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

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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 man- ganotantalite 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 radio- genic 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 mi- crofractures;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, respective- ly. 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.
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