American Mineralogist, Volume 77, pages l,79-188, 1992

Geochemical alteration of pyrochlore group minerals: Microlite subgroup

Gnrconv R. LuurpxrNAdvanced Materials Program, Australian Nuclear Science and Technology Organisation, Private Mail Bag I,

Menai, New South Wales 2234, Ans1nalla

RooNev C. EwrucDepartment 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 assemblagesand reactions in the simplified system Na-Ca-Mn-Ta-O-H. Results suggest that microliteis stable under conditions of moderate to high 4*"* and aq^z* and low to moderate 4rnz*.Microlite is often replaced during the latter stages of granitic pegmatite evolution by man-ganotantalite and fersmite or rynersonite, indicating increasing dq^z* ?fid 4Mn2* relative toa*.-. Primary (hydrothermal) alteration involves replacement of Na, F, and vacancies byCa and O, represented by the coupled substitutions ANaYF - ACaYO and AtrY! - ACaYO.

Exchange reactions between microlite and fluid suggest conditions of relatively high pH,high aa,^z*,low to moderate a.r, and low a".. during alteration by evolved pegmatite fluidsat 350-550'C and 2-4 kbar. Secondary (weathering) alteration involves leaching ofNa,Ca, F, and O, represented by the coupled substitutions ANaYF - AEY!, ACaYO - AEYII

and ACaxO - Alxfl. Up to 800/o of the A sites may be vacant, usually accompanied by acomparable number of anion (X + Y) vacancies and HrO molecules. Secondary alterationresults 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 radio-genic Pb due to long-term difftrsion overprints changes in Pb content associated withprimary alteration in most samples.

INrnouucrroN

Microlite, the Ta-rich member of the pyrochlore group(Hogarth, 1977), is largely restricted to evolved graniticpegmatites of the beryl-columbite, complex spodumene,and complex lepidolite types (Cerni, 1989). The crystalstructure is cubic (Fd3m, Z: 8), a derivative of the fluo-rite structure type, and has the general formulaA2 *B'2X6-'Y,_,.pHrO (Lumpkin, 1989). End-membermicrolite has the ideal composition NaCaTarO.F. Anumber of simple and coupled substitutions are possible,primarily involving Na, Ca, U, and vacancies at the Asite; Ta, Nb, and Ti at the B site; and O, OH, F, andvacancies at the Y site (Lumpkin et al., 1986). The X siteis normally fully occupied by O; however, small amountsof F and OH and vacancies may occur on the X site innatural samples (Lumpkin, 1989), consistent with pre-vious work on synthetic pyrochlores (Groult et al., 1982;Rotella etal., 1982; Subramanian et al., 1983). Vacanciesare tolerated mainly as defect components that can bewritten as ArBrXu[J, !AB2X6I, and trrBrX.M (Chakou-makos, 1984). In the latter case, the Y site is occupied bylarge monovalent cations like K, Rb, and Cs. The abilityto accept a variety of elements, vacancies, and HrO mol-ecules suggests that microlite may be a useful indicatorof relative changes in fluid composition in granitic peg-matites.

0003-{04x/92l0 l 02-o l 79$02.00

Alteration processes have been divided into primaryand secondary types by Van Wambeke (1970) and Ewing(1975). Primary alteration is hydrothermal in nature andis usually associated with emplacement of the host rocks.This type of alteration is most likely to occur prior tosignificant radiation damage due to a decay of 238IJ, 235IJ,

and 232Th, resulting in either a change in composition orreplacement by one or more phases. Secondary alterationis a near-surface phenomenon associated with weatheringand often occurs after the crystal structure has been ren-dered fully aperiodic (metamict) by radiation damage.Secondary alteration leads to major increases in vacan-cies due to leaching of A-site cations and Y-site anionsand to increased hydration (Van Wambeke, 1970; Ewing,I 97 5 ; von Knorring and Fadipe, I 98 I ; Lumpkin and Ew-ing, 1985; Lumpkin et al., 1986).

This paper has four purposes: (l) to describe potentialreplacement reactions and chemical changes that accom-pany primary alteration, (2) to determine the maximumnumber of A-site vacancies resulting from secondary al-teration along with any attending cation exchange effects,(3) to estimate the maximum number of anion vacanciesand determine whether they correlate with cation vacan-cies; and (4) to outline the P-T-X conditions of alterationfor the microlite subgtoup in granitic pegmatites. Resultsofthis study are also relevant to the disposal ofnuclearwaste in ceramic materials that contain pyrochlore as a

t79

180 LUMPKIN AND EWING: PYROCHLORE GROUP MINERAIS

TABLE 1. Localities, mineral associations, and descriptions of microlite specimens

Sample nos. tocality ancl lithologic unit Alteration pattem and mineral association ilh,

080, 130

185

188

202

231

324

327

153, 260, P5.1

154,264,269, P15.1,P17 .1 , P18 .1

P21, P2.2, P7.1

Rutherford pegmatite, Amelia, VA, second in-termediate zone

Tin Mountain pegmatite, Custer, SD, quartz-spodumene-mica zone

Volta Grande, Minas Gerais, Brazil

Brown Derby pegmatite, Gunnison County,CO, f epidolite{u aftz zone

Opportunity pegmatite, Gunnison County, CO,quartz zone

Pidlite pegmatite, Mora County, NM, lepido-lite4uartz zone

Fungwe district, Zimbabwe

Harding pegmatite, Taos County, NM, cleave.landite unit

Harding pegmatite, Taos County, NM, micro-dine.s@umene zone

Harding p€gmatite, Taos County, NM, lepido-lits-cleavelandite subunit

Crystals 1-2 cm with secondary alteration along micrG 0.05--0.12fractures; Ab + Qtz + Cbt a Fgn + Mzt + Ryn

Crystals 0.4-1.0 mm, heavily microfractured, complete 0.00secondary alteration; Lp + Otz + Ms

Crystals 4-6 mm with thin, pink secondary alteration 0.20-0.60rinds: Wgn + Qtz a Cbt

Crystals 0.6-1.0 mm with secondary alteration along frac- 0.(x)tures; Lp + Ms + Qtz

Crystds N mm with secondary alteration along trac- 0.03tures: Qtz t Ms

Crystals 1-2 mm with primary alteration as uniform rims; 0.204.70Q t z + L p + Z r c + C b t

A crystaf 1 x 1 x 2 cm, with patchy, primary alteration; 0.90-1.00mineral association unknown

Crystals 1-6 mm with primary alteration as irr€ular rims, 0.00--0.15minor s€@ndary alteration along fractures; Ab + QE +M s t M c + ; _ p + g o e t h i t e

Crystals 0.5-6.0 mm with primary alteration as inegular 0.00rims and patches, minor secondary alteration along mi-crofractures; Mc + Qtz + Spd + Ms + Lp + Ab +776 + goethite

Crystals 0.5-2.0 mm with primary alteration as inegularrims, minor secondary alteration along microfractures;Lp + Ab + Ms + Qtz + Ryn + goethite

' ,//o ranges from 0.00 for fully metamict samples to 1.00 for highly crystalline samples. All samples from the UNM collection except tor 080 (USNM96739), 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-rynersonite or fersmite, Spm-spodumene, Wgn-wodginite, Zrc-zircon.

constituent phase (e.g., Harker, 1988; Ringwood et al.,l 988).

ExpenrunNTAL PRocEDUREs

Electron microprobe analysis

Analyses were perfonned using a JEOL 733 Super-probe operated at 15 kV and 20 nA with a probe diameterof l0 pm. Data were corrected for drift and dead timeand then reduced using an empirical c-factor approach(Bence and Albee, 1968; Albee and Ray, 1970). Each el-ement was counted for 40 s or until a standard deviationof 0.5V0 was reached. The elements F, Na, Mg, Al, K, Ca,Ti, Mn, and Fe were analyzed using Ka spectral lines. Zalines were employed for Sr, Y, Zr, Nb, Sn, Sb, Cs, Ba,and REEs. For the elements Ta, W, Pb, Bi, Th, and U,Ma lines were used. Empirical overlap corrections weredetermined and used for peak interference problems.Minimum detection limits are typically 0.02-0.04 wto/ofor the oxides of Mg, Ca, Mn, Fe, Sr, Ba, Pb, Cs, and K;0.04-0.06 wto/o for the oxides of Nb, Ta, and Na; 0.06-0.08 wto/o for the oxides of Ti, Zr, Sn, Al, Y, REEs, andSb; and 0.08-0. 12 wto/o for the oxides of W, Bi, Th, IJ,and elemental F. Standards included natural microlite,manganotantalite, cassiterite, stibiotantalite, olivine,benitoite, cerussite, pollucite, anorthite, gadolinite, andsynthetic NaNbOr, CaWOo, SrMoOo, UOr, CaTiOr,ThSiO4, ZrSiOo, KTaOr, YPO., and REEPO..

X-ray diffraction analysis

Powder diffraction patterns were obtained using a Scin-tag diffractometer operated at 40 kV and 30 mA. Patternswere recorded from l0 to 70" 20 at l" per minute using

graphite-monochromatized CuKa radiation. BaF, and Siwere used as internal and external standards, respective-ly. Lattice parameters were refined by the method ofleast-squares (Appleman and Evans, 1973). The level ofradiation damage (1//o) was estimated from the total in-tegrated intensity of all diffraction lines in the patternrelative to hlghly crystalline standards. Metamict and al-tered specimens were heated in air or N, for I h at 1000qC and reanalyzed.

Transmission electron microscopy

Powders of selected samples were dispersed in acetoneand deposited on holey-carbon filmed Cu grids. Sampleswere examined using a JEOL 2000FX transmission elec-tron microscope (TEM) operated at 200 kV. High-reso-lution images were taken at a magnification of 410000using axial illumination and objective lens defocus valuesof - 60 to - I 50 nm. A Tracor-Northern TN-5500 energydispersive X-ray analyzer (EDS) was used to obtain anal-ysos ofspecific areas. Spectra were acquired for 300 s realtime (20-300/o dead time) using an effective probe di-ameter of 10 nm. Data reduction was accomplished usingthe Tracor software package SMTF with empirical k fac-tors determined from standards.

Cnrrrnrl FoR REcocNTTroN oF ALTERATToN

Three basic criteria were used to differentiate betweenprimary and secondary alteration: field relationships, mi-croscopic observations, and alteration mechanisms. Thefirst two criteria involve the use of mineral assemblagesas a guide for the identification of hydrothermal alter-ation and weathering. In granitic pegmatites, primary

LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS l 8 r

lNa-l

1lNa.l

i

(hydrothermal) alteration is commonly indicated by re-placement of earlier mineral assemblages by albite andlithium mica (Jahns, 1982; Hawthorne and Cernj', 1982;Cernj'and Burt, 1984). Secondary alteration is indicatedby the breakdown of feldspars and micas to clay minerals,a process often associated with the formation of iron oxy-hydroxides like goethite.

Alteration mechanisms include intracrystalline diffir-sion and fluid transport through preexisting cracks andvoids. Diffusion is an effective mechanism at high tem-peratures and is expected to be the dominant process dur-ing primary alteration. Fluid transport thrcugh cracks andvoids with limited intracrystalline diffirsion is probablythe dominant mechanism operative at the relatively lowtemperatures characteristic of secondary alteration. Thetiming of alteration may be estimated where clear cross-cutting relationships with radiation-induced microfrac-turing exist (Lumpkin and Ewing, 1985).

The above criteria were used to classifu alteration in20 specimens (Table l). Typical examples of primary al-teration are specimens from the Harding, Pidlite, andZimbabwe pegmatites. Alteration patterns in these mi-crolite samples are characterized by irregular rim diffir-sion, uniform rim diftrsion, or a patchy interior distri-bution, respectively. Primary alteration appears to predatemost of the radiation-induced microfracturing. The bestexamples of secondary alteration occur in microlite sam-ples from Amelia, Virginia, in which alteration is local-ized along radiation-induced microfractures with limitedintracrystalline diffi.rsion. Individual microlite crystals insample 185 from the Tin Mountain pegmatite, South Da-kota, are heavily microfractured and completely altered.Alteration was classified as secondary (see Table l) onthe basis of small crystal size, density of microfracturing,and proximity to a l-cm layer of kaolinite in the quartz-spodumene-mica core of the pegmatite (Spilde andShearer, 1987).

Investigation of the samples listed in Table I by opticalmicroscopy, X-ray diffraction, and TEM shows that mosthave received extensive radiation damage. In many sam-

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

lMn"l

1

--------+ [Ca]l

ples the structure has been rendered completely aperiodic(metamict) as the result of cumulative a-decay events(Lumpkin and Ewing, 1988). This study has failed to re-veal conclusive evidence for natural recrystallization ofchemically altered, metamict microlite. Recrystallizationeffects appear to be more common in radiation-damagedsilicate minerals and have been observed, for example,in altered thorite group minerals from the Harding peg-matite (Lumpkin and Chakoumakos, 1988).

T^c.NrAl-uru oxrDE MTNERAL REAcrroNs

Based on our observations of oxide mineral associa-tions in the granitic pegmatites of northern New Mexicoand Amelia, Virginia, together with those of previous in-vestigators (e.g., von Knorring and Fadipe, l98l; Foord,1982; Cerni and Ercit, 1985, 1989; Ercit, 1986), we pre-sent here a summary of some possible reactions betweenmicrolite and other tantalum oxide minerals. Results areconfined to idealized phase relations in the system Na-Ca-Mn-Ta-O-H. This system is represented by the com-mon association of microlite, manganotantalite, fersmite,and rynersonite-vigezzite. Exotic associations reviewedby Cern!'and Ercit (1985, 1989) include tantite (TarOr),calciotantite (CaTaoO,,), and natrotantite (NarTaoO,,).

In Figure I we have constructed schematic phase re-lations as a function of the activities of Na*, Ca2t, andMn2* based on idealized compositions, including Na-TaO, and CarTarOr. For simplicity, solid solution amongmicrolite (NaCaTarOu r), rynersonite (CaTarOu), andCarTarO, has been omitted. Our aim is to illustrate qual-itatively how changes in fluid composition affect tanta-lum oxide mineral assemblages. Simple mineral reactionswere used (Table 2), guided by known mineral associa-tions or replacement features, to arrive at the topologiesshown in Figure l. For example, Figure la shows howthe assemblages microlite * calciotantite + tantite, mi-crolite + natrotantite. microlite * calciotantite, and mi-crolite + rynersonite constrain the topology of the dia-gram.

The relations shown in Figures la-lc were combined

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

182

[Mn2+]I

tl,laiFig.2. Three-dimensional activity relations derived from the

sections shown in Figure l. Hypothetical reaction path A-B-C-Ddepicts the interaction of microlite !\dth a fluid characterized byincreasing Ca2* and Mn2+ and decreasing Na* activities. Abbre-viations: Ct : calciotantite, Mic : microlite, Mnt : mangano-tantalite, Nt: natrotantite, Ryn: rynersonite, Tn: tantite, I: NaTaOr, 2: CarTarOr.

to provide a three-dimensional view of mineral compat-ibility in the Na-Ca-Mn-Ta-O-H system. Figure 2 indi-cates that microlite and manganotantalite may coexistover a range of fluid compositions characterized by mod-erate to high a""-, ecaz*, ?trd aea,z-. The actual stabilityfield of microlite may be extended by incorporation ofCaTarOu and CarTarOT components in solid solution (cf.Lumpkin et al., 1986).

A hypothetical example of the breakdown of microliteis shown in Figure 2. Microlite initially in equilibriumwith a fluid at point A is exposed to a fluid whose com-position is characterized by increasing as^r* and cr". and

TlaLE 2. Mineral reactions in the system Na-Ca-Mn-Ta-O-H

1. 0.5HrO + Ca2+ + 2NaTaO3: NaCaTa"Ouu + Na* + H+2. 2H* + 4NaTaOo: NarTaoO,, + 2Na* + HrO3. 2H2O + 2Ca2* + NaJa4O1, :2NaCaTarO6s + 4H*4.2H* + NarTa4O11 :2Taoos + 2Na* + HrO5. 'l.sHro + Na* + Ca2* + Ta2Os: NaoaTazOos + 3H*6. HrO + Ca2* + 2'fa2os: CaTa*O,i + 2Ht7. 2H2O + 2Na* + Ca2* + CaTa4O11 : 2NaCaT&O65 + 4H*8. HrO + Ca'z* + CaTa4Or, : 2CaTa2O6 + 2H*9. 0.5HrO + Na+ + CaTa2O6: NaoaTazOos * H*

10, HrO + Ca2+ + CaTarO" : Ca"Ta"O, + 2H*11. H* + Na+ + CarTarO,: NaCaTarO"u + Car+ + 0.5HrO12. Mn'?* + 2NaTaO3: MnTarO6 + 2Na*13. HrO + 2Mn2* + NaJaqoli :2MnTarO" + 2Na* + 2H*14. H,O + Mn2+ + TarO.: MnTa"Ou * 2H*15. 2H* + Mn'?* + CaJarOr: MnTaoO" + 2Ca2' + H2O16. Mn'z* + CaTarO6: MnTa2O6 + Ca,*17. H2O + 2Mn2* + CaT&O, : 2MnTaoO" + Ca* + 2H*18. 0.5HrO + Na+ + Caz* + MnTa2O6: NaCaTarO". + Mn2+ + H+

Note: NaCaTa,O6j : microlite, MnTarO6 : manganotantalite, CaTa.Oli: calciotantite, CaTa.g": rynersonite or fersmite, NarTaoO,r : natrotan-tite, Ta2Os : tantite.

LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

- [Ca2+]

A2* AT

Fig. 3. Triangular plot of divalent A-site cations (mainly Ca),monovalent A-site cations (mainly Na), and A-site vacancies inunaltered and altered microtte samples.

by decreasing aNur. At point B, rynersonite begins to formby reaction of microlite with the fluid (Table 2, Reaction9). Manganotantalite begins to crystallize at point C byanother reaction (Table 2, Reaction l8). These threephases coexist and finally equilibrate with the fluid atpoint D. For d*"* values below that of point D, microlitewill be completely replaced by rynersonite + mangano-tantalite.

Crrnvrrcll EFFECTS oF ALTERATToN

Primary alteration

Electron microprobe analyses of microlite samples fromthe Harding pegmatite (Tables 3 and 4r) demonstrate thatthe major chemical effects of primary alteration are de-creased Na and increased Ca, Fe, Mn, and O. The amountsof IJ, F, and inferred HrO (estimated by difference) tendto remain relatively constant, although lower F and high-er inferred HrO contents were noted in a few samples.Similar results were found in sample 324 from the Pidlitepegmatite, Mora County, New Mexico, and in sample327 from the Fungwe district, Zimbabwe. Structural for-mulas of sample 324 indrcate that Ca, Mn, Fe, and Oincreased and that Na, AE, and Yfl decreased as a resultof alteration. For sample 327, structural formulas clearlyindicate strong Ca enrichment and Na depletion duringprimary alteration. Following a typical pattern, theamount of O increased in the altered areas, whereas AE

and YE decreased. The Mn content and inferred HrO re-mained constant, but Fe decreased during alteration.

' To receive a copy of Table 4, order Document AM-92-488from the Business Oftce, Mineralogical Society of America, I130Seventeenth Street NW, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

TABLE 3. Representative electron microprobe analyses of unaltered and altered areas of six microlite samples

130 130s 202 2O2s 188 188s 231 231s 324 324p 327 327p

183

WO"Nb2o5TarOuTio,SnO,Tho,UO,Alro3Y.o"REE,O3sbro3BirosCaOMnOFeOBaOPboNaroCs2OFSumO = F

Total

0.4310.662.01 . 1 02.010.201.570.040.150.140.000.00

12.60.000.060.000.223.620.002.7

97.44- 1 . 1 396.31

0.28 0.06 0.16 0.1011 .5 9.24 9.2',1 2.0364.5 58.9 61.6 76.31 .10 0.03 0.04 0.042.28 2.4't 2.19 1.750.09 0.00 0.00 0.001.28 9.28 8.74 2.220.06 0.00 0.00 0.080.20 0.06 0.07 0.030.25 0.26 0.11 0.300.03 0.08 0.14 0.000.00 0.04 0.08 0.005.83 10.0 8.14 11.20. ' t7 0.20 0.00 0.110.58 0.52 0.13 0.030.00 0.00 0.00 0.000.19 0.20 0.40 0.01o.20 4.08 0.13 4.080.00 0.00 0.00 0.000 j2 2 .1 1 .6 2 .4

88.66 97.46 92.74 100.68-0.05 -0.88 -0.67 -1.0188.61 96.58 92.07 99.67

0.31 0.31 0.51 0j22.05 3.82 3.91 14.3

72.7 71.1 71.6 61.90 .14 0 j7 0 .16 0 .112.28 0.46 0.76 0.120.00 0.01 0.00 0.123.20 3.85 4.24 3.250.20 0.00 0.00 0.090.18 0.07 0.09 0.090.80 0.10 0.20 0.140.06 0.46 0.30 0.060.00 0.01 0.00 0.040.01 10.9 8.57 12.40.00 0.30 0.47 0.100.83 0.56 0.25 0.004.00 0.05 0.05 0.00o.21 0.13 0.23 0.290.00 3.82 2.72 2.92o.12 0.00 0.01 0.000.01 2.6 1.2 1.5

87.10 98.72 95.27 97.55-0.00 -1.09 -0.50 -0.6387.10 97.63 94.77 96.92

0.0020.0981.8170.0090.0510.0230.0000.0020.0270.0100.0200.0080.0000.0020.0010.0070.1120.0390.0020.2885.2150.0115.226

0.007 0.012 0.0030.161 0.162 0.5481.803 1.787 1.4290.012 0.01 1 0.0070.017 0.028 0.0040.000 0.000 0.0090.000 0.000 0.0000.000 0.000 0.0020.080 0.087 0.0610.003 0.004 0.0040.003 0.007 0.0040.018 0.01 1 0.0020.000 0.000 0.0011.089 0.843 1.1280.024 0.037 0.0070.044 0.019 0.0000.002 0.002 0.0000.003 0.006 0.0070.690 0.484 0.4801 .956 1.499 1.6976.289 6.170 6.3180.746 0.343 0.3897.035 6.513 6.707

0.00 0.04 0.0014.4 6.15 5.2060.3 76.5 76.10.10 1.35 1.060.16 0.26 0.280.15 0.00 0.003.29 0.00 0.000.08 0.01 0.010.09 0.05 0.000.17 0 .10 0 .14oj2 0.07 0.130.08 0.10 0.11

14.8 11.2 16.00.49 0.15 0.180.49 012 0.020.00 0.03 0.000.16 0.00 0.011.29 1.82 0.590.00 0.00 0.001.2 1 .3 1 .3

97.35 99.47 101.25-0.50 -0.55 -0.5s96.85 98.92 100.70

0.000 0.001 0.0000.562 0.225 0.1961.417 1.683 1.7260.007 0.082 0.0660.006 0.008 0.0090.008 0.001 0.0010.000 0.000 0.0000.003 0.000 0.0000.063 0.000 0.0000.004 0.002 0.0000.005 0.003 0.0040.004 0.002 0.0010.002 0.002 0.0021.370 0.971 1.4300.036 0.010 0.0130.03s 0.008 0.0010.000 0.001 0.0000.004 0.000 0.0000.216 0.285 0.0951.744 1.285 1.5476.532 5.930 6.2880.328 0.343 0.3506.860 6.273 6.6i|8

Stluctural formulas based on > B: 2.00

wNbTaTiSnAIFe3*ThU

REESbBiCaMnFEP-BaPbNa> AoF>(x + Y)

0.010 0.006 0.002 0.004 0.0020.409 0.422 0.391 0.381 0.0811 .439 't.425 1 .499 1 .533 1 .8430.071 0.067 0.002 0.003 0.0030.068 0.074 0.089 0.080 0.0620.004 0.006 0.000 0.000 0.0080.000 0.000 0.017 0.000 0.0000.004 0.002 0.000 0.000 0.0000.030 0.023 0.193 0.178 0.0440.007 0.009 0.003 0.003 0.0010.004 0.007 0.009 0.004 0.0100.000 0.001 0.003 0.00s 0.0000.000 0.000 0.001 0.002 0.0001.152 0.507 1 .002 0.798 1 .0660.000 0.012 0.016 0.000 0.0080.004 0.039 0.025 0.010 0.0020.000 0.000 0.000 0.000 0.0000.005 0.004 0.007 0.010 0.0000.599 0.032 0.741 0.023 0.7021.805 0.636 2.000 1 .033 1.8346.111 5.565 6.412 5.940 6.1580.729 0.031 0.624 0.454 0.6696.841 5.596 7.036 6.395 6.827

Note.'A letter at the end of a sample number indicates alteration: s : secondary, p : primary. Mg, K, Sr, and Zr were below detection limits and

arenotreported.Al l Feal locatedtoinensi teexceptforanalyseswith>A>2.dO,forwhichenoughFewasal locatedtotheBsi tetogive>A:2.00.

A triangular plot of A-site cations and vacancies (Fig.3) demonstrates the consistent increase in divalent cat-ions during primary alteration. Monovalent cations (es-sentially Na) usually decrease but in a few cases remainrelatively constant. A-site vacancies tend to decrease orremain relatively constant. Two samples showed minorincreases in the number of A-site vacancies. A similarplot of anions and vacancies (Fig. 4) indicates that Oincreases, F tends to decrease or remain relatively con-stant, and anion vacancies usually decrease. The U con-tents of altered areas remain unchanged in comparisonto unaltered areas (Fig. 5). Apart from Pb, which showsminor decreases in some samples, the divalent cationsCa, Mn, and Fe generally increase during alteration. Av-erage increases in these cations are approximately 0.25Ca, 0.05 Mn, and 0.03 Fe atoms per formula unit (Fig.

6). Major substitutions inferred from these results areAEYfI + ACa"O, ANaYF - ACaYO, and ANa"OH - ACaYO.

The latter substitution is postulated for the cases whereF remains relatively constant and O increases. The sim-ple substitution YF - YOH is also possible but is largelymasked by coupled substitutions.

The substitutions noted above can be related to per-vasive, subsolidus alteration by exchange reactions oftheform:

HrO + Caz' I [CaTarOu]*""

: lCatTarOr]-* + 2H*

HrO + Ca2" * [NaCaTarO.fl-""

: lCarTarOrl-"" * Na* + H* + HF

( le)

(20a)

XalYp

1 8 4 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

0.15

0.10

0.05

.9)

. Y

c)

o-5 FFig. 4. Triangular plot of X- and Y-site anions and vacancies

in unaltered and altered microlite samples. The O content isplotted as O-5 to give the same scale as in Figure 3.

0 .15 o #to trtrn

ETo o t r Bo o t r -' o - u t ro o no o o - -

"-':T go o gt:r'::.".t lTor Ab"o o .

q 0 .10

0.05

0.4 0.8 1.2

Ca (atoms per formula unit)

Fig. 6. Plots of Mn and Fe vs. Ca in unaltered and alteredmicrolite samples.

reaction is favored by relatively high a*2, and pH and bylow ar,* in the fluid phase. Equations 19, 20a, and,20cindicate that temperature and the activities of HrO andHF also play a role in controlling the composition ofmicrolite. In situations where F remains relatively con-stant, Reactions 20b and 20c best account for chemicalchanges attending primary alteration.

Secondary alteration

Examination of Figure 3 shows that alteration causescompositions to move toward the AE-A2* join as Na isremoved at a faster rate than divalent cations. Amongthe divalent cations, Figure 6 shows that Ca is lost,whereas Mn and Fe usually show slight to moderate gains(up to 0. 15 atoms per formula unit). Minor increases inPb also occur in some specimens. With the exception ofsample 188, Ba shows slight increases in only a few ofthe microlite samples. Once Na is completely leached fromthe A site, compositions then move toward the A! vertexof Figure 3 as Ca continues to be removed. In sample080, the compositions plot along a curved trend, indicat-ing a gradual increase in the relative rate of Ca loss withprogressive alteration.

Alteration trends represented by the anions (Fig. a)closely follow those of the cations (Fig. 3). Figure 4 showsthat, initially, F is removed from the Y site, causing com-positions to move toward the (xtr + Y!)-(O-5) join. Asalteration proceeds, O is removed from the X and Y sites,causing compositions to approach the x! * YD vertex of

Ca2t + [NaCaTarO.OH]-""

: lCarTarOrL* * Na* * H+

0.5HrO + Ca2* + [NaCaTarOur]-""

: [CarTarOrl-"" + Na+ + H+.

(20b)

(20c)

Reaction 20 is given in three forms to account for occu-pancy of the Y site by F, O, or OH. The right side of each

Atr

A4* A2'

Fig. 5. Triangular plot of tetravalent A-site cations (mainlyU), divalent A-site cations (mainly Ca), and A-site vacancies inunaltered and altered microlite samples.

. UNALTERED(o PRIMARY

ALTERATTON lo sECONDARYt

oo

oo

o ^9o _oo v _ o t r

o o t r o -O o u

o o o o ^ : . t o t r o f t

o o o ^ 3 trrU Oo I | | r .

a

LUMPKIN AND EWING: PYROCHLORE GROUP MINERAIS 185

Figure 4. Notice in Figure 4 that data for sample 080follow a curved path complementary to the cation trendshown in Figure 3.

Combined results for cations and anions indicate thatSchottky defects are produced by substitutions ofthe formANaYF - 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-tions. Also, structural considerations indicate that onlyminor amounts of OH- will bo tolerated at the Y site inconjuction with large numbers of cation vacancies (Su-bramanian et al., 1983). The major substitutions resultin highly defective, hydrated microlite samples charac-terized by increases of 4 to 13 wto/o HrO. Furthermore,Figure 5 shows that the U content of microlite remainsremarkably constant as a result ofsecondary alteration.

In most samples, vacancies reach maximum values ofAtr : 1.5, YE : 1.0, and xtr : 0.5 per formula unit (Ta-bles 3 and 4). Sample 188 from Minas Gerais, Brazil,consists of crystals of unaltered microlite with thin alter-ation rinds of bariomicrolite in which vacancies reachmaximum values of AE: 1.8, Y! : 1 .0, and xt r :0.8per formula unit. Structural formulas indicate virtuallycomplete leaching of Ca, Na, and F, compensated in partby entry of minor amounts of Ba, Pb, Fe, REEs, Cs, andl0-12 wto/o HrO.

Secondary alteration, or weathering, of microlite canbe approximated by reactions of the form:

3Ht + HrO + [NaCaTarO.Fl-*

: [TarOr'2HrO]-* * Na* * Ca2* * HF (21)

2}I* + l.5HrO * 0.5Ba2* + [NaCaTarOufl_*

: [BaorTarO55.2H2O]-* + Na+ + Ca2* + HF. (22)

The right side of each reaction is favored by relativelylow , low pH, and low ay^4 aqyz+1 and a". in the fuidphase. Additionally, Reaction 22 is favored by relativelyhigJr ar"r. in solution.

U-Pb systematics

Microprobe analyses were used to evaluate the behav-ior of U and Pb in microlite subsequent to crystallization.The Harding suite was selected because a large numberof samples are available, the geologic age is well estab-lished at 1300 m.y. (Aldrich et al., 1958; Brookins et al.,1979), significant metamorphic overprinting is absent, andthe microlite samples have low Th and high U contents(Lumpkin et al., 1986). Figure 7 shows a U-Pb plot in-cluding both unaltered and altered microlite samples fromthe Harding pegmatite. Most of the data fall below the1300-m.y. reference line. Except for specific examples,the magnitude of Pb loss is poorly correlated with alter-ation. The maximum degree of Pb loss approaches 800/oin both unaltered and altered material, suggesting thatlong-term difrrsion is a viable explanation of the results.The high-Pb data points in Figure 7 are from the core ofa microlite grain in sample 269 and represent either Pb

{ '$:

,/3 "o.fi ;

o -^q"po-r9gs.---./.""

- _so1-3b)")--

"@oo

9'" 'q"- ''i .8a: -- -

= v . u 9

o

o u . u +

0.o2

0.1 0.2 0.3 0.4

U (atoms per fonnula unit)

Fig. 7. U-Pb systematics of unaltered and altered microlitesamples.

gain during alteration or common Pb incorporated duringcrystallization.

Data for microlite samples from other approximately1300-m.y.-old pegmatites are generally consistent withthe Harding suite. Microlite crystals from the Pidlite peg-matite exhibit Pb loss of 0-350/o in unaltered cores and6O-75o/o in altered rims. These results are consistent witheither episodic Pb loss during primary alteration or a sub-sequent long-term diftrsion gradient preserved over adistance of 0.5-1.0 mm from core to rim. In comparison,heavily microfractured microlite crystals (samples 202 and,231) from the Quartz Creek district, Colorado, consis-tently show 60-800/o Pb loss in both unaltered and alteredareas. In this case the alteration is classified as secondary(Table l). The U-Pb systematics reported above indicatethat radiation-induced microfracturing plays an impor-tant role in controlling the long-term loss of radiogenicPb in microlite.

DrscussroNChemical effects of alteration

Chemical changes that accompany alteration are sum-marized in Figure 8 using idealized end-members to showmajor element trends. Most unaltered compositions fallnear the NaCaTarO.F-CaTarOu join or just within theNaCaTarOuF-CarTarOr-UlarO. composition field. Fouralteration vectors are indicated by the data in Figures 3and 4: (l) filling of Schottky defects by CaO, (2) NaF 'CaO exchange, (3) formation of Schottky defects by re-moval of NaF, and (4) creation of Schottky defects byremoval of CaO. Altho'gh there are obvious differencesbetween individual samples, primary alteration paths lieconsistently between trends I and 2 (Fig. 8).

tr2Ta2O5tr2

r86 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

Ca2TarOt NaCaTa206F

Fig. 8. Ternary plot surnmarizing the major element trendsfor primary (P) and secondary (S) alteration of microlite. Vectorsl-4 are described in the text.

Secondary alteration follows a distinctly different pathbetween trends 3 and 4, gradually approaching trend 4 asalteration proceeds to completion. In cases of extremesecondary alteration, this path ultimately requires thatsome O be removed from the X site. The occurrence ofpaired vacancies (Schottky defects) is consistent with thegeometry of the pyrochlore structure type, in which YAotetrahedra and AX.Y, distorted cubic sites exist withinlarge cavities of a stable BrXu octahedral framework (Sub-ramanian et al., 1983; Chakoumakos, 1984). These largecavities are interconnected in three dimensions by chan-nels, through which A- and Y-site ions can be removedin combinations that maintain overall charge balance.

Microlite also commonly exhibits slight to moderateincreases in Mn and Fe during primary or secondary al-teration. In the Harding microlite samples, values of 2 A> 2.00 atoms per formula unit require that essentially allof the Fe and part of the Mn incorporated during primaryalteration to be located at the B site as Fe3* and Mn3*(Lumpkin et al., 1986). During secondary alteration, mi-crolite may exhibit slight to moderate increases in the Bacontent. A slight increase in Cs was noted in the bariomi-crolite sample from Brazll.

The exceptional stability of U in the A site results inpart from the coordination geometry, where true leachingis restricted to cations of low formal valence. Removalof a IJa* or IJ6* ion would be diftcult to charge balancein combination with Y-site anions alone. Long-term lossof radiogenic Pb in microlite is supported by the work ofAldrich et al. (1958), who found that microlite samplesfrom the Harding pegmatite give discordant U-Pb ages.The authors investigated two samples, one with 0.7 wto/o

U and 0.I wt0/o Pb and another with 7 .7 wto/o U and l.lwto/o Pb. Both gave 23su'-2o6Pb and2'rsu-2oiPb isotopic agesof 900-1000 m.y., consistent with loss of 20-300/o of theradiogenic Pb.

Conditions of alteration

Members of the microlite subgroup show well-definedprimary and secondary alteration in terms of both tex-tural and chemical features. We suggest that this is relatedto the depth of emplacement of the host rocks. Most gra-nitic pegmatites of the rare-element class are emplaced atdepths of 4-12 km (Jahns, 1982; London, 1984; Cernj',1989; Chakoumakos and Lumpkin, 1990; Norton andRedden, 1990). Becaue of the small size of granitic peg-matites, the cooling rate is rapid in comparison with muchlarger granitic plutons. On the basis of cooling models fora finite slab with dimensions of 2 x 2 x 0.02 km, Cha-koumakos and Lumpkin (1990) estimated that the mag-matic phase of the Harding pegmatite lasted only 100-1000 yr. Emplacement at relatively deep crustal levelsmeans that subsequent hydrothermal activity will usuallyhave ended long before granitic pegmatites reach the sur-face following uplift and erosion.

Primary alteration of microlite occurred during the latemagmatic to hydrothermal stages of pegmatite emplace-ment at P : 2-4 kbar and f : 350-550 "C (cf. Chakou-makos and Lumpkin, 1990). Alteration is influenced bythe presence and evolution ofa dense silicate liquid andsupercritical HrO-COr-NaCl"n fluid rich in volatiles andfluxing elements such as F, Li, Be, B, P, Rb, and Cs (Jahns,1982; London, 1986, 1987; Cerni, 1989). Exchange re-actions between microlite and fluid suggest conditions ofrelatively hrgh pH, high ag^z-, and low 4pur, oft€l accom-panied by elevated ey1^z* urrd Qp"z*. Tllre requirement ofFe3* and Mn3* to occupy the B site in some of the Har-ding microlite samples suggests that relatively hrgh "f",conditions prevailed during alteration. This is supportedby hieh IJ6+/LJ4+ ratios in microlite (Jahns and Ewing,1976) and the incorporation of V5* and Bi3* in thoriteduring primary alteration (Lumpkin and Chakoumakos,1988). Modeling of mineral reactions in the system Na-Ca-Mn-Ta-O-H indicates that if the increases in a*z- and4y"z* proceed far enough, microlite will be replaced byrynersonite (or fersmite) and manganotantalite.

Secondary alteration is characteristic of near-surfaceconditions with Z < 100 'C and P < I kbar. Alterationoccurs in the presence of relatively large volumes of me-teoric HrO and is mainly controlled by radiation-inducedmicrofractures. Exchange reactions between microlite andmeteoric HrO suggest conditions of relatively low 4""*,og22*1 espl and pH, resulting in leaching of Na, Ca, F, andO accompanied by hydration. In special circumstances,cations like Cs, Ba, or Pb may be picked up by meteoricHrO through dissolution of unstable silicate minerals andsubsequently exchanged with microlite during secondaryalteration.

LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS 1 8 7

CoNcr,usroxs

This investigation shows that the microlite subgroup isa useful indicator ofgeochemical processes occurring athydrothermal temperatures during the latter stages ofpegmatite emplacement. Relative changes in fluid com-position can be assessed through the use oftantalum ox-ide mineral associations, replacement reactions, intra-crystalline zoning patterns, and subsequent primaryalteration effects.

Results of this study are also relevant to nuclear wastedisposal using ceramic waste forms and show that pyro-chlore structure types are susceptible to leaching of cat-ions and anions at low temperatures. This effect may beenhanced by prior radiation damage and microfracturing.The next two papers in this series will address alterationeffects in the pyrochlore and betafite subgroups, encom-passing a broader range ofcompositions and geologic en-vironments, including carbonatites and nepheline syenitepegmatltes.

AcxNowlr,ocMENTS

We thank Carl Francis of Harvard University, John White, Jr. of theSmithsonian Institution, and George Harlow of the American Museurnof Natural History for providing some of the samples. Much of this workis based on the collection of microlite samples from the Harding peg-matite at the University of New Mexico, for which we acknowledge theeffons of Art Montgomery, B.C. Chakournakos, and all those involved inpreserving this remarkable locality. We thank E.R. Vance, T.J. White,and an anonymous referee for providing critical reviews of this manu-script. Electron microprobe analyses and transmission electron micros-copy were completed in the Electron Microbeam Analysis Facility in theDepartment of Geology and Institute of Meteoritics at the University ofNew Mexico, supported in part by NSF, NASA, DOE-BES, and the Stateof New Mexico. This work was supported by the Department of Energy,Office ofBasic Energy Sciences, under grant DE-FG04-84ER45099.

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1 8 8 LUMPKIN AND EWING: PYROCHLORE GROUP MINERALS

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ERRATUM

Manganese, ferric iron, and the equilibrium between garnet and biotite, by M. L. Williams andJ. A. Grambling (v. 75, p. 886-908). The final term in Equations 12 and l3 should be 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 thecalibration experiments. Note that, for clarity, the final Serm has been maintained as in Equa-tion 12. Also, in Table 10, Wff"*, should be changed to 7285,20775, and 10031 for Models l,2, and 3, respectively.