrite-normalized abundance patterns of unalteredand altered areas of pyrochlore might reflect relativechanges in pH (Burt and London, 1982; Burt, 1989).

Even though the total REEs in pyrochlore may increasein abundance by as much as a factor of two as a result ofalteration, data shown in Figure 7 suggest that chondrite-normalized abundance patterns do not change signifi-cantly. Unfortunately, REEs heavier than Gd are near orbelow detection limits in the samples analyzed in thisstudy, thus limiting the usefulness of abundance patterns.

The observed Y/Ce ratios of most of the pyrochlores an-alyzed in this study did not change significantly as a resultof primary or transitional alteration. However, secondaryalteration commonly resulted in an increase in the Y/Ceratio. As a result of alteration, the Y/Ce ratio of samples298, 214, and 299A,8 increased from 0.14 to 0.23, 1.2to 2.1, and 0.5 to 2.0, respectively. These results are con-sistent with the relatively lower values of pH predicted

by Reactions l0-13.

: (SrorNbrO55'2H2O)o". * Na+ + Ca'?+ + HF (12) U_pb systematics

2I{* + l.sHro + K+ + (NaCaNbrOuF)o"" Uranpyrochlore from 1000 m.y. old Grenville rocks in

: (KNb,o55 2H,o)""" * Na+ + ca2+ + HF. (r3) tTffiJil3:Tiat:iT";:;TfSj:XT1tJ"XT,fiil,tJ;Reactions I l-13 require a source of Ba, Sr, and K. In metamict. The U and Pb contents plotted in Figure 8

carbonatites, these elements are normally supplied by the indicate Pb loss of up to 80-850/o in both altered and

dissolution of feldspars and carbonate minerals during unaltered material, consistent with a model of long-termlaterite formation. Breakdown ofcarbonate minerals (i.e., Pb loss. However, there is a tendency for unaltered sam-ankerite, siderite, bastnaesite, etc.) may also provide much ples to fall closer to the 1000 m.y. reference line. Three


1 0 4

0 .08

0 .06

0 .04

0 .02

. unaltered

tr primary altsration

A transitional alteration

O secondary alteration

a

a

1000 m.y. U-Pb l in€ a t t t '

AA$a aAa.a

"s:t o80% Pb loss s

U (atoms pfu)

Fig. 8. U-Pb systematics of unaltered and altered areas ofuranpyrochlore samples from approximately 1000 rn.y. oldGrenville rocks, eastern Canada. In these samples, the alteredareas tend to plot further below the 1000 m.y. U-Pb line.

for whole rock samples of carbonatite and pyroxenite(Amaral et al., 1967) and U-Pb isotopic ages of 127 + 5and 132 + 5 m.y. for two grains of zirconolite from un-weathered pyroxenite (Oversby and fungwood, l98l).Altered rims on the same grains consistently give lowerPb contents, leading to an average U-Pb age of 75 + 20m.y. X-ray diffraction data show that the crystals are par-tially crystalline (see Table l), but the calculated dose of7-8 x lO'u a/mgis well above the saturation dose of Ix 10'6 a/mg and therefore the crystals should be fullymetamict (Lumpkin and Ewing, 1988). Subsequent TEMinvestigation revealed that the unaltered cores are in factmetamict, but the rims exhibit a recrystallized micro-structure. These results indicate -40o/o Pb loss duringprimary alteration and recrystallization of the Jacupiran-ga uranpyrochlore.

DrscussroxChemical effects of alteration

The major chemical changes that accompany alterationare summarized in terms of idealized end-members inFigure 9. Unaltered compositions generally project with-in the NaCaNbrOuF-CarNbrOr-CaNbrOu compositionfield. Composition vectors in this system correspond tofour substitutions: I : AEY! + ACaYO, 2 : ANaYF +ACaYO,3 : ANaYF+A!YE, and4 : ACaYO ) AEYI :- I (above 50 molo/o of the defect pyrochlore component,vector 4 : ACaxO - AEXtr). Primary alteration paths(Pl) of uranpyrochlore from granitic pegmatites and car-bonatites tend to fall between vectors I and 2. much likethose of the microlite subgroup (Lumpkin and Ewing,1992).In contrast, primary alteration paths (P2) of pyro-

o-oEo(E

-oo-

1 0 3

0 .30 .2 0 .40 . 1

oo=ItEoE(,gCIEEo

1 0 4

1 0 3

L a C e P r M S m B D y y E r \ b

Fig. 7. Chondrite-normalized REE distribution patterns ofunaltered and altered pyrochlores. (a) Sample 084. (b) Sample18 l. ktters at the end of the sample number refer to the typeof alteration: p: primary, t: transitional, and s : secondary.MDLs : rninimum detection limits.

samples in particular show Pb loss associated with alter-ation. Assuming all Pb to be radiogenic, unaltered areasof sample 289 give an average U-Pb age of 1005 + 65m.y. In contrast, altered areas of the sample give an av-erage age of 560 a ll5 m.y., indicating 400/o Pb lossduring alteration. Average U-Pb ages for sample 090 are870 t 55 m.y. for unaltered and 635 + 85 m.y. for al-tered areas, consistent with Pb loss of 250lo during alter-ation. Analyses of sample l9l give U-Pb ages df 910 +50 m.y. and 100 + 25 m.y. for unaltered and alteredmaterial, respectively. This is the most extreme case en-countered, indicating loss ofabout 900/o ofthe radiogenicPb during secondary alteration.

Disturbance of U-Pb systematics can also occur in rel-atively young specimens as indicated by electron micro-probe analyses of uranpyrochlore 325 from the carbon-atite of Jacupiranga, Brazil. Unaltered cores of severalgrains give an average U-Pb age of 125 + 20 m.y., con-sistent with K-Ar isotopic age determinations of 136 m.y.


chlore from nepheline syenites and carbonatites fall be-tween vectors 3 and 4. Transitional alteration paths (T)scatter in a broad area between vectors 2 and 4, generallyfalling between the alteration paths observed for primaryand secondary alteration (S).

With progressive primary and transitional alteration,minerals of the pyrochlore subgroup exhibit A-site cationexchange for Sr, Fe, and Mn relatively early. During thelater stages of alteration, Sr and Mn tend to decrease,whereas Fe continues to increase along with Ba and REEs.Significant changes in the REE abundance patterns orY/Ce ratios of pyrochlore were not observed for primaryor transitional alteration, indicating that actual pH changeswere minor and that alteration was largely controlled byvariations in the activities of cations, HF, and HrO. Mod-erate to major increases in K, Sr, and Ba are typical ofsecondary alteration in tropical climates (Jiiger et al., 1959;Harris, 1965; Van Wambeke, 1978). As a result of sec-ondary alteration, a few samples exhibit changes in Y/Ceratio that are qualitatively consistent with relatively lowvalues ofpH inferred from exchange reactions. However,the use of REEs to monitor pH during alteration of pyro-chlore requires caution because of crystal chemical fac-tors (Fleischer, 1965; Burt, 1989; Mariano, 1989).

Although recent evidence indicates that high valencecations like Ti and Zr (along with REEs, Th, and U) aremobile under hydrothermal conditions (Gier6, 1986,1990; Williams and Gier6, 1988; Gier6 and Williams,1992; Flohr, 1994), the major B-site cations Nb, Ta, Ti,andZr are relatively immobile once incorporated into thestable framework structure of pyrochlore. Although theresults of this investigation also show that Th and Uare immobilized quite effectively by pyrochlore, the be-havior ofradiogenic Pb is variable. Analyses ofunaltereduranpyrochlore suggest that up to 800/o ofthe radiogenicPb may have been lost by long-term diffusion and re-moval from the radiation damaged samples by fluids mi-grating along cracks and microfractures (Lumpkin andEwing, 1992). In many of these samples there is also ev-idence for episodic loss of 25-90o/o of the radiogenic Pbas a result of primary, transitional, or especially second-ary alteration.

Most of the results described above are related to theremarkable ability of the pyrochlore structure to toleratevacancies at the A, X, and Y sites stabilized by the in-corporation of HrO molecules and OH groups (up to l0-15 wto/o total HrO). Both the A and Y sites are locatedwithin channels parallel to {ll0} formed by the stableBrXu framework of pyrochlore. Cation-anion pairs (e.g.,Na-F, Ca-O) located on these sites can be removed fromthe structure in response to chemical potential gradients,resulting in paired vacancies (Schottky defects). Addi-tional Schottky defects are produced by limited removalofCa-O pairs from the A and X sites. Loss ofanions canbe partially offset by exchange for large monovalent anddivalent cations like K, Sr, and Ba. Ionic conduction bycharge-balanced A-Y and A-X pairs provides a first orderconstraint on the cation exchange capacity ofpyrochlore

Ca2Nb2Q NaCaNb2QF Na2Nb2O5F2

Fig. 9. Composition vectors and triangular plot of pyro-chlore end-members summarizing observed geochemical alter-ation patterns. Pl : primary alteration ofuranpyrochlore fromgranitic pegmatites, p2 : primary alteration of sodium calciumpyrochlore from nepheline syenites and carbonatites, T : tran-sitional alteration, and S : secondary alteration.

on the basis of the formal valence of the A-site cation:A+ > A2+ >> A3+ >> A4*. This sequence, modified bythe additional effect ofcation size, gives a reasonable ex-planation for the observed cation mobility sequence inpyrochlore: Na > Ca > Sr > Ba > K > REEs > Th, U.

Conditions of alteration

As summarized previously by Lumpkin and Ewing(1992), members of the microlite subgroup undergo pri-mary alteration during the late magmatic to hydrother-mal stages of highly fractionated, complex pegmatites ofthe spodumene and lepidolite type. The alteration typi-cally occurs at P : 24kbar and T :350-550 "C in thepresence of a Na-K-Al silicate liquid and an exsolved,saline H,O-CO, fluid rich in Li, Be, B, F, P, Mn, Rb, Cs,and Ta. Members of the pyrochlore subgroup (mainlyuranpyrochlore) are generally restricted to geochemicallymore primitive granitic pegmatites of the rare earth, ber-yl, and beryl-columbite type (Cernj,, 1989; Cerni and Er-cit, 1989). These rock types typically have higher Ti, Fe,Nb, and REE contents and lower levels of volatiles, Mn,and Ta than their more highly fractionated counterparts.As a result, the magmatic-hydrothermal conditions char-acteristic of uranpyrochlore crystallization and subse-quent primary alteration may be shifted to higher tem-peratures toward the granite solidus, giving anapproximate temperature range of 450-650 .C at 2-5 kbar(see eernj', 1989, Fig. 2). Exchange reations betweenuranpyrochlore and the hydrothermal fluid suggest con-ditons of relatively high pH, high as^,*, and low 4*,*,accompanied by elevated ap.zt a,r;.d c."'* during primaryalteration.

In contrast to granitic pegmatites, carbonatites andnepheline syenites are typically emplaced at shallowerdepths of 0-5 km (P = 0-2 kbar) as part of compara-tively large, alkaline igneous complexes. Experimentalwork and field observations suggest that the magmaticphase of carbonatite emplacement may have concluded

trzNbzOstrz


at temperatures as low as 450-500 'C, with continuedhydrothermal activity to 200-300'C (Aleksandrov et al.,1975; Heinrich, 1980). The magmatic phase of host rockemplacement is of relatively long duration, and associ-ated hydrothermal activity may continue throughout theperiod ofuplift and erosion, thus accounting for the rangeof alteration effects observed in minerals of the pyro-chlore subgroup. During this time, the hydrothermal fluidmay evolve from primarily magmatic fluid at deepercrustal levels to one dominated by ground water near thesurf,ace (Andersen, 1984,19871, Flohr, 1994).

In carbonatites, most of the pyrochlore crystallized inthe presence of an alkali-carbonate magma and exsolvedfluid rich in COr, F, P, Ti, Fe, Sr, Zr, Nb, Ba, REEs, Th,and U after crystallization of diopside, forsterite, phlogo-pite, and most of the calcite. Primary alteration of pyro-chlore probably occurred during subsequent formation oftremolite, dolomite, and ankerite. The subsolidus re-actions tremolite + calcite : diopside * forsterite andtremolite * dolomite : forsterite + calcite (T = 450-550 qC, X.o, = 0.05-0.95, P : I kbar: Rice and Ferry,1982) provide an estimate of the upper limit of primaryalteration. Exchange reactions between pyrochlore andthe hydrothermal fluid are generally consistent with rel-atively low pH, low 4"u, low a"u* , low e*,* , and elevatedactivities of Mn, Fe, and Sr during alteration.

The next stage of carbonatite emplacement is charac-terized by alteration of olivine to serpentine, pyroxene tosodium amphibole, tremolite to talc, and phlogopite totalc or chlorite, together with the formation of bastnae-site, monazite, barite, fluorite, qlrartz, and sulfides. Atlow pressure, alteration of tremolite to talc can occur attemperatures below 30G450 qC over a range of X.o,(-0.H.8), but serpentinization of olivine requires a flu-id composition with very low values of X"o,(<0.05) andtemperatures below 300-350 'C (Winkler, 1979). Re-placement reactions involving pyrochlore, columbite, andfersmite occurred at this time (Heinrich, 1980), indicat-ing that the fluid had relatively high ao.u and low d"u*.Transitional alteration of pyrochlore probably took placeduring this stage in the presence of a medium- to low-temperature (200-350'C) hydrothermal fluid with rela-tively low pH, low d"., low 4*"*, low eg^,*,high as,,*,high a*,*, and high a*uu,,.

Secondary alteration ofpyrochlore mainly occurred atlow temperatures (< 150 "C) by exposure to ground waterwith relatively low pH, low concentrations of Na, Ca, andF, and significant amounts of Mg, Al, K, Fe, Sr, Ba, andREEs derived from dissolution of silicate and carbonateminerals. In some carbonatites, especially those exposedto large volumes of ground water in tropical climates,large quantities ofaltered pyrochlore are concentrated inlaterite horizons (e.g., Jiiger et al., 1959; Harris, 1965;Van Wambeke, l97l; von Maravi6, 1983), demonstrat-ing that the ion exchange rate far exceeds the dissolutionrate of pyrochlore under these conditions. The resultspresented in this study are in accordance with experi-mental studies, which have shown that defect pyrochlore

readily undergoes ion exchange and hydration at temper-atures below 500'C in acid or salt solutions (e.g., Micheletal., 1975; Goodenough etal., 1976; Groult etal., 1982;see Subramanian et al., 1983, for a review).

CoNcr,usroxsMembers of the pyrochlore subgroup serve as useful

indicators of geochemical processes over a broad rangeof PTX conditions involving granitic pegmatites, nephe-line syenites, carbonatites, and their evolved hydrother-mal fluids. Although the basic alteration mechanisms ofcation exchange and leaching are similar to those of themicrolite subgroup, many samples of the pyrochlore sub-group exhibit alteration effects that are transitional be-tween the previously identified extremes of primary @y-drothermal) and secondary (near surface) alteration. Abetter understanding of the conditions of alteration canbe obtained only through a combination of experimentalwork and detailed studies of individual mineral deposits.

With regard to the disposal of nuclear waste using ce-ramic materials such as Synroc (Ringwood et al., 1988),various tailored ceramics (Harker, 1988), and lanthanumzirconium pyrochlore (Hayakawa and Kamizono, 1993),this study demonstrates that minerals of the pyrochloresubgroup are susceptible to alteration over a considerablerange of conditions. However, the alteration mechanismsare subject to charge-balance constraints that limit themobility of REEs and, in particular, Th and.U. Anotherpaper will address the geochemical stability of the betafitesubgroup, providing information on U-rich, heavily ra-diation-damaged samples in which Ti is a major B-siteframework cation.

AcxxowI,Eocunnrs

This paper is dedicated to the late Doug Brookins and his many accom-plishments in the field of geochemistry. We thank Carl Francis (HarvardUniversity), John White (Smithsonian Institution), George Harlow(American Museum of Natural History), and Anthony Mariano for pro-viding many of the pyrochlore samples. The manuscripl was improvedthrough reviews by Don Burt and one anonymous referee. Most of theresearch was completed in the Electron Microbeam Analysis Facility inthe Department ofEarth and Planetary Sciences at the University ofNewMexico, supported in part by NSF, NASA, DOE-BES (gant DE-FG03-93ER45498). and the State of New Mexico

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743

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Mer.ruscnrrr RECETvED SBpleMseR. 22, 1994M,unncnrrr AccEprED MencH 18. 1995


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Geochemical alteration of pyrochlore group minerals: Pyrochlore