Geochemical Alteration of Pyrochlore Group Minerals: Microlite Subgroup Gnrconv R. Luurpxrn Roonev C. Ewruc Ansrru.Cr

AmericanMineralogist, Volume 77, pages l,79-188,1992

Geochemical alteration of pyrochlore group minerals: Microlite subgroup

Gnrconv R. LuurpxrN Advanced Materials Program, Australian Nuclear Science and Technology Organisation, Private Mail Bag I, Menai, New South Wales 2234, Ans1nalla RooNev C. Ewruc Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

Ansrru.cr A qualitative picture of microlite stability is derived from known mineral assemblages and reactions in the simplified system Na-Ca-Mn-Ta-O-H. Results suggestthat microlite is stable under conditions of moderate to high 4*"* and aq^z*and low to moderate 4rnz*. Microlite is often replaced during the latter stagesof granitic pegmatite evolution by manganotantalite and fersmite or rynersonite, indicating increasingdq^z* ?fid 4Mn2*relative to a*.-. Primary (hydrothermal) alteration involves replacementof Na, F, and vacanciesby Ca and O, representedby the coupled substitutions ANaYF- ACaYOand AtrY! - ACaYO. Exchangereactions between microlite and fluid suggestconditions of relatively high pH, high aa,^z*,lowto moderate a.r, and low a".. during alteration by evolved pegmatite fluids at 350-550'C and 2-4 kbar. Secondary(weathering) alteration involves leaching ofNa, Ca, F, and O, representedby the coupled substitutions ANaYF - AEY!, ACaYO- AEYII and ACaxO - Alxfl. Up to 800/oof the A sites may be vacant, usually accompaniedby a comparablenumber of anion (X + Y) vacanciesand HrO molecules.Secondary alteration results from interaction with relatively acidic meteoric HrO at temperatures below 100 "C. In both types of alteration, the U content remains remarkably constant.Loss of radiogenic Pb due to long-term difftrsion overprints changesin Pb content associatedwith primary alteration in most samples.

INrnouucrroN Alteration processeshave been divided into primary and secondarytypes by Van Wambeke (1970) and Ewing Microlite, the Ta-rich member of the pyrochlore group (1975). Primary alteration is hydrothermal in nature and (Hogarth, 1977), is largely restricted to evolved granitic is usually associatedwith emplacementof the host rocks. pegmatitesof the beryl-columbite, complex spodumene, This type of alteration is most likely to occur prior to and complex lepidolite types (Cerni, 1989). The crystal significant radiation damagedue to a decayof 238IJ,235IJ, structure is cubic (Fd3m, Z: 8), a derivative of the fluo- and 232Th,resulting in either a changein composition or rite structure type, and has the general formula replacementby one or more phases.Secondary alteration A2 *B'2X6-'Y,_,.pHrO (Lumpkin, 1989). End-member is a near-surfacephenomenon associated with weathering microlite has the ideal composition NaCaTarO.F. A and often occurs after the crystal structure has been ren- number of simple and coupled substitutions are possible, dered fully aperiodic (metamict) by radiation damage. primarily involving Na, Ca, U, and vacanciesat the A Secondaryalteration leads to major increasesin vacan- site; Ta, Nb, and Ti at the B site; and O, OH, F, and cies due to leaching of A-site cations and Y-site anions vacanciesat the Y site (Lumpkin et al., 1986).The X site and to increasedhydration (Van Wambeke, 1970;Ewing, is normally fully occupiedby O; however, small amounts I 975 ; von Knorring and Fadipe, I 98 I ; Lumpkin and Ew- of F and OH and vacanciesmay occur on the X site in ing, 1985;Lumpkin et al., 1986). natural samples (Lumpkin, 1989), consistent with pre- This paper has four purposes:(l) to describepotential vious work on synthetic pyrochlores (Groult et al., 1982; replacementreactions and chemical changesthat accom- Rotella etal., 1982;Subramanian et al., 1983).Vacancies pany primary alteration, (2) to determine the maximum are tolerated mainly as defect components that can be number of A-site vacanciesresulting from secondaryal- written as ArBrXu[J, !AB2X6I, and trrBrX.M (Chakou- teration along with any attending cation exchangeeffects, makos, 1984).In the latter case,the Y site is occupied by (3) to estimate the maximum number of anion vacancies large monovalent cations like K, Rb, and Cs. The ability and determine whether they correlate with cation vacan- to accepta variety of elements,vacancies, and HrO mol- cies;and (4) to outline the P-T-X conditions of alteration ecules suggeststhat microlite may be a useful indicator for the microlite subgtoup in granitic pegmatites.Results of relative changesin fluid composition in granitic peg- ofthis study are also relevant to the disposal ofnuclear matites. waste in ceramic materials that contain pyrochlore as a 0003-{04x/92l0l 02-o l 79$02.00 t79 180 LUMPKIN AND EWING: PYROCHLORE GROUP MINERAIS

TABLE1. Localities,mineral associations,and descriptions of microlite specimens

Sample nos. tocality ancl lithologicunit Alteration pattem and mineralassociation ilh, 080, 130 Rutherford pegmatite,Amelia, VA, second in- Crystals1-2 cm with secondaryalteration along micrG 0.05--0.12 termediate zone fractures;Ab + Qtz + Cbt a Fgn+ Mzt + Ryn 185 Tin Mountain pegmatite,Custer, SD, quartz- Crystals0.4-1.0 mm, heavily microfractured, complete 0.00 spodumene-mica zone secondaryalteration; Lp + Otz + Ms 188 Volta Grande, Minas Gerais, Brazil Crystals4-6 mmwith thin,pink secondary alteration 0.20-0.60 rinds:Wgn + Qtz a Cbt 202 Brown Derby pegmatite,Gunnison County, Crystals0.6-1.0 mm with secondaryalteration along frac- 0.(x) CO, fepidolite{u aftz zone tures;Lp + Ms + Qtz 231 Opportunity pegmatite,Gunnison County, CO, Crystds N mm with secondaryalteration along trac- 0.03 quartz zone tures:Qtz t Ms 324 Pidlite pegmatite, Mora County, NM, lepido- Crystals1-2 mmwith primaryalteration as uniformrims; 0.204.70 lite4uartz zone Qtz+Lp+Zrc+Cbt 327 Fungwe district, Zimbabwe A crystaf1 x 1 x 2 cm,with patchy,primary alteration; 0.90-1.00 mineralassociation unknown 153,260, P5.1 Harding pegmatite,Taos County, NM, cleave. Crystals1-6 mmwith primaryalteration as irr€ular rims, 0.00--0.15 landite unit minors€@ndary alteration along fractures; Ab + QE + MstMc+;_p+goethite 154,264,269,P15.1, Harding pegmatite,Taos County, NM, micro- Crystals0.5-6.0 mm with primaryalteration as inegular 0.00 P17.1,P18.1 dine.s@umene zone rimsand patches, minor secondary alteration along microfractures;Mc + Qtz + Spd + Ms + Lp + Ab + 776+ goethite P21, P2.2,P7.1 Harding p€gmatite,Taos County, NM, lepido- Crystals0.5-2.0 mm with primaryalteration as inegular lits-cleavelanditesubunit rims,minor secondary alteration along microfractures; Lp + Ab + Ms + Qtz + Ryn + goethite ' ,//oranges from 0.00 for fullymetamict samples to 1.00for highlycrystalline samples. All samplesfrom the UNMcollection except tor 080(USNM 96739),185 (AMNH 34311), 188 (AMNH 24560) and 202 (HU 106185). Ab-albite, Cbt-columbite,Fgn-fergusonite, Lp-lepidolite, Mc-micaodine, Ms-muscovite,Mzt-monazite, Qtz-quartz, Ryn-rynersoniteor fersmite,Spm-spodumene, Wgn-wodginite, Zrc-zircon.

constituent phase (e.g., Harker, 1988; Ringwood et al., graphite-monochromatizedCuKa radiation. BaF, and Si l 988). were used as internal and external standards, respectively. Lattice parameters were refined by the method of ExpenrunNTAL PRocEDUREs least-squares(Appleman and Evans, 1973). The level of Electron microprobe analysis radiation damage (1//o) was estimated from the total in- pattern Analyses were perfonned using a JEOL 733 Super- tegrated intensity of all diffraction lines in the probe operatedat 15 kV and 20 nA with a probe diameter relative to hlghly crystalline standards. Metamict and al- of l0 pm. Data were corrected for drift and dead time tered specimenswere heated in air or N, for I h at 1000 qC and then reduced using an empirical c-factor approach and reanalyzed. (Benceand Albee, 1968; Albee and Ray, 1970). Each el- Transmission electron microscopy ement was counted for 40 s or until a standard deviation of 0.5V0was reached.The elementsF, Na, Mg, Al, K, Ca, Powders of selectedsamples were dispersedin acetone Ti, Mn, and Fe were analyzedusing Ka spectrallines. Za and deposited on holey-carbon filmed Cu grids. Samples lines were employed for Sr, Y, Zr, Nb, Sn, Sb, Cs, Ba, were examined using a JEOL 2000FX transmission elec- and REEs. For the elements Ta, W, Pb, Bi, Th, and U, tron microscope (TEM) operated at 200 kV. High-reso- Ma lines were used. Empirical overlap corrections were lution images were taken at a magnification of 410000 determined and used for peak interference problems. using axial illumination and objective lens defocusvalues - - Minimum detection limits are typically 0.02-0.04 wto/o of 60 to I 50 nm. A Tracor-Northern TN-5500 energy for the oxides of Mg, Ca, Mn, Fe, Sr, Ba, Pb, Cs, and K; dispersive X-ray analyzer(EDS) was used to obtain anal- 0.04-0.06 wto/ofor the oxides of Nb, Ta, and Na; 0.06- ysosofspecific areas.Spectra were acquired for 300 s real 0.08 wto/ofor the oxides of Ti, Zr, Sn, Al, Y, REEs, and time (20-300/odead time) using an effective probe di- Sb; and 0.08-0. 12 wto/ofor the oxides of W, Bi, Th, IJ, ameter of 10 nm. Data reduction was accomplishedusing and elemental F. Standards included natural microlite, the Tracor software package SMTF with empirical k fac- manganotantalite, cassiterite, stibiotantalite, olivine, tors determined from standards. benitoite, cerussite,pollucite, anorthite, gadolinite, and synthetic NaNbOr, CaWOo, SrMoOo, UOr, CaTiOr, Cnrrrnrl FoR REcocNTTroN oF ALTERATToN ThSiO4, ZrSiOo, KTaOr, YPO., and REEPO.. Three basic criteria were used to differentiate between primary and secondaryalteration: field relationships,mi- X-ray diffraction analysis croscopic observations,and alteration mechanisms.The Powder diffraction patterns were obtained using a Scin- first two criteria involve the use of mineral assemblages tag diffractometer operatedat 40 kV and 30 mA. Patterns as a guide for the identification of hydrothermal alter- were recorded from l0 to 70" 20 at l" per minute using ation and weathering. In granitic pegmatites, primary LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS l8r

lNa-l lNa.l lMn"l 1 i 1

+ [Ca'.] + [Mn,-] ------+ [Ca]l

Fig. l. Qualitative activity diagrams for the system Na-Ca-Mn-Ta-O-H based on known mineral paragenesesand idealized reactions listed in Table 2. (a) Na-Ca, (b) Na-Mn, and (c) Mn-Ca sections. Known associations are indicated by circles, replacement features by arrows.

(hydrothermal) alteration is commonly indicated by re- ples the structure has been renderedcompletely aperiodic placement of earlier mineral assemblagesby albite and (metamict) as the result of cumulative a-decay events lithium mica (Jahns, 1982; Hawthorne and Cernj', 1982; (Lumpkin and Ewing, 1988). This study has failed to re- Cernj'and Burt, 1984). Secondaryalteration is indicated veal conclusive evidence for natural recrystallization of by the breakdown of feldsparsand micas to clay minerals, chemically altered, metamict microlite. Recrystallization a processoften associatedwith the formation of iron oxy- effects appear to be more common in radiation-damaged hydroxides like goethite. silicate minerals and have been observed, for example, Alteration mechanisms include intracrystalline diffir- in altered thorite group minerals from the Harding peg- sion and fluid transport through preexisting cracks and matite (Lumpkin and Chakoumakos, 1988). voids. Diffusion is an effective mechanism at high tem- peraturesand is expectedto be the dominant processdur- T^c.NrAl-uru oxrDE MTNERALREAcrroNs ing primary alteration. Fluid transport thrcugh cracks and Based on our observations of oxide mineral associa- voids with limited intracrystalline diffirsion is probably tions in the granitic pegmatitesof northern New Mexico the dominant mechanism operative at the relatively low and Amelia, Virginia, together with those of previous in- temperatures characteristic of secondary alteration. The vestigators(e.g., von Knorring and Fadipe, l98l; Foord, timing of alteration may be estimated where clear cross- 1982;Cerni and Ercit, 1985, 1989;Ercit, 1986),we pre- cutting relationships with radiation-induced microfrac- sent here a summary of some possiblereactions between turing exist (Lumpkin and Ewing, 1985). microlite and other tantalum oxide minerals. Results are The above criteria were used to classifu alteration in confined to idealized phase relations in the system Na- 20 specimens(Table l). Typical examplesof primary al- Ca-Mn-Ta-O-H. This system is representedby the com- teration are specimens from the Harding, Pidlite, and mon associationof microlite, manganotantalite,fersmite, Zimbabwe pegmatites. Alteration patterns in these mi- and rynersonite-vigezzite.Exotic associationsreviewed crolite samples are characterized by irregular rim diffir- by Cern!'and Ercit (1985, 1989)include tantite (TarOr), sion, uniform rim diftrsion, or a patchy interior distri- calciotantite (CaTaoO,,),and natrotantite (NarTaoO,,). bution, respectively. Primary alteration appearsto predate In Figure I we have constructed schematic phase re- most of the radiation-induced microfracturing. The best lations as a function of the activities of Na*, Ca2t, and examplesof secondaryalteration occur in microlite sam- Mn2* based on idealized compositions, including Na- ples from Amelia, Virginia, in which alteration is local- TaO, and CarTarOr.For simplicity, solid solution among ized along radiation-induced microfractures with limited microlite (NaCaTarOur), rynersonite (CaTarOu),and intracrystalline diffi.rsion. Individual microlite crystals in CarTarO, has been omitted. Our aim is to illustrate qual- sample 185 from the Tin Mountain pegmatite,South Da- itatively how changes in fluid composition affect tanta- kota, are heavily microfractured and completely altered. lum oxide mineral assemblages.Simple mineral reactions Alteration was classified as secondary (see Table l) on were used (Table 2), guided by known mineral associa- the basis of small crystal size, density of microfracturing, tions or replacement features, to arrive at the topologies and proximity to a l-cm layer of kaolinite in the quartz- shown in Figure l. For example, Figure la shows how spodumene-mica core of the pegmatite (Spilde and the assemblagesmicrolite * calciotantite + tantite, mi- Shearer,1987). crolite + natrotantite. microlite * calciotantite, and mi- Investigation of the sampleslisted in Table I by optical crolite + rynersonite constrain the topology of the dia- microscopy, X-ray diffraction, and TEM showsthat most gram. have received extensive radiation damage. In many sam- The relations shown in Figures la-lc were combined 182 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

[Mn2+] I

- [Ca2+]

tl,lai Fig.2. Three-dimensionalactivity relations derived from the A2* AT sectionsshown in Figurel. Hypotheticalreaction path A-B-C-D depictsthe interactionof microlite !\dth a fluid characterizedby Fig.3. Triangularplot of divalentA-site cations (mainly Ca), increasingCa2* and Mn2+and decreasingNa* activities.Abbre- monovalentA-site cations (mainly Na), and A-site vacancies in viations:Ct : calciotantite,Mic : microlite,Mnt : mangano- unalteredand altered microtte samples. tantalite,Nt: natrotantite,Ryn: rynersonite,Tn: tantite,I : NaTaOr,2: CarTarOr. by decreasingaNur. At point B, rynersonitebegins to form by reaction of microlite with the fluid (Table 2, Reaction to provide a three-dimensionalview of mineral compat- 9). Manganotantalite begins to crystallize at point C by ibility in the Na-Ca-Mn-Ta-O-H system. Figure 2 indi- another reaction (Table 2, Reaction l8). These three cates that microlite and manganotantalite may coexist phases coexist and finally equilibrate with the fluid at over a rangeof fluid compositions characterizedby mod- point D. For d*"* values below that of point D, microlite erate to high a""-, ecaz*,?trd aea,z-.The actual stability will be completely replaced by rynersonite + mangano- field of microlite may be extended by incorporation of tantalite. CaTarOuand CarTarOTcomponents in solid solution (cf. Lumpkin et al., 1986). Crrnvrrcll EFFECTSoF ALTERATToN A hypothetical example of the breakdown of microlite Primary alteration is shown in Figure 2. Microlite initially in equilibrium Electron microprobe analysesof microlite samples from with a fluid at point A is exposedto a fluid whose com- the Harding pegmatite (Tables3 and 4r) demonstratethat position is characterizedby increasingas^r* and cr". and the major chemical effects of primary alteration are de- creasedNa and increasedCa, Fe, Mn, and O. The amounts TlaLE2. Mineralreactions in the systemNa-Ca-Mn-Ta-O-H of IJ, F, and inferred HrO (estimatedby difference)tend to remain relatively constant,although lower F and high- 1. 0.5HrO+ Ca2++ 2NaTaO3: NaCaTa"Ouu+ Na* + H+ 2. 2H* + 4NaTaOo: NarTaoO,,+ 2Na* + HrO er inferred HrO contents were noted in a few samples. 3. 2H2O+ 2Ca2*+ NaJa4O1, :2NaCaTarO6s + 4H* Similar resultswere found in sample 324 from the Pidlite 4.2H* + NarTa4O11:2Taoos + 2Na* + HrO 'l.sHro pegmatite, Mora County, New Mexico, and in sample 5. + Na* + Ca2*+ Ta2Os: NaoaTazOos+ 3H* 6. HrO + Ca2* + 2'fa2os: CaTa*O,i + 2Ht 327 from the Fungwe district, Zimbabwe. Structural for- 7. 2H2O+ 2Na* + Ca2* + CaTa4O11: 2NaCaT&O65+ 4H* mulas of sample 324 indrcate that Ca, Mn, Fe, and O : 8. HrO + Ca'z*+ CaTa4Or, 2CaTa2O6+ 2H* increasedand that Na, AE, and Yfl decreasedas a result 9. 0.5HrO+ Na++ CaTa2O6: NaoaTazOos* H* 10, HrO + Ca2++ CaTarO": Ca"Ta"O, + 2H* of alteration. For sample 327, structural formulas clearly 11. H* + Na+ + CarTarO,: NaCaTarO"u+ Car++ 0.5HrO indicate strong Ca enrichment and Na depletion during 12. Mn'?*+ 2NaTaO3: MnTarO6+ 2Na* primary pattern, 13. HrO + 2Mn2*+ NaJaqoli :2MnTarO" + 2Na* + 2H* alteration. Following a typical the 14. H,O + Mn2++ TarO.: MnTa"Ou* 2H* amount of O increasedin the altered areas,whereas AE 15. 2H* + Mn'?*+ CaJarOr: MnTaoO"+ 2Ca2' + H2O and YE decreased.The Mn content and inferred HrO re- 16. Mn'z*+ CaTarO6: MnTa2O6+ Ca,* 17. H2O + 2Mn2* + CaT&O, : 2MnTaoO"+ Ca* + 2H* mained constant, but Fe decreasedduring alteration. 18. 0.5HrO+ Na++ Caz*+ MnTa2O6: NaCaTarO".+ Mn2++ H+ ' To receive a copy of Table 4, order Document AM-92-488 : : Note: NaCaTa,O6j microlite,MnTarO6 manganotantalite,CaTa.Oli from the BusinessOftce, Mineralogical Society of America, I130 : calciotantite, : CaTa.g": rynersoniteor fersmite, NarTaoO,r natrotan- Washington, DC 20036, tite, Ta2Os: tantite. Seventeenth Street NW, Suite 330, U.S.A. Pleaseremit $5.00 in advance for the microfiche. LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS 183

TABLE3. Representativeelectron microprobe analyses of unaltered and altered areas of six microlite samples 130 130s 202 2O2s 188 188s 231 231s 324 324p 327 327p WO" 0.43 0.28 0.06 0.16 0.10 0.31 0.31 0.51 0j2 0.00 0.04 0.00 Nb2o5 10.6 11.5 9.24 9.2',1 2.03 2.05 3.82 3.91 14.3 14.4 6.15 5.20 TarOu 62.0 64.5 58.9 61.6 76.3 72.7 71.1 71.6 61.9 60.3 76.5 76.1 Tio, 1.10 1.10 0.03 0.04 0.04 0.14 0j7 0.16 0.11 0.10 1.35 1.06 SnO, 2.01 2.28 2.4't 2.19 1.75 2.28 0.46 0.76 0.12 0.16 0.26 0.28 Tho, 0.20 0.09 0.00 0.00 0.00 0.00 0.01 0.00 0.12 0.15 0.00 0.00 UO, 1.57 1.28 9.28 8.74 2.22 3.20 3.85 4.24 3.25 3.29 0.00 0.00 Alro3 0.04 0.06 0.00 0.00 0.08 0.20 0.00 0.00 0.09 0.08 0.01 0.01 Y.o" 0.15 0.20 0.06 0.07 0.03 0.18 0.07 0.09 0.09 0.09 0.05 0.00 REE,O3 0.14 0.25 0.26 0.11 0.30 0.80 0.10 0.20 0.14 0.17 0.10 0.14 sbro3 0.00 0.03 0.08 0.14 0.00 0.06 0.46 0.30 0.06 oj2 0.07 0.13 Biros 0.00 0.00 0.04 0.08 0.00 0.00 0.01 0.00 0.04 0.08 0.10 0.11 CaO 12.6 5.83 10.0 8.14 11.2 0.01 10.9 8.57 12.4 14.8 11.2 16.0 MnO 0.00 0.'t7 0.20 0.00 0.11 0.00 0.30 0.47 0.10 0.49 0.15 0.18 FeO 0.06 0.58 0.52 0.13 0.03 0.83 0.56 0.25 0.00 0.49 012 0.02 BaO 0.00 0.00 0.00 0.00 0.00 4.00 0.05 0.05 0.00 0.00 0.03 0.00 Pbo 0.22 0.19 0.20 0.40 0.01 o.21 0.13 0.23 0.29 0.16 0.00 0.01 Naro 3.62 o.20 4.08 0.13 4.08 0.00 3.82 2.72 2.92 1.29 1.82 0.59 Cs2O 0.00 0.00 0.00 0.00 0.00 o.12 0.00 0.01 0.00 0.00 0.00 0.00 F 2.7 0j2 2.1 1.6 2.4 0.01 2.6 1.2 1.5 1.2 1.3 1.3 Sum 97.44 88.66 97.46 92.74 100.68 87.10 98.72 95.27 97.55 97.35 99.47 101.25 -0.50 -0.55 -0.5s O=F -1.13 -0.05 -0.88 -0.67 -1.01 -0.00 -1.09 -0.50 -0.63 Total 96.31 88.61 96.58 92.07 99.67 87.10 97.63 94.77 96.92 96.85 98.92 100.70 Stluctural formulas based on > B: 2.00 w 0.010 0.006 0.002 0.004 0.002 0.002 0.007 0.012 0.003 0.000 0.001 0.000 Nb 0.409 0.422 0.391 0.381 0.081 0.098 0.161 0.162 0.548 0.562 0.225 0.196 Ta 1.439 't.425 1.499 1.533 1.843 1.817 1.803 1.787 1.429 1.417 1.683 1.726 Ti 0.071 0.067 0.002 0.003 0.003 0.009 0.012 0.011 0.007 0.007 0.082 0.066 Sn 0.068 0.074 0.089 0.080 0.062 0.051 0.017 0.028 0.004 0.006 0.008 0.009 AI 0.004 0.006 0.000 0.000 0.008 0.023 0.000 0.000 0.009 0.008 0.001 0.001 Fe3* 0.000 0.000 0.017 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Th 0.004 0.002 0.000 0.000 0.000 0.002 0.000 0.000 0.002 0.003 0.000 0.000 U 0.030 0.023 0.193 0.178 0.044 0.027 0.080 0.087 0.061 0.063 0.000 0.000 0.007 0.009 0.003 0.003 0.001 0.010 0.003 0.004 0.004 0.004 0.002 0.000 REE 0.004 0.007 0.009 0.004 0.010 0.020 0.003 0.007 0.004 0.005 0.003 0.004 Sb 0.000 0.001 0.003 0.00s 0.000 0.008 0.018 0.011 0.002 0.004 0.002 0.001 Bi 0.000 0.000 0.001 0.002 0.000 0.000 0.000 0.000 0.001 0.002 0.002 0.002 Ca 1.152 0.507 1.002 0.798 1.066 0.002 1.089 0.843 1.128 1.370 0.971 1.430 Mn 0.000 0.012 0.016 0.000 0.008 0.001 0.024 0.037 0.007 0.036 0.010 0.013 FEP- 0.004 0.039 0.025 0.010 0.002 0.007 0.044 0.019 0.000 0.03s 0.008 0.001 Ba 0.000 0.000 0.000 0.000 0.000 0.112 0.002 0.002 0.000 0.000 0.001 0.000 Pb 0.005 0.004 0.007 0.010 0.000 0.039 0.003 0.006 0.007 0.004 0.000 0.000 Na 0.599 0.032 0.741 0.023 0.702 0.002 0.690 0.484 0.480 0.216 0.285 0.095 >A 1.805 0.636 2.000 1.033 1.834 0.288 1.956 1.499 1.697 1.744 1.285 1.547 o 6.111 5.565 6.412 5.940 6.158 5.215 6.289 6.170 6.318 6.532 5.930 6.288 F 0.729 0.031 0.624 0.454 0.669 0.011 0.746 0.343 0.389 0.328 0.343 0.350 >(x + Y) 6.841 5.596 7.036 6.395 6.827 5.226 7.035 6.513 6.707 6.860 6.273 6.6i|8 Note.'A letter at the end of a sample number indicatesalteration: s : secondary,p : primary. Mg, K, Sr, and Zr were below detection limits and arenotreported.AllFeallocatedtoinensiteexceptforanalyseswith>A>2.dO,forwhichenoughFewasallocatedtotheBsitetogive>A: 2.00.

A triangular plot of A-site cations and vacancies(Fig. 6). Major substitutions inferred from these results are ACaYO, ANa"OH ACaYO. 3) demonstratesthe consistent increasein divalent cat- AEYfI + ACa"O, ANaYF - and - ions during primary alteration. Monovalent cations (es- The latter substitution is postulated for the caseswhere sentially Na) usually decreasebut in a few casesremain F remains relatively constant and O increases.The sim- relatively constant. A-site vacanciestend to decreaseor ple substitution YF - YOH is also possible but is largely remain relatively constant. Two samples showed minor masked by coupled substitutions. increasesin the number of A-site vacancies.A similar The substitutions noted above can be related to per- plot of anions and vacancies (Fig. 4) indicates that O vasive, subsolidusalteration by exchangereactions ofthe increases,F tends to decreaseor remain relatively con- form: stant, and anion vacanciesusually decrease.The U contents of altered areas remain unchanged in comparison HrO + Caz' I [CaTarOu]*"" to unaltered areas(Fig. 5). Apart from Pb, which shows : lCatTarOr]-* + 2H* (le) minor decreasesin some samples, the divalent cations Ca, Mn, and Fe generallyincrease during alteration. Av- HrO + Ca2" * [NaCaTarO.fl-"" erage increasesin these cations are approximately 0.25 : (20a) Ca, 0.05 Mn, and 0.03 Fe atoms per formula unit (Fig. lCarTarOrl-"" * Na* + H* + HF 184 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

XalYp . UNALTERED (o PRIMARY ALTERATTONlo sECONDARY t

0.15 o o .9) o 0.10 ov_otr^9o _o o o tro- Oou 0.05 ooo .totroft o ^: ooo^3 trrU O o I | | r. .Y c)

0.15 o# to trtrn ET q 0.10 o o trB ootr- - o-5 F ' o utr 0.05 oooon o-- Fig. 4. plot Triangular of X- and Y-site anions and vacancies oo g in unaltered and altered microlite samples. The O content is or Ab"o o . gt:r'::.".t"-':T lT plotted give as O-5 to the same scale as in Figure 3. 0.4 0.8 1.2 Ca (atomsper formula unit) Ca2t + [NaCaTarO.OH]-"" Fig. 6. Plots of Mn and Fe vs. Ca in unalteredand altered : lCarTarOrL* * Na* * H+ (20b) microlitesamples.

0.5HrO + Ca2*+ [NaCaTarOur]-"" reaction is favored by relatively high a*2, and pH and by : [CarTarOrl-"" + Na+ + H+. (20c) low ar,* in the fluid phase. Equations 19, 20a, and,20c HrO Reaction 20 is given in three forms to account for occu- indicate that temperature and the activities of and play role in the of pancy of the Y site by F, O, or OH. The right side of each HF also a controlling composition microlite. In situations where F remains relatively con- Atr stant, Reactions 20b and 20c best account for chemical changesattending primary alteration.

Secondary alteration Examination of Figure 3 shows that alteration causes compositions to move toward the AE-A2* join as Na is removed at a faster rate than divalent cations. Among the divalent cations, Figure 6 shows that Ca is lost, whereasMn and Fe usually show slight to moderategains (up to 0. 15 atoms per formula unit). Minor increasesin Pb also occur in some specimens.With the exception of sample 188, Ba shows slight increasesin only a few of the microlite samples.Once Na is completely leachedfrom the A site, compositions then move toward the A! vertex of Figure 3 as Ca continues to be removed. In sample a 080, the compositions plot along a curved trend, indicating a gradual increasein the relative rate of Ca loss with progressivealteration. Alteration trends represented by the anions (Fig. a) closely follow those of the cations (Fig. 3). Figure 4 shows A4* A2' that, initially, F is removed from the Y site, causingcom- Y!)-(O-5) Fig. 5. Triangular plot of tetravalent A-site cations (mainly positions to move toward the (xtr + join. As U), divalent A-site cations (mainly Ca), and A-site vacancies in alteration proceeds,O is removed from the X and Y sites, unaltered and altered microlite samples. causingcompositions to approach the x! * YD vertex of LUMPKIN AND EWING: PYROCHLORE GROUP MINERAIS 185

Figure 4. Notice in Figure 4 that data for sample 080 follow a curved path complementary to the cation trend shown in Figure 3. Combined results for cations and anions indicate that Schottky defects are produced by substitutions ofthe form ANaYF - A[Y[], ACayO - AlyE, and ACaxO r ADxf]. {' Simple substitutions such as YF J YOH and ACa2+- 2H* $: are possible but tend to be masked by coupled substitu- = v.u9 tions. Also, structural considerations indicate that only minor amounts of OH- will bo tolerated at the Y site in o conjuction with large numbers of cation vacancies (Su- o u.u+ bramanian et al., 1983). The major substitutions result in highly defective, hydrated microlite samples charac- "o terized by increasesof 4 to 13 wto/oHrO. Furthermore, ,/3 o 0.o2 .fi Figure 5 shows that the U content of microlite remains ;- -^q"po-r9gs.--- remarkably constant as a result ofsecondary alteration. ./."" _so1-3b)")-- In most samples,vacancies reach maximum values of .8a:-- - 9'"'q"-' Atr : 1.5,YE : 1.0, and xtr : 0.5 per formula unit (Ta- 'i"@oo bles 3 and 4). Sample 188 from Minas Gerais, Brazil, 0.1 0.2 0.3 0.4 consists of crystals of unaltered microlite with thin alter- U (atomsper fonnula unit) ation rinds of bariomicrolite in which vacancies reach AE: Y!: xtr:0.8 Fig. 7. U-Pb systematicsof unalteredand alteredmicrolite maximumvalues of 1.8, 1.0,and samples. per formula unit. Structural formulas indicate virtually complete leachingof Ca, Na, and F, compensatedin part by entry of minor amounts of Ba, Pb, Fe, REEs, Cs, and l0-12 wto/oHrO. gain during alteration or common Pb incorporated during Secondary alteration, or weathering, of microlite can crystallization. be approximated by reactions of the form: Data for microlite samples from other approximately 1300-m.y.-old pegmatites are generally consistent with 3Ht + HrO + [NaCaTarO.Fl-* the Harding suite. Microlite crystals from the Pidlite pegmatite exhibit Pb loss of 0-350/oin unaltered cores and : [TarOr'2HrO]-* * Na* * Ca2** HF (21) 6O-75o/oin altered rims. Theseresults are consistentwith 2}I* + l.5HrO * 0.5Ba2* + [NaCaTarOufl_* either episodic Pb loss during primary alteration or a sub- sequent long-term diftrsion gradient preserved over a : [BaorTarO55.2H2O]-*+ Na+ + Ca2*+ HF. (22) distanceof 0.5-1.0 mm from core to rim. In comparison, The right side of each reaction is favored by relatively heavily microfractured microlite crystals (samples 202 and, low , low pH, and low ay^4 aqyz+1and a". in the fuid 231) from the Quartz Creek district, Colorado, consis- phase.Additionally, Reaction 22 is favored by relatively tently show 60-800/oPb loss in both unalteredand altered higJr ar"r. in solution. areas.In this casethe alteration is classifiedas secondary (Table l). The U-Pb systematicsreported above indicate U-Pb systematics that radiation-induced microfracturing plays an impor- Microprobe analyses were used to evaluate the behav- tant role in controlling the long-term loss of radiogenic ior of U and Pb in microlite subsequentto crystallization. Pb in microlite. The Harding suite was selected because a large number DrscussroN of samples are available, the geologic age is well estab- lished at 1300m.y. (Aldrich et al., 1958;Brookins et al., Chemical effects of alteration 1979), significant metamorphic overprinting is absent, and Chemical changes that accompany alteration are sum- the microlite samples have low Th and high U contents marized in Figure 8 using idealized end-members to show (Lumpkin et al., 1986). Figure 7 shows a U-Pb plot in- major element trends. Most unaltered compositions fall cluding both unaltered and altered microlite samples from near the NaCaTarO.F-CaTarOujoin or just within the the Harding pegmatite. Most of the data fall below the NaCaTarOuF-CarTarOr-UlarO. composition field. Four 1300-m.y. reference line. Except for specific examples, alteration vectors are indicated by the data in Figures 3 the magnitude of Pb loss is poorly correlated with alter- and 4: (l) filling of Schottky defectsby CaO, (2) NaF ' ation. The maximum degree of Pb loss approaches 800/o CaO exchange, (3) formation of Schottky defects by re- in both unaltered and altered material, suggesting that moval of NaF, and (4) creation of Schottky defects by long-term difrrsion is a viable explanation of the results. removal of CaO. Altho'gh there are obvious differences The high-Pb data points in Figure 7 are from the core of between individual samples, primary alteration paths lie a microlite grain in sample 269 and represent either Pb consistently between trends I and 2 (Fig. 8). r86 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

tr2Ta2O5tr2 U and 0.I wt0/oPb and another with 7.7 wto/oU and l.l wto/oPb. Both gave 23su'-2o6Pband2'rsu-2oiPb isotopic ages of 900-1000 m.y., consistentwith lossof 20-300/oof the radiogenic Pb.

Conditions of alteration Members of the microlite subgroup show well-defined primary and secondary alteration in terms of both tex- tural and chemical features.We suggestthat this is related to the depth of emplacementof the host rocks. Most granitic pegmatitesof the rare-elementclass are emplacedat depthsof 4-12 km (Jahns,1982; London, 1984;Cernj', 1989; Chakoumakosand Lumpkin, 1990; Norton and Redden, 1990). Becaueof the small size of granitic pegmatites, the cooling rate is rapid in comparison with much larger granitic plutons. On the basis of cooling models for a finite slab with dimensions of 2 x 2 x 0.02 km, Cha- Ca2TarOt NaCaTa206F koumakos and Lumpkin (1990) estimated that the magmatic phase of the Harding pegmatite lasted only 100- Fig. 8. Ternaryplot surnmarizingthe major elementtrends 1000 yr. Emplacement at relatively deep crustal levels primary (P) (S)alteration of microlite. Vectors for and secondary meansthat subsequenthydrothermal activity will usually l-4 aredescribed in the text. have ended long before granitic pegmatitesreach the surface following uplift and erosion. Primary alteration of microlite occurred during the late Secondaryalteration follows a distinctly different path magmatic to hydrothermal stagesof pegmatite emplace- betweentrends 3 and 4, gradually approachingtrend 4 as ment at P : 2-4 kbar and f : 350-550 "C (cf. Chakou- alteration proceeds to completion. In casesof extreme makos and Lumpkin, 1990). Alteration is influenced by secondary alteration, this path ultimately requires that the presenceand evolution ofa densesilicate liquid and some O be removed from the X site. The occurrenceof supercritical HrO-COr-NaCl"n fluid rich in volatiles and paired vacancies(Schottky defects)is consistentwith the fluxing elementssuch as F, Li, Be, B, P, Rb, and Cs (Jahns, geometry of the pyrochlore structure type, in which YAo 1982; London, 1986, 1987; Cerni, 1989).Exchange re- tetrahedra and AX.Y, distorted cubic sites exist within actions betweenmicrolite and fluid suggestconditions of largecavities of a stableBrXu octahedralframework (Sub- relatively hrgh pH, high ag^z-,and low 4pur,oft€l accom- ramanianet al., 1983;Chakoumakos, 1984). These large panied by elevated ey1^z*urrd Qp"z*.Tllre requirement of cavities are interconnectedin three dimensions by chan- Fe3*and Mn3* to occupy the B site in some of the Har- nels, through which A- and Y-site ions can be removed ding microlite samples suggeststhat relatively hrgh "f", in combinations that maintain overall chargebalance. conditions prevailed during alteration. This is supported Microlite also commonly exhibits slight to moderate by hieh IJ6+/LJ4+ratios in microlite (Jahns and Ewing, increasesin Mn and Fe during primary or secondaryal- 1976) and the incorporation of V5* and Bi3* in thorite teration. In the Harding microlite samples,values of 2 A during primary alteration (Lumpkin and Chakoumakos, > 2.00 atoms per formula unit require that essentiallyall 1988). Modeling of mineral reactions in the system Na- of the Fe and part of the Mn incorporated during primary Ca-Mn-Ta-O-H indicates that if the increasesin a*z- and alteration to be located at the B site as Fe3* and Mn3* 4y"z* proceed far enough, microlite will be replaced by (Lumpkin et al., 1986). During secondaryalteration, mi- rynersonite (or fersmite) and manganotantalite. crolite may exhibit slight to moderate increasesin the Ba Secondary alteration is characteristic of near-surface 'C content. A slight increasein Cs was noted in the bariomi- conditions with Z < 100 and P < I kbar. Alteration crolite sample from Brazll. occurs in the presenceof relatively large volumes of me- The exceptional stability of U in the A site results in teoric HrO and is mainly controlled by radiation-induced part from the coordination geometry,where true leaching microfractures. Exchange reactions between microlite and is restricted to cations of low formal valence. Removal meteoric HrO suggestconditions of relatively low 4""*, of a IJa* or IJ6* ion would be diftcult to charge balance og22*1espl and pH, resulting in leaching of Na, Ca, F, and in combination with Y-site anions alone. Long-term loss O accompanied by hydration. In special circumstances, of radiogenic Pb in microlite is supported by the work of cations like Cs, Ba, or Pb may be picked up by meteoric Aldrich et al. (1958), who found that microlite samples HrO through dissolution of unstable silicate minerals and from the Harding pegmatite give discordant U-Pb ages. subsequentlyexchanged with microlite during secondary The authors investigated two samples,one with 0.7 wto/o alteration. LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS 187

CoNcr,usroxs - (1989) Mineralogy of niobium and tantalum: Crystal chemical relationships, paragenetic aspectsand their economic implications. In P. This investigation showsthat the microlite subgroupis Mdller, P. Cem!, and F. Saup6, Eds., Ianthanides, tantalum and nio- a useful indicator ofgeochemical processesoccurring at bium, p. 27-79. Springer-Verlag,Berlin. hydrothermal temperatures during the latter stages of Chakoumakos,B.C. (1984) Systematicsofthe pyrochlore structure type, StateChemistry, 53, 120-129. pegmatite emplacement. Relative changesin fluid com- ideal ArBrXuY. Journal of Solid Chakoumakos, 8.C., and Lumpkin, G.R. (1990) Pressure-temperature position can be assessedthrough the use oftantalum ox- constraints on the crystallization of the Harding pegmatite, Taos Coun- ide mineral associations, replacement reactions, intra- ty, New Mexico. Canadian Mineralogist, 28,2E7-298. crystalline zoning patterns, and subsequentprimary Ercit, T.S. (1986) The simpsonite pa.ragenesis:The crystal chemistry and alteration effects. geochemistry of extreme Ta fractionation. Ph.D. dissertation, Univer- sity of Manitoba, Winnipeg. Results of this study are also relevant to nuclear waste Ewing, R.C. (1975) Alteration of melamict, rare-earth,ABro6-type Nb- disposal using ceramic waste forms and show that pyro- Ta-Ti oxides. Geochimica et CosmochimicaActa, 39, 521-530. chlore structure types are susceptibleto leaching of cat- Foord, E.E. (1982) Minerals of tin, titanium, niobium and rantalum in ions and anions at low temperatures.This effect may be granitic pegrnatites.In Mineralogical Association ofCanada Short Course enhancedby prior radiation damageand microfracturing. Handbook,8, lE7-238. Groult, D., Pannetier,J., and Raveau,B. (l982) Neutron di&action study The next two papers in this serieswill addressalteration of the defect pyrochloresTaWOr,,HrTarOu, and HTaWO.'H,O. Jour- effectsin the pyrochlore and betafite subgroups,encom- nal of Solid State Chemistry, 41, 277-285. passinga broader range ofcompositions and geologicen- Harker, A.B. (1988)Tailored ceramics.In W. Lutze and R.C. Ewing, Eds., vironments, including carbonatitesand nephelinesyenite Radioactive waste forms for the future, p. 335-t92. North-Holland, pegmatltes. Amsterdam. Hawthorne, F.C., and Gmf, P. (1982) The mica group. In Mineralogical Association ofCanada Short CourseHandbooh 8, 63-98. AcxNowlr,ocMENTS Hogarth, D.D. (1977) Classification and nomenclature of the pyrochlore We thank Carl Francis of Harvard University, John White, Jr. of the group. American Mineralogist, 62, 403-410. pegmatite In Mineral- Smithsonian Institution, and George Harlow of the American Museurn Jahns, R.H. (1982) Internal evolution of bodies' 293-327. of Natural History for providing some of the samples. Much of this work ogical Associationof CanadaShort Course Handbook, 8, (1976) mine, Taos County, is based on the collection of microlite samples from the Harding peg- Jahns, R.H., and Ewing, R.C. The Harding Eds., New Mexico Geo- matite at the University of New Mexico, for which we acknowledge the New Mexico. In R.C. Ewing and B.S. Kues, p. 263-276' New effons of Art Montgomery, B.C. Chakournakos, and all those involved in logical Society Guidebook, 27th Field Conference, preserving this remarkable locality. We thank E.R. Vance, T.J. White, Mexico Geological Society, Socorro, New Mexico. phase in systemLiAlSiOo- and an anonymous referee for providing critical reviews of this manu- Ilndon, D. (1984) Experimental equilibria the petrogenetic grid pegmatites. American script. Electron microprobe analyses and transmission electron micros- SiOr-H,O: A for lithium-rich copy were completed in the Electron Microbeam Analysis Facility in the Mineralogist,69, 995-1004. -(1986) in the Tanco rare-ele- Department of Geology and Institute of Meteoritics at the University of Magmatic-hydrothermal transition phase-equilibrium New Mexico, supported in part by NSF, NASA, DOE-BES, and the State ment pegimatite: Evidence from fluid inclusions and of New Mexico. This work was supported by the Department of Energy, experiments. Arnerican Mineralogist, 7 l, 37 6-395. - ( pegmatites: Effects Office ofBasic Energy Sciences,under grant DE-FG04-84ER45099. I 987) Intemal differentiation of rare-element of boron, phosphorus, and fluorine. Geochimica et Cosmochimica Acta, 5r.403-420. RnrnnnNcns crrED Lurnpkin, G.R. (1989) Alpha-decay damage, geochemical alteration, and crystal chemistry of natural pyrochlores. Ph.D. dissertatioo, University (1970) for probe Albee, A.L., and Ray, L. Conection factors electron of New Mexico, Albuquerque. phosphates, microanalysis of silicates, oxides, carbonates, and sulfates. Lumpkin, G.R., and Chakoumakos,B.C. (1988) Chemistry and radiation Analytical Chernistry, 42, | 408-1414. effects of thorite-group minerals from the Harding pegmatite, Taos Tilton, (195E)Ra- Aldrich, L.T., Wetherill, G.W., Davis, G.L., and G.R. County, New Mexico. American Mineralogist, 73, 1405-1419. granitic K-Ar meth- dioactive agesof micas from rocks by Rb-Sr and Lumpkin, G.R., and Ewing, R.C. (1985) Natural pyrocblores:Analogues ods.Transactions ofthe American GeophysicalUnion,39, lL24-1134. for actinide host phasesin radioactive waste forms. Materials Research (1973) Indexing least-squares Appleman, D.E., and Evans, H.T., Jr. and Society Symposium Proceedings,44, 647-654. X-ray powder data. United States National Technical refinement of -(1988) Alpha-decay damage in minerals of the pyrochlore group. Document Pb-2 16-188. Information Service. Physicsand Chemistry of Minerals, 16,2-20. Bence,A.E., and Albee, A.L. (1968) Empirical correction factors for the Lumpkin, G.R., Chakoumakos,B.C., and Ewing, R.C. (1986) Mineralogy electron microanalysis of silicates and oxides. Journal of Geology, 76, and radiation effects of microlite from the Harding lrcgmatite' Taos 382-403. County, New Mexico. American Mineralogist, 71, 569-588' Brookins, D.G., Chakoumakos,B.C., Cook, C.W., Ewing, R.C., Iandis, and Redden, J.A. (1990) Relations ofzoned pegrnatitesto G.P., and Register,M.E. (1979) The Harding pegmatite:Summary of Norton, J.J., pegmatites, ganite, and metamorphic rocks in the southem Bliack recent research. In R.V. Ingersoll and L.A. Woodward, Eds., New Mex- other Hills, South Dakota. American Mineralogist, 75,631-655. ico Geological Society Guidebook, 30th Field Conference, p. 127-133- I*vins, D.M', and Ramm, New Mexico Geological Society, Socorro, New Mexico. Ringwood, A.E., Kesson, S.8., Reeve, K.D., R.C. Ewing, Eds., Radioactive Cern!, P. (1989) Characteristicsofpegmatite depositsoftantalum. In P. E.J. (1988) Synroc. In W. Lutze and p.233-334. Amsterdam. Mdller, P. Cern!, and F. Saup6, Eds., Ianthanides, tantalum and nio- waste forms for the future, North-Holland, bium, p. 196-219. Springer-Yerlag, Berlin. Rotella, F.J., Jorgensen,J.D., Biefeld, R.M., and Morosin, B. (1982) I-o- Gmi, P., and Burt, D.M. (1984) Paragenesis,crystallochemical charac- cation of deuterium sites in the defect pyrochlore DTaWOu from neu- teristics and geochemical evolution of micas in granitic pegmatites. In tron powder diffraction data. Acta Crystallographica,838, 1697-1703- Mineralogical Society of America Reviews in Minenlogy, 13, 257 -298. Spilde, M.N., and Shearer,C.K. (1987) A cornparison of tantalum-nio- Grni, P., and Ercit, T.S. (1985) Somerecent advances in the mineralogy bium mineral assemblagesin two mineralogically distinct rare-element and geochemistry of Nb and Ta in rare-element granitic pegm.atites. pegmatites. Geological Society of America Abstracts $,ith Programs, Bulletin de Min6ralogie, lO8, 499-532. 19,853. 188 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

Subramanian,M.A., Aravamudan, G., and Subba Rao, G.V. (1983) Ox- von Knorring, O., and Fadipe, A. (1981) On the mineralogy and geo- ide pyrochlores-A review. Progress in Solid State Chemistry, 15, 55- chemistry of niobium and tantalum in some granite pegmaties and 143. alkali granitesofAfrica. Bulletin de Min6ralogie, 104,496-507. Van Wambeke, L. (1970) The alteration proc€ssesofthe complex titano- niobo-tantalates and their consequences.Neues Jahrbuch ffiLr Miner- M,cNuscRtpr RECETVEDAucusr 9, 1990 alogie Abhandlungen, I12, ll7-149. Mem.ncnn'r AccEprEDSsFrEMsrn II, l99l

ERRATUM

Manganese,ferric iron, and the equilibrium betweengarnet and biotite, by M. L. Williams and J. A. Grambling (v. 75, p. 886-908). The final term in Equations12 and l3 shouldbe negative. Also, the first term is slightly low due to rounding errors. The correct form of Equation 13 is 1"(K): [-17368 - 79.5(P)+ 1579 - wro..(X,,- - X,*) - 12550(Xs.) - 9230(X"*)l - R[n KD - 0.782 - ln(Fe2+BVFerd'Bt)"] (13) where Ko includes only Fe2*and starred terms refer to the composition of the minerals in the calibration experiments. Note that, for clarity, the final Sermhas been maintained as in Equa- tion 12. Also, in Table 10, Wff"*, shouldbe changedto 7285,20775, and 10031for Models l, 2, and 3, respectively.