Mineralogy and Radiation Effects of Microlite from the Harding Pegmatite' Taos County, New Mexico

American Mineralogist, Volume 71, pages 569-588, 1986

Mineralogy and radiation effects of microlite from the Harding pegmatite' Taos County, New Mexico

G. R. LurvrpxrN.B. C. CnlxouMAKos" aNo R. C. EwINc Deparfinent of Geology, University of New Mexico, Albuquerque, New Mexico 87131

Ansrucr Microlite, rangrngfrom crystalline to metamict, is a principal accessoryin several lithologic units of the Harding pegmatite, Taos County, New Mexico. From the sequenceof lithologic units within the pegmatite,crystallization of microlite from the pegmatitemagma is inferred to have begun relatively late, after the formation of the beryl and quartz zones, and continued throughout the formation of the core zones and subsolidus replacement units. Over 200 chemical analysesof microlite determined by electron microprobe are reported, and they are consistent with the acceptedstructural formula, A2-^B2X6Y.-,' pHrO, where principally A : Ca, Na, IJ, Mn; B : Ta, Nb, Ti; X : O; and Y : F, OH, O. General featuresof microlite crystal chemistry identified include (l) a positive correlation between A-site vacanciesand the maximum Y-site vacancies,(2) a positive correlation betweenNa and F, and (3) a negativecorrelation betweenU and F. The latter is consistent with the interpretation that U at the A site in the pyrochlore structure is analogousto the uranyl group, UO3* , requiring O in place of F at the Y site. Microlite compositions from four of five lithologic units examinedare chemically distinct in terms of linear combinations of the variablesIJ, Fe, Ti, Bi, Ca, Ce, Pb, F, Mn, Ba, Sb, Th, Ta, and Na. The cleavelanditeunit microlites, the exception, not surprisingly have chemistries like those of the quartz- lath spodumenezone, which the cleavelanditeunit in part hasreplaced. The earliest-formed microlites from the quartz-lath spodumenezone have the highest Ta, Na, and F and are low in U and Mn. In contrast, microlites from the later microcline-spodumenezone and the replacement units are generally higher in Ti, Mn, and U and lower in F and Na. Chemical changesascribed to primary hydrothermal alteration from residual pegmatitic fluids include increasesin Ca, Mn, and Ti and an overall loss of A-site cations. Alteration effectsand chemical zoning within crystals, analyzedin terms of simple end members, ACaYO identifies the following principal substitution schemes:BNb- BTa,BTa- BNb, + ANaYF ANaYFand AEYE - aNaYF for core-to-rim zoning in crystals and ACaYO- and ACaYO- AEY! for primary alteration. Secondary (weathering) alteration results in decreasesin Na, Ca, and F and increasesin HrO. The effectsof alpha-recoildamage due to the decayof constituent U have beenexamined. Becauseof the wide variation in U content (0.1-10 wto/oUOr), the microlites exhibit the full rangeof structural periodicity from completely crystalline (< I dpa), partially crystalline (up to lf20 dpa) to completely X-ray and electron diffraction amorphous (>20 dpa). Based on X-ray and electron ditrraction analysis, the progressivestructural modification of microlite udth increasingalpha dose involves (1) formation of isolated defect aggregates (i.e., individual alpha-recoil tracks) up through dosesof l0'4 alphas/mg with no detectable effect on the materials's ability to diffract X-rays or electrons,(2) continued damageand overlap of thesedefect aggregatesyielding coexistingregions of amorphous and crystalline domains at l0rs to 1016alphas/mg, and (3) complete amorphization at dosesgreater than 10'7alphas/mg.

IxrnonucrroN tions have <10 molo/oTi and <25 molo/oNb (Lumpkin Minerals of the pyrochlore group (Fd3m, Z: 8) have and Ewing, 1985).Except for a single report of microlite the generalformula Ar-. B rXuY,-,'pHrO, whereA : Na, occurring in a granite (Sitnin and Bykova, 1962), most Ca, IJ, Pb, Sr, Th, REE, Bi, Sn2+,Ba, Mn, Fe2+;B: Ta, microlites are found in granitic pegmatites of the rare- Nb, Ti, h, Fe3+,Sna+, W; X : O; and Y : O, OH, F. element class(Grnj', 1982; Cernf and Burt, 1984). The Hogarth (1977) defined the microlite subgroup to have typical accessoryminerals associatedwith microlite in- Ta > Nb and Nb + Ta > 2Ti. Most natural composi- clude zircon, stibiotantalite, bismutotantalite, columbite- 0003-o04x/86/0304-0569$02.00 569 570 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

tantalite, rynersonite, wodginite, and simpsonite (Von Ewing, I 976). Bedding (So)and transposition layering (So/ Knorring and Fadipe, l98l; Foord, 1982). S,) trend N8"E in Vadito rocks north of the pegmatite. Microlite was first described from the Harding peg- The primary foliation is slaty cleavageor schistosity (Sr) matite by Hirschi (1931), and during the period 1942- which trends Nl0'W and intersectsbedding at a low angle 1947 over I I tons were produced by hand-mining meth- (Holcombe and Callender, 1982).The pegmatitetruncates ods (Jahns and Ewing, 1976, 1977). The microlite con- So/S,and S, and must postdate F, and F, folding. centratesaveraged 68 wto/oTarO, and 7 wto/oNbrOr. The Severaltypes ofgranitic rocks are present in the area. first complete chemical analysescorrelated the changefrom On the basis of field evidence and limited radiometric light to dark color in five microlites with decreasingTal dating, Long (1974) considered the Harding and other Nb ratio and increasing OH/F ratio and the U, Ti, and pegmatitesto be late in the history ofgranitic magmatism. Mn content (Jahns and Ewing, 1976). Partial electron- He suggesteda separatemagmatic event for the pegmatites microprobe analyses(Suchomel, 1976) confirmed these 100 m.y. later than the youngest(unfoliated) granite and trends for Ta,/Nb, Ti, and Mn. Suchomeldid not analyze as much as 300-400 m.y. later than the oldest (foliated) for F and U, so the analysesdo not conform to the py- granitic rocks. Aldrich et al. (1958) determined K-Ar and rochlore structural formula (Chakoumakos, I 978). Rb-Sr agesof 1260 m.y. for lepidolite from the Harding Microlites are stable from ca. 500'C and 5 kbar down mine. RecentRb-Sr agesof 1336 + 73 for the "spotted to ambient, near-surfaceconditions and can accommo- rock" core unit,1264 + 128 for rose muscovite, and 1246 date a variety of rare elements as a function T, P, and + 40 m.y. for lepidolite were determined by Register(in melVfluid composition. Microlites from African localities Brookins el al., 1979). were broadly classifiedas primary, secondary,and uran- iferous types by Von Knorring and Fadipe (1981). They INrnnNx, zoNING AND REpLACEMENTuNrrs suggestedthat with time, microlites generally show de- The main pegmatite dike is characterizedby an asym- creasingamounts of Ta, Ca, and F with increasing con- metric sequenceof lithologic units from top to bottom. centrationsof Ti, U, Sb, Bi, Pb, and HrO. However, little These consist of six primary zones and two replacement is known about chemical variations in microlites within units. The following description has beencondensed from individual pegmatites(cf., Mih6lik, 1967; Foord,, 1976). Jahnsand Ewing (1976, 1977),Suchomel (1976), Chak- The abundanceof microlite in severallithologic units of oumakos(1978), and Norton (1983). the Harding pegmatite provides an excellent opportunity The hanging-wall portion of the pegmatite consists of to examine thesechemical trends. three primary zones.The continuous beryl zone includes Prior to this work, Suchomel(1976) noted higherNbrO, a thin, fine-grainedborder rind and coarser-grainedwall and lower TarO, contents in microlites from the micro- zonecomposed ofquartz, albite, and muscovite.Common cline-spodumenezone ("spotted rock") relative to those accessoryminerals are beryl, microcline, apatite, and co- from other units. Chakoumakos( I 978) made an extensive lumbite-tantalite. Below this is the quartz zone, a fairly petrographic and X-ray diftaction study of 30 located continuous unit of massive quartz with accessorymus- samples,three high-gradeore samples,and two analyzed, covite, microcline, and albite. The quartz zone is under- samples.No relationship between degreeof crystallinity lain by a spectacularquartz-lath spodumenezone char and lithologic unit was noted, implying that U does not acleized by spodumenecrystals up Io 2 m in length (see vary systematicallywithin the pegmatite. Most of Chak- Fig. 6, Jahns and Ewing, 1976). Microlite, beryl, apatite, oumakos' samples have been used in this study, along microcline, and lepidolite arecommon accassoryminerals with additional specimensdonated by Arthur Montgom- near the baseofthe discontinuous zone. The core ofthe ery. The purpose of this paper is threefold: (l) to discuss pegmatite in the western thick end is dominated by the generalfeatures of microlite crystal chemistry, (2) to ex- microcline-spodumenezone, otherwise known as "spotted amine relationships between composition and lithologic rock." Microcline, spodumene,and quartz are the major unit using statisticalanalysis, and (3) to describeradiation minerals with minor amounts of apatite, albite, microlite, effectsin microlites. and columbite-tantalite. Fine-grained lepidolite and Li- bearing muscovite commonly replace microcline and L,oc.c.rroN AND GENERALGEoLocy spodumene. Locally, the unit grades into nearly pure The Harding pegmatite is located in the Picuris Range massesof lepidolite. Two primary zones occur beneath l0 km eastofDixon and 30 km southwestofTaos in sec. the microcline-spodumenezone in the footwall portion 29, T23N, Rl lE. The pegmatite lies within Precambrian of the pegmatite.The perthite zoneis a discontinuousunit rocks of the Vadito Group. The main dike is about 370 of blocky, perthitic microcline with minor quartz and al- m Iong and up to 80 m wide in outcrop. The body is bite. Beneath this is the aplite zone, composed of fine- roughly tabular in shapewith a maximum thicknessof 25 grained albite and quartz with accessory beryl, apatite, m and an averageplunge of I 0"S(Jahns and Ewing, I 976, and columbite-tantalite. 1977).Exposures occur primarily along the boundary be- The primary zones have been replaced to varying de- tween amphibolite (hangingwall) on the south and pelitic greesby two units. The cleavelanditeunit is composedof schists (footwall) on the north (Long, 1974; Jahns and platy albite, qluarlz,and minor muscovite. It occurs pri- LUMPKIN ET AL,: MICROLITEFROM HARDING PEGMATITE 571 t\

l._., .1 at l'..F' i2 ,.' ,| .1 A! ,F .8- w4 I

Fig. 1. Photomicrographsof microlite from the quartz-lath spodumenezone (a, b, c), cleavelanditereplacement unit (d)' and (CT) microcline-spodumenezone ("spotted rock") (e, 0. Scalebars : 0.3 mm (a) Euhedral microlite (270) and columbite-tantalite (c) in cleavelandite (Ab) in quartz (a). 0) A cluster of subhedral microlite crystals (271) typical of high-grade ore. Microlite pr.udo111orpi1icafter spodumene.Crystals are zoned (arrows)toward the quartz interface.(d) An altered (A) microlite (P05.1) from a cleavelanditemass in the beryl zone. Matrix minerals are cleavelanditeand lepidolite (Lp). (e) Anhedral microlite (Pl5.1) showing hydrothermal alteration along grain boundaries and fractures. Matrix mineral is microcline (Mc). (f) Altered and microfractured microlite (P17.1) in a microcline matrix. Unaltered areasof some grains are Pb-rich (seetext). 572 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

and associatedwith minor lepidolite, muscovite, and quartz within the cleavelanditeunit (Fig. ld). Large eu- hedralcrystals (up to I 5 mm) occur wherethe unit replaces ----r-r Q the beryl zone. They are often zoned from yellow cores tcm to dark brown rims. Other crystals display an unusual mottled yellow/brown exterior coloration. In the core of '! y'r{i1l pegmatite, 'itor.'/// the the cleavelanditeunit mainly replacesthe quartz-lath spodumene zone. Microlite is found in this unit aseuhedral to subhedralcrystals (up to 3 mm) ranging ker in color from pale yellow to black. Many crystals show this color change in core-to-rim zoning. Submillimeter Fig.2. Schematiccross sections of cleavelandite(C) replace- yellow and brown microlites are associatedwith cleave- mentsalong possible pre-existing spodumene (S) laths. The ma- landite pseudomorphsafter spodumene(Fig. 2). Crystals trix is pale smokyquartz (Q), which is dark smokycolor (darker tend to be aligned near the edgesof pseudomorphswhere Q) adjacentto microlite(M) crystalssegregated along the base quartz of each"pseudomorph." The photomicrographin Fig. lc is an adjacent is darker in color. Color zoning is not enlargeddetail of the microlite.Sketches are from handsamples prevalent, but a few grains show nonconcentric zoning Pl 1.3(Chakoumakos, 1978). involving a light-to-dark color changetoward the cleavelandite-quartz interface (Fig. lc). Microlite also occurs within a lepidolite-cleavelandite marily in the core of the pegmatite, but also is found as subunit in easternextensions of the pegmatitedike (Chak- scatteredmasses in the beryl zone (Chakoumakos, 1978). oumakos, 1978).The microlite is distributed as brown to The rose mu"scovitHleavelandite unit mainly replaced black, 0.5 to 5.0 mm, euhedral to subhedralcrystals in a parts of the quartz-lath spodumene zone (Heinrich and matrix of lepidolite, bladed albite, and quartz. Accessory Levinson, 1953). columbite-tantalite completesthe association. Mrcnor,rrn pETRocRApHy The major occurrenceof microlite is within the quartz- ExprnrvrnNTAl PRoCEDURES Iath spodumene and microcline-spodumene ("spotted X-ray diffraction studies rock") zones,It is rare in the quartz zone where it occurs performed as honey-brown, microcrystalline aggregatesalong grain A variety of analyses was using either a Philips or a Scintagdiffractometer and CuKa radiation: (1) A suite of single boundaries of accessoryapatite (Suchomel, 1976). Mi- crystalswas hand picked from a microlite concentrate,mounted crolite occurs with columbite-tantalite quartz-lath in the on a Gandolfi attachment, and X-rayed in a 114.6-mm Debye- spodumenezone as euhedral to subhedralcrystals (0.1-5 Scherrercamera. Films were correctedfor shrinkage,and lattice mm) poikilitically enclosedby quartz (Fig. I a). The quartz constants were refined by least-squaresanalysis. The crystals were fills intersticesbetween interlocking spodumenelaths. Mi- then mounted and polished for electron-microprobeanalysis. (2) crolite crystals tend to form dense aggregates(Fig. lb) Powder-diftactometer traces were recorded for larger specimens which provided most of the high-gradeTa-Nb ore. Colors with the Philips unit. Patterns were calibrated with BaFr(a : range from pink, colorless, and pale green to shadesof 0.6198 nm) as an external standard, and lattice constantswere yellow and brown. Somecrystals are zoned with light cores refined by least-squaresanalysis. (3) Powder-ditrraction patterns for microlites and dark rims. Lepidolite pods occurring near the baseof five ranging from crystalline to metamict were collected with a Scintagdifractometer (seeFig. l2), and lattice the unit contain abundant, euhedral to subhedral,yellow constantswere refined using the program LATCoN(Scintag soft- microlite crystals (Montgomery, 1950). ware). In addition, samplesannealed during the courseof TGA In the microcline-spodumenezone, microlite is found measurementswere X-rayed and found to have recrystallizedto as disseminated,euhedral to anhedral crystalsenclosed in microlite plus minor CaTarOuand CaTaoO,,. coarse-grainedmicrocline (Figs. le, lf). The color is dark brown to black. Mostgrains are 1.0 to 5.0 mm in size. Transmissionelectron microscopy Associatedminerals include columbite-tantalite, hafnian Three microlite samples(highly crystalline,partially metamict, zircon, and lepidolite. The Ta-Nb minerals average0.150/o and fully metamict) were dispersed onto holey carbon grids in in the microcline-spodumenezone, enough to consider methanol. The sampleswere examined with a rrol 2000 rx rBvr this as low-gradeore (Jahnsand Ewing, 1976, 1977).The operatedat an acceleratingpotential of200 keV. Standardbright- most prominent alteration effectsare observed in these field (BD, selected-areadiffiaction (SAD), and high-resolution microlites. Reddish, hydrothermal (primary) alteration (HRTEM) techniqueswere employed. SAD patterns were cali- commonly occurs along grain margins and may or may brated using a gold film under the sameinstrumental conditions. not showfracture control. Brownish (secondary)alteration A double-tilting stagewas used to bring the [1 I l] zone axis into the diffracting condition such that HRTEM images show pri- is confined to <2o-pm-wide regions along fractures and marily 0.6-nm (l 1l) and 0.3-nm (222) finges.Image resolution is believed to be the result of weathering (Lumpkin and and magnificationwere checkedusing a graphitizedcarbon (0.34 Ewing, 1985;Ewing, 1975;Van Wambeke, 1970). nm) standard. HRTEM images were taken at magnifications of Replacementunits also contain microlite as an acces- 410000 or 500000. The samplesappeared to be stable for at sory phase. The microlites are enclosedin bladed albite least l0 min under normal operating conditions. LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE 573

et al., I 983; Chakoumakos' Thermogravimehic analysis Aleshin and Roy, I 962; Subramanian 19841.We calculatetwo end-member formulas that bracket the a DuPont Water contents were estimated for a few samplesusing rangeof possibleanion chemistries:one that maximizes OH and 20 mg were 951 TGA and 990 Recorder.Samples weighing 10 to anoiher that includesall water as molecular HrO and maximizes was heated in air to 1000"C at a rate of 10'/min. The system Y-site vacancies(Table 2). For the analysesin which HrO has op- calibrated with a calcium oxalate standard using the same not be€n estimated by TGA, a single formula is calculatedand by reweighing erating parameters.The calibration was checked Y-site vacanciesare allocated when F and O are less than 7'0 percentage each sample after cooling to room temperature. The (Tables I and 3). Assumptions involved in calculating structural within 50/oof of weight loss calculatedin this way was usually iormulas mainly affect O, OH, and HrO and Y-site vacancies' that measured. The reader should be aware ofthese uncertainties in the following discussion. Elecbon microprobe analysis have been obtained using an au- Over 200 microlite analyses Mrcnor.rrn cHEMrsrRY tomated r/Eor733operated at an acceleratingvoltage of 15 keV and a samplecurrent of 20 nA. Standardsinclude natural samples General relationshiPs of microlite (F, Na, Ca, Nb, Ta), manganotantalite(Mn), stibi- effects,the major-elementcontents otantalite (Sb, Bi), benitoite (Ti, Ba), olivine (Fe), cerussite(Pb), Excludingalteration microlites pollucite (Cs), and cassiterite(Sn). Synthetic crystalsof CaWOn' are not unusual when compared to analyzed Von YPO., CePOo,ThSiO., and UO, were used for W, Y, Ce, Th, from other localities (Bonshtedt-Kupletskaya,1966; and U. Each element was counted until a relative standard de' viation of 1.00/owas reached,up to a maximum counting time of 30 s. To minimize volatilization induced by electron-beam heating,all analyseswere performed with a beam diameter of l0 pm. Data were corrected for drift, deadtime, absorption, fluo- rescence,and atomic number efects using a theoretical (ZAF) procedurebased on the program MAGICIV (Colby, 1968)' The completeset ofanalyses is presentedin Table I .1Average analyses not determined (Tables I and 3)' Major (including HrO) are given in Table 2, and representativeanalyses yseswhere HrO is range of known microlite com- are grouped accordingto lithologic unit in Table 3. elementsnearly cover the positions. appear to be enriched or de- Structural formulas Certain minor elements All microlite analyses were converted to a structural formula basedon a total of2.0 B-site cations(cf., Borodin and Nazarenko, 1957;Van Wambeke,1970). This allowsan estimateof the A- site vacanciesfor eachanalysis. The major problem in assigning a structural formula is determination of the oxidation state of Fe, which can enter either the A site (Fe'?+)or the B site (Fe3*). Chemicalanalyses by C. O. Ingamells(in Jahnsand Ewitg,1976) reported Fe as FeO and/or FerO, in no systematicfashion; however, for all five analyses,FeO averages0.07 and FerO, averages 0.02 wt0/0.Consequently, we report all Fe as FeO and allocate it to the A site, which minimizes A-site vacancies.This effectwill be slight since the Fe content is very low in most of the probe analyses.On the basis of chemical analysesin Jahns and Ewing (1976),all U is calculatedas UOr, which increasesthe calculated O content in the anion group and minimizes Y-site vacancies. The amount of O, OH, and F can be calculatedin conjunction with HrO contents estimated by TGA. A major difrculty is the determination of whether the water is present as OH or molecular HrO. Our TGA data aloneare not sufrcient in this respect.Water releasedat ca. 150'C is most likely molecular HrO, but much of the water was releasedgradually up to 1000'tCand cannot be distinguishedas OH- or molecularHrO. Borodin and Nazarenko WOr. (1957)assumed a total ofseven anionsand calculatedthe amount of OH necessaryto achieve charge balance. Excesswater was Stoichiomehy allocated as HrO. In general, this procedure is not strictly valid becauseof anion vacanciesat the Y site [Pyatenko, I 959 (l 960); B-site cations.Pentavalent Ta and Nb are the dominant cations occupying the octahedral B site (Fig. 3). The amount of Ta rangesfrom 1.35to l'85 and Nb rangesfrom 0.10 to 0.45 atoms per formula unit. Substitution of Ti is lim- I AM-86-297 To receive a copy of Table I, order Document ited to a maximum of 0.25 atoms per formula unit. The from the BusinessOffice, Mineralogical Societyof America, 1625 W Sn rarely attain 0.01 and 0.003 atoms I Street,N.W., Suite 414, Washington,D.C. 20006. Pleaseremit contents of and $5.00 in advancefor the microfiche. per formula unit, respectively. 574 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

Table 2' Averageelectron-microprobe analyses for Harding pegmatite microlites v/ith HrO contents estimatedby TGA (formulas basedon >B : 2.00 include two possibleanion distributions; seetext)

I48 I49 150 153 I54 288 wo" o,26 0.08 0.03 o.23 0.06 0.07 0.20 TaÔ- 75.r 74.1 75.6 74.1 70.2 6r.7 7r.2 Nbô- z) 5.68 4.36 5.66 4.77 7.61 5.28 T102 o.o2 0. 03 0.20 0.05 0.40 0.90 o.52 Sn02 0.0I 0. 00 0.00 0.0t 0, 00 0.00 0.00 Tho2 0.03 0. 04 0.00 0 .04 0. 00 0.10 0.05 uo. 0, 00 2.20 0.69 I. 84 9.07 10.0 3.07 Y 2o3 0. 00 0.07 0,10 0.02 0.00 0.08 0.07 c.203 0. 14 0. 09 0.12 0. I8 0.13 0.20 0.17 sb203 0. 09 0.11 0,09 0. 08 0.05 0.08 0.14 Bi203 0. 04 0. 00 0.05 0.10 0. l7 0.02 0.00 Mn0 0,03 0.19 0.06 0. 03 0. 09 0.54 0.16 FeO 0.03 0. 00 0.00 0, 04 0.06 0.04 0.12 CaO I0.4 I1.0 I1,6 9.65 8.62 10,5 Il.9 Ba0 0. 00 0. 00 0,03 0. 00 0. 00 0.00 Pb0 0. 04 0. 03 0.02 0,10 1,85 o,29 0.06 Na2O 5. r0 4.25 3.70 4.77 3.1,7 3.09 csro 0. 00 0.04 0.00 0. 00 0.02 0.00 0.00 F 3.4r 2.04 2.44 2.51 1.25 r ,43 1.65 Hzo '1.2 0.1 1.1 o.7 t.2 4,0 3.2 sttl4 r00.48 r00. 34 r00.65 t00.6I tot.26 r00.73 r00.98 -1 .43 -0.86 -r.02 -r.05 -0.50 -0. TOTAL 69 99.05 99.48 99.63 99.56 100.74 i00.13 100.29 t^' 0 .006 0.002 0,001 0.005 0.00I 0.002 0.005 1.770 1.820 r.782 t.167 r.77| 1.607 1.739 Nb o.223 0.t77 0.204 0.225 0.200 o.328 o.2t4 Ti 0. 001 0. 002 0.0I3 0.003 0.028 0.063 0.042 Sn c. 000 0. 000 0.000 0. 000 0.000 0.000 0.000 Th 0.001 0,001 0.000 0.00I 0.000 0.002 0.00t U 0. 000 0.041 0.013 0. 035 0.177 0.213 0.058 Y 0. 000 0. 003 0.005 0.001 0.000 0.004 0. 003 Ce 0. 004 0, 003 0.004 0. 006 0.004 0.007 0.006 sb 0.003 0. 004 0.003 0. 003 0.002 0.003 0.005 Bi 0.001 0. 000 0.001 0.002 0.004 0,000 0.000 tin 0, 002 0.014 0.004 0. 002 0.007 0.044 0.012 Fe 0. 002 0. 000 0.000 0.002 0.00s 0.003 0.009 Ca 0.965 1.U)) r.o71 0.910 0.856 L.O77 r. 145 Ba 0. 000 0. 000 0.000 0. 000 0.000 0.000 Pb 0,001 0.001 0.000 0. 003 0.046 0.008 0.001 Na 0.857 0. 738 o.622 0.8I2 o.597 0.585 0.538 Cs 0.00I 0. 002 0.000 0.000 0.001 0.000 0.000 !A 0.16 0.16 0.14 0.14 o.27 0.27 0.22 0.22 0.30 0.30 0,05 0.05 0.22 0.22 o 5.92 5.95 6.00 6.00 5.94 6.00 5.95 6.00 6.00 6.00 6.56 6.28 5.00 6.00 0 0. 00 0. 00 0.08 0,29 0.00 0.11 0.00 0.I1 o.25 0.55 0.00 0.57 0.00 0.38 ott 0.07 0.00 0. 34 0. 00 0.33 0.00 0.3i 0.00 0,39 0.00 0.57 0.00 0.53 0.00 0.93 0.93 0.58 0,58 0.61 0.67 0.69 0.69 0.37 0.37 0.43 0.43 o.47 0.47 uY 0.00 0.07 0.00 0.13 0.00 0,22 0. 00 0. 20 0.00 0.07 0.00 0.00 0,00 0. 15 HÔ 0.00 0.03 0. 12 0. 33 0.04 0.20 0.20 0.35 0.06 0.37 0.99 r.28 0.69 0.95

A-site cations. The major cations in the eight-coordi- atoms per formula unit. Basedon total A-site cations, the nated A site are Ca and Na (Fig. 4). Typical rangesfor number of vacanciesranges from 0.0 to nearly 0.4. A few thesecations are 0.85 to 1.30for Ca and 0.30 to 0.95 for of the analysesof hydrothermally altered microlites from Na. No other cations achieve 20o/oof the A-site total: the microcline-spodumenezone give A-site totals in ex- therefore, all of the Harding samples are microlite, as cessof2.00 (up to 2.13).These A-site excessescould be defined by Hogarth (1977). The minor elementsU, Mn, due to either B-site vacanciesor an A-site cation located Fe, and Pb approachmaximum amounts of 0.25, 0.15, at the B site. BecauseFe and Sb are low in thesesamples, 0.06, and 0.10 atoms per formula unit, respectively. Fe the most likely candidatesare Mn3+ and U6* (Subraman- is below 0.01 in most of the formulas. The maximum ian et al., 1983;Chakoumakos, 1984; Chakoumakos and y, content of Sb is 0.02 atoms per formula unit. Ce, Bi, Ewing, 1985). Although site occupancy determinations and Cs are usually less than 0.01 and Ba is below 0.003 are neededto prove this, enoughMn is presentto account LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE 575

Table 3. Representativeelectron-microprobe analyses of microlites from lithologic units of the Harding pegmatite (formulas basedon >B : 2.00){'

Quartz - Lath Spodumenezone CleaveLandite Unit (rep1. Quartz-Lath Spodunene Zone) 26Ic 2611 27lc 27LE 272c 272r 267 265 P03 ' lc P03. 1r P04c PO4r

0. 14 0. 16 0.38 0.28 0.14 0. L4 0.25 0.23 0.48 0.25 0.38 0.29 "o3 TaZo5 75.1 67.7 14.4 77.3 74,7 74.9 75.2 67,4 72.8 62.0 76.0 73.4 Nb205 6'08 9.35 5,66 2.72 5.27 5.0r 5.51 9.96 7.49 r0.5 5.64 4.48 Tio2 0'03 0.30 0.07 0.42 0.00 0. 16 o.02 0.47 0.03 r.59 0.03 0.67 SnO2 0,00 0.00 0.0i 0.04 0.00 0 .00 0.03 0.00 0.01 0.o2 0.00 0,03 Tho2 0.00 0.00 0.03 0.00 0,07 0 .00 0.05 0.04 0.00 0.04 0,03 0,01 uo3 0.06 2.18 0,31 0,66 0.03 0.78 0.00 2.75 1.28 4.23 0.05 2.61 YzO3 0,00 0.00 0.05 0.0s 0.0s 0.08 0.02 0.03 0.09 0.07 0.06 0.07 ce203 O,25 0.2I 0.00 0.05 0.1,2 0.08 0.r2 0.16 0.21 0.i1 0.14 0.21 sb2o3 0.17 0.31 0.1I 0,19 0.09 0.14 o.12 0.15 0.21 0.48 0,14 0.23 Bi2o3 0.r2 0. 20 0, 12 0.00 0.09 0. 16 0.02 0.06 0.12 0.00 0.07 0.06 Mno 0.03 0, 10 0,03 0,t2 0.20 0.06 0.o2 0.18 0.00 0. 36 0.00 0.02 FeO 0.00 0.04 0,00 0.01 0.00 0 .00 0.00 0.00 0.14 0.34 0.05 0.11 CaO 10.7 11.0 1I.7 I2.8 11.8 r1.7 Il.l 11.4 10.4 r3.1 10.4 10.5 Ba0 0,00 0.00 0.01 0.00 0.01 0.00 0.01 Pbo 0,00 0.03 0.03 0.03 0.00 0.0r 0.03 0.12 0.06 0.10 0.03 0.07 NarO 5.28 4.64 4. 38 2.25 4 .34 4.27 5.57 4.26 5.09 2.90 5.24 4.68 csio 0,04 0.00 0.00 0.02 0.00 0 .00 0.00 0.00 0,01 0.00 0.03 0.02 F' 3.23 2,33 2.94 2.45 3.03 2.47 3.40 2.25 2.88 2.L2 3.35 2.41 suM 101.23 98.59 r00.23 99.39 99.84 99.96 101,46 99.41 lot .36 98.22 rot.64 99,88 -0.89 -1.41 -1,01 0=F -1,36 -0.98 -r.23 -1.03 -r.27 -r .o4 -L,43 -0.95 -1.2r TOTAL 99.87 97,6r 99.00 98. 36 98.57 98.92 100.03 98.52 i00. I5 97.33 100.23 98,87

l,J 0.003 0,004 0.008 0.006 0.003 0 .003 0.006 0.005 0.0i I 0 . 006 0.008 0 .007 Ta L758 1.608 t.764 I .856 r.787 r.788 |,776 \.577 r.697 I.474 1.77r 1.768 Nb 0.237 0.369 o.223 0.109 0.210 0.199 o .2t6 0.387 0,290 0.415 0.218 0.179 Ti 0.002 0.020 0.005 0.028 0.000 0.01I 0.00r 0.030 0,002 0 . I05 0 .002 0, 045 sn 0.000 0.000 0.000 0.001 0.000 0 .000 0.001 0.000 0.000 0.00r 0.000 0.001

Th 0 ,000 0 .000 0.00I 0.000 0.00r 0 .000 0.001 0,001 0,000 0.001 0.001 0.000 U 0.001 0.040 0.006 0.012 0.001 0.014 0.000 0.050 0.023 0.078 0.001 0.048 Y 0.000 0.000 0.003 0.002 0.002 0 ,004 0.001 0.001 0,004 0.003 0.003 0.003 Ce 0.008 0.007 0.000 0.002 0.004 0 .003 0.004 0.005 0.007 0.004 0.004 0.007 Sb 0.006 0,0I 1 0,004 0.007 0.003 0 .005 0.004 0.00s 0.010 0.017 0.005 0.008 B1 0 .003 0 .005 0.003 0.000 0.002 0.004 0.000 0.00r 0.003 0.000 0.002 0.001 Mn 0.002 0.007 0.002 0.009 0.015 0. 004 0.001 0.0I3 0,000 0 .027 0 .000 0.002 Fe 0.000 0,003 0.000 0.001 0.000 0.000 0 ,000 0 .000 0 .0 10 0.025 0.004 0.008 Ca 0.987 r.029 1.093 1,210 I.I12 1.100 r.032 r.051 0.955 r.227 0,955 0,996 Ba 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Pb 0.000 0.001 0.00I 0.001 0.000 0 .000 0.00I 0.003 0.00I 0.002 0.001 0.002 Na 0.88I O.792 0.740 0.385 0.740 0.726 0.938 0.711 0.846 0.491 0.870 0.804 Cs 0.00I 0.000 0.000 0.001 0.000 0 .000 0.000 0.000 0.000 0.000 0.001 0.001 EA 0.11 0.11 0.15 0.37 0 .12 0.14 0.02 0.16 0.14 0.12 0. 15 0.r2

o 6.02 6.26 6.09 6.11 6.10 6.19 5.05 6.27 6.1I 6.41 5,97 6.22 F 0.88 0.64 0.81 0.68 0.84 0.69 0.93 0.61 0.78 0.59 0,91 0.68 m 0. I0 0,10 0.10 0.2r 0.06 0.t2 0.02 0.12 0.11 0.00 0.09 0. I0

dose 0.026 0.90 0,12 0.27 0.013 0.032 <0.01 1,1 0.51 r,7 0.02 I.1 dpa 0.3 Il o.2 4 <0.I L4 6 2L 0.3 13 *"="or",r=rln,p=prlpryatteratton,s=secondaryalteratlon,dosexl0lTalphas/mg,dpa=displacem€ntsperatoh.

for )A in excessof 2.00 in each case and is positively spodumene zone and the cleavelandite unit which ap- correlatedwith >A. Furthermore, the ionic radius (Shan- proach stoichiometric NaCa(Ta' BNbo.r)O5F.Microlites non, 1976) of Mn3+(r":0.0645 nm) is nearly identical from lepidolite pods near the baseofthe quartz-lath spod- to thoseof Nb5+and Tas+(r": 0.064 nm). The radiusof umene zone are virtually identical in composition. The U6+ is significantly larger (r" : 0.073 nm). most unusual microlites are rich in Pb and poor in F. Anion group. The only anion determined by analysis They occur in the microcline-spodumenezone, approach- was F, which rangesfrom 0. I to 0.9 atoms per formula ing the composition trdNa".CaoJbo 'Mno 'U8.f (Ta' 'Nbo ,) unit. The calculatedamount of O rangesfrom 5.9 to 6.9 OuuFo,trIr.l.2H2O (HrO estimatedby difference). atoms per formula unit. Formulas presentedin Table 2 bracket the OH content at 0.M.6 atoms and HrO at 0.0- Alteration effects 1.3 moleculesper formula unit. The maximum number Processesofgeochemical alteration are generallydivid- ofY-site vacanciesapproaches 0.4. )Y tends to be greater ed into primary (hydrothermal) and secondary (weath- than 1.00where IA exceedsor is near 2.00.The highest ering)types (e.9., Van Wambeke,1970; Ewing, 1975).For contents of F occur in microlites from the quartz-lath the complex,,{8206, Ti-Nb-Ta oxidesEwing (1975)ob- 576 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

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primary servedconsistent increases in Ca associatedwith Legend alteration. Samplesthought to reflect secondaryalteration o Core showed significant leaching of A-site cations such as Y, o Rim B.SITE REE, U, and Th and increasedHrO content. Van Wam- . Olher CATIONS beke (1970) found similar results for a uranpyrochlore a Allered and a betafite from Madagascar. Lumpkin and Ewing (1985)used a combination of scanning electron microscopy, electron-microprobe analysis, and TGA to characterize altered pyrochlores, betafites, and microlites. Four Harding pegmatite microlites were included. In general,the results showed that primary alteration produced minor increasesin Fe and Mn, de- - creasesin Na, and variable to minor changesin Ca, F, QUARTZ LATH SPODUMENEZONE HrO, and A-site vacancies.Secondary alteration resulted in leaching of Na, Ca, and F, with extensive hydration (up to l2 wto/oHrO). This processmaximizes the number ofA and Y vacanciestolerated by the pyrochlore structure and leads to cation exchangefor small amounts of large CLEAVELANDITE UNIT cationslike K Sr, Ba, Pb, Bi, and Cs (Harris, 1965;Van Wambeke, 1978; Saffiannikoffand Van Wambeke, 196l; Von Knorring and Fadipe, 1981; Lumpkin and Ewing, l 985). Electron-microprobe analysesof altered and unaltered MICROCLINE-SPODUMENE ZONE areasof Harding microlites are given in Table 3. Speci- = "spottro nocx" mens showing primary alteration (153, 154, 260, 264, 269,P07.1,Pl5. l, Pl7. l, P18.1)give consistent increases in Mn, Ca, and O in the structural formula. IncreasingCa (1970) supportsthe findings of Van Wambeke and Ewing LEPIDOLITE- CLEAVELANDITE (1975). The typical increasesin these elementsare 0.05- SUBUNIT 0.10 Mn, 0.20-0.40Ca, and 0.10-0.40 O atoms per formula unit. The content of Fe increasesin some of the samplesby as much as 0.03 atoms per formula unit. Sim- ilar increasesin Ti and U compensatedby decreasingTa and Nb were noted in a few cases,but could be the result oo o oo o of alteration superimposedon original compositional het- o oo erogeneity. The F content remains approximately con- CLEAVELANDTTEUN|T I ( " stant. Considering analytical totals, the probe data indi- tN BERYLZONE) . cate that total HrO also remains nearly constant, except " 07 08 09 (Pl5.l, in a few cases P07.1)where an increaseof up to lo 2 wto/ois inferred. This is equivalent to 0.7 OH anions or Fig. 3. Major-elementB-site chemistry by lithologicunit for 0.35 HrO moleculesin the formula. The amount of Pb is the Hardingpegmatite microlites. Minor amountsof Sn and W ifthe unusualPb-rich grains(269) are considered constant are excluded.The points labeledother include unzonedgrains not to have resultedfrom alteration (seeFig. lf). Primary andanalytical points between the coreand rim ofzonedgrains. alteration also leads to consistentdecreases of0.l0-0.35 Na, 0.1-0.4 trA, and 0.24.4 trY. As noted above, some of the analysessuggest that Mn might be located at the B contents of 8-13 wt0/0,consistent with TGA results on site. If so, the inferred decreasesin A and Y vacancies other pyrochlores,betafites and microlites (Lumpkin and would be diminishedby 0.1-0.2. Ewing, 1985; Lumpkin, unpub. data). Assuming an HrO Microprobe analysesof secondary alteration (weath- content of I 2 wt0/0,a tlpical formula is Dl, Ca ou Na., Uo' - ering) adjacent to fractures were obtained in a few ofthe (Ta, rNborrTio or)Or rFo rtrIo' 3.5HrO. samples(153, 269, P02.2). Pronounced decreasesof up to 0.7 Na, 0.6 O, 0.5 Ca, and 0.5 F atoms per formula Intracrystalline zoning patterns unit are observed.The amount of Pb decreasesby as much As expectedfrom the occuffenceof distinct color zon- as 0.03 atoms per formula unit. These changesare ac- ing, many of the Harding microlites show core-to-rim companied by increasesin vacanciesof up to 0.9 trA and variation in both major and minor elements.Three gen- 0.7 trY. Minor increasesin Mn and Fe occur, but signif- eral typesofzoning patternshave beenobserved: (l) Crys- icant increasesin Ba, Pb, Bi, and Cs were not observed. tals in which Nb, Na, and F decreaseand Ta, Ca, IJ, Ti, Low analytical totals ofthe probe resultssuggest total HrO and HrO increasefrom core to rim. IncreasingHtO con- 578 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

- Legend A SITE CATIONS o Core o Rim . Olher A Alfered

Other

ur/ 3

Oo . .qa o7 A ^a ,tr 4^ .F A AA

CLEAVELANOITE QUARTZ-LATH SPODUMENE,'SPOTTEDZONE, UNtI SPODUMENEZONE .= ROCK.' (IN BERYLZONE) o6 o1 oa o9 o8 o.9 o8 o9 o8 o9 o8 o9 to Co Fig. 4. A-site chemistry by lithologic unit for the Harding pegmatite microlites. The ternary end-member o/fter includes Th, IJt*, Y, Ce, Sb, Bi, Mn, Fe2*, Ba, Pb, and Cs. The points labeled other include unzoned grains and analytical points betweenthe core and rim ofzoned grains.

tent is inferred from lower analytical totals of rim com- occur. In turn, end-member compositional variations al- positions. (2) Microlites with cores relatively rich in Nb, low a determination of substitution mechanisms. The Ta, Na, and F and rims relatively rich in U, Ti, and HrO. simplest case,Ta-Nb substitution, is obvious from intra- Ca content remains fairly constant as does the Nb/Ta crystalline zoning and has a correlation coefficient (r) of ratio. (3) Grains of microlite that have decreasingcontents -0.92. Other substitutionsare more complex and require of Ta, Na, and F coupled with increasingamounts of Nb, detailed evaluation in terms of end-member composi- Ca, IJ, Ti, and HrO from core to rim. tions. In addition, the minor elements Mn, Fe, Sb, and Pb A convenient starting point is the ideal microlite com- tend to be enriched in crystal rims. U-Ti and Na-F have position NaCa(Ta,M)rOuF or Ar+A2+B5+X?-Y-. This a closepositive correlation in all three types ofzoning. In component accounts for 40-950/oof the composition of zoning patterns I and 3, Nb-Ta and Ca-Na show a neg- the samplesand is supported by positive correlation of ative correlation. Note also that thesetwo patterns differ Na-F shown in Figure 5 (r: 0.71). Much of the scatter mainly in the reversed core-to-rim behavior of Nb and in this plot results from primary alteration, in which Na Ta.7-oning types I and2 are characteristicof microlites decreasesas F remains nearly constant. Intracrystalline from the quartz-lath spodumene zone. Zoning pattern 3 zoning patterns show a close positive correlation of Na- is typical of microlites from the cleavelanditeunit, lepid- F. Variation of divalent cations Ca, Mn, and Pb can be olite

o o E f o E o o

o o E t

o o z o ^^4- - oo 02 0.4 0.6 o'8 ro F ofoms/ formulounit Fig. 5. Na versusF in atomsper formula unit for the Harding oro- oJo.on.l"1 pegmatitemicrolites. .'r" , rorl,.,tu,oun,oto Fig. 6. A-site vacanciesversus maximum Y-site vacancies per pegmatitemicrolites. determined for theseanalyses). For the mean analysesfor formula unit for the Harding which HrO was determined (Table 2), there is enough water to completely fill the Y site with hydroxyl and in Y site is 0.7 atoms (the differencebeing vacanciesor hy- many casesto have excessmolecular HrO. Not deter- droxyl). The first line (shallower slope) is for two Y-site mining HrO/OH maximizesthe Y-site vacancies,whereas oxygensfor each go+, which would be expectedfor low allocating as much OH as possibleto the Y site will min- concentrationsof U (i.e.,widely scatteredUOr polyhedra). imize the Y-site vacancies.Therefore, to establish a cor- The secondline (steeperslope) is one Y-site O for each relation betweenA-site and Y-site vacancies,as proposed U6*, which would be the limit approachedas the U con- by Pyatenko(1959) andAleshin and Roy (1962),hydroxyl tent increasedand UOr polyhedra sharedcorners (Y-site and molecular HrO must be determined, otherwise the oxygens).Actually, an infinite number of lines can be conelation is dependenton assumptionsin the calculation drawn parallel to those in Figure 7, each for a different of the structural formula. amount of vacancy plus OH substitution at the Y site. An inversecorrelation betweenU and F was found (Fig. Note that the data roughly trend along the relationship of 7). The wet chemical analysesof microlites by C. O. In- U6* : 2(Y-site oxygens),as expectedfor low U content. gamellsreported in Jahns and Ewing (1976) indicate that The correlation should shift toward the relationship of the Harding microlites have high U6+/U4+ ratios. In the go+ : (Y-site oxygens)as the U content increases,finally absenceof wet chemical analysesfor these samples, we assumethat all the uranium is U6*. The correlation, between (J6+ and F is noteworthy becauseit supports the proposition that U6+ at the cubic A site in the pyrochlore structure is much like the uranyl group (UO]*) and its characteristic coordination polyhedron, which has been c suggestedby Greegoret al. (1985b) basedon extendedX- J ray absorption fine structureofUl-edges in metamict and o annealedpyrochlore. The uranyl group is a common struc- l E =(y-sireO) tural unit of many uranium compounds and is also stable o in the aqueousenvironment. The uranyl ion is linear with U6+=2(Y-siie O) U-O distancesshorter than the sum of radii for 02- and o IJ6+. Four, five, or six ligand atoms typically lie in the E equatorial plane of the O-U-O group, with these U-O o distances longer than radii sums. In comparison, the o pyrochlore + rhombohedrally distorted cubic A site of has @ a similar structure, with two short bonds in a linear Y- f A-Y arrangement and a puckered equatorial girdle of six long A-X bonds (Chakoumakos, 1984, Subramanian et al., 1983);see inset of Figure 7. With Y-A-Y as the uranyl group, the amount of F at the Y site will be inversely correlatedwith U6+ (Fig. 7). The large amount of scatter F otoms/ formulounit in Figure 7 is expectedbecause hydroxyls and vacancies Fig. 7. U versus F in atoms per formula unit for the Harding also substitute for F at the Y site. The lines plotted in microlites. The two lines are theoretical relationships assuming Figure 7 illustrate two theoretical relationships between all the U is hexavalent and requires oxygen in the Y site; seetext IJ6+ and F at the Y site, assumingthat each U5+ belongs for discussion. The insert illustrates the A-site coordination rel- to a uranyl group at the A site and the maximum F at the ative to a reference cube in the pyrochlore structure. 580 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

U To 20, \ 50 mo le . cores o nms

807060+ No CoTo20.F Co2To207

Fig. 8. Intracrystalline chemical variations (zoning) plotted in terms of hypothetical end-member compositions. Mn and Pb are included with Ca, and Nb with Ta.

reachingthe approximate limits of 0.4 and 0.6 atoms per uranyl group ifU is consideredto have enteredthe struc- formula unit for a defect (AtrBrO?) and ideal (ArBrO?) ture as U6+ al the A site. However, data for A and Y microlite, respectively.Note that the U6+ values may be vacancies(Fig. 6) suggestthat at least some of the U was slightly overestimated in calculating all the U (4+ and originally incorporated as IJa*. Furthermore, when we 6+) as hexavalent.With increasedhexavalent U at the A form the component U6*Ti2O, (or Uo*TirOu), there is site, increaseddivalent anions are neededin the Y site to usually excessU remaining that must be consideredto be preserveelectrostatic neutrality as well as possible;alter- Ua* in a compon€nt like Ua*TarO, (disregardingprimary natively, the stableuranyl group simply persistsas a fun- alteration effects).Iacking more precisedata on the initial damental unit in the crystal. An alternative diagram that oxidation state of U, we arbitrarily use the end members illustrates this same relationship is a plot of U6+ versus IJ6*Ti2O7and Ua*TarO, for purposesof plotting in Fig- the O content of the Y site (r: 0.93). However, the true ures 8 and 9. Theseaccount for up to 300/oofthe observed values of the Y-site O are dependent on the water or chemical variation. Intracrystalline zoning patterns are hydroxyl content and the specific method of calculating displayed in Figure 8 based on the end members Na- the structural formula. CaTarOuF,CarTarO.,, CaTarOu, UTarOr, and UTirOr. In Other microlite end membersinvolve the incorporation all cases,Mn and Pb are included with Ca and Nb with ofU at the A site and Ti at the B site. A close positive Ta. Core-to-rim trends indicate increasesin the compo- correlation of U-Ti in zoning patterns can be used to nentsCarTarOr, CaTarOu,UTarOr, and UTirO, relative postulate the components CaUTirOr, Ua+TaTiO.F, and to NaCaTarOuF.The substitution schemesderived from IJ6+Ti2O?(or Ua+TirOu).The first two are excludedowing these trends are listed in Table 4 in order of decreasing to a lack ofcorrelation observedfor Ca-U and the negative importance. Alteration effectshave beenevaluated on the correlation found for U-F (r : -0.79). The U-F data plot- samebasis. Figure 9 showsthat the end membersUTarO, ted in Figure 7 are consistent with the structure of the and CarTarO, increase in relation to NaCaTarOuF. In

U CoTozoe \': 50 mole % ./ o oltered . unollered

NoCoTo2O5F

Fig. 9. Primary (hydrothermal) alteration effectsplotted in terms of hypothetical end-member compositions. Mn and Pb are included with Ca, and Nb with Ta. Apparent increases in U end members are the result of major increases in the CarTarO, component. LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE 581

AIJ4+BTa Table 4. Substitution schemesfor microlites from the like ACarBMn- oNaz"Ta or AU'6+BMn+ be- Harding pegmatite come plausible, the second scheme involving oxidation ofu. zonlng Trends Prifiary Alteration

AcrYo Cnnurcll, vARTATtoNSBETwEEN LrrHolocrc UNITS "lib + -Ta ' \.Yr AcuYo -Ta + "Nb . hY! The major-elementchemistry, displayedin typical fash- AcuYo - \.Yr if Mn- : ion on ternary diagramsfor the A- and B-site cations(Figs. AgYg rru ' \.Yr \6* fu" * \_,4* 3 and 4), does not allow the clear chemical distinction A..L.B-. Y^ A. A- B- Y. uurlru+Nauafa?t betweenmicrolites from different lithologic units. In order ffi;Yo . \.oc."" to determine if the microlites from diferent lithologic units are chemically distinct, the five groups of 217 mi' crolite analyseswere subjected to discriminant analysis some cases,UTirO? increasesrelative to NaCaTarOuF, on the basisof 20 variablesconsisting of the atomic com- and CarTarO, increaseswith respectto CaTarO.. Because positions for all the elementsdetermined and the sum of of the major substitution of Ca.rTarOr,increases in the U A-site cations. Discriminant analysis is a powerful tool componentsare more apparent than real. Corresponding for multivariant analysis(Davis, 1973) and classification substitution schemesare rankedin Table 4 for comparison improvement of complex mineral groups (e.g. Ewing, to zoning trends. The major similarity seemsto be the I 976).The purposehere is simply to describethe chemical coupled substitution ACaYO * rNaYF (type 5, Aleshin differencesamong microlites from different lithologic units and Roy, 1962). Differences include schemesinvolving within the pegmatiteand to identify the chemical changes vacancies;for example, AEY! - ANaYF is common in due to primary hydrothermal alteration. zoning patterns and ACaYO- AEYII occurs as a result of alteration. These substitution schemesrepresent forma- The discriminant analysisprocedures used are those in the tion of Schottky-type defects during crystal growth and StatisticalAnalysis System software (SAS, 1982). One ofseveral their filling during hydrothermal alteration. Both schemes assumptionsconcerning the variablesemployed in multivariant fall undertype 8 ofAleshin and Roy (1962).IfMn3* enters proceduresnecessary to test the significanceof diferencesbe- the B site during alteration, then coupled substitutions tweengroups is that the variableshave a multivariant normal

Table 5. Means and standard deviations of the most discriminating chemical variables by lithologic unit for the Harding Pegmatitemicrolites

Quartz-1ath Cleavelandite Microcllne- Lepldolite- cleavelandlte unlt Variable spodumene zone unlt spodunene zone cleavelandlte aubunlt (1n beryl zone)

0 .02 0.05 0.14 0.13 0.07 !0.01 !0 .05 !0 .05 !0.03 r0 ,04

0.001 0 .01 0.004 0.03 0.03 i0 .002 !0.01 !0.007 lo,02 r0.02

T1 0 .01 o.o2 0 .04 0.04 0.09 10.01 !0 .02 t0 .03 10.03 i0.07

I .09 L.O2 i .09 I .03 I .07 r0. 07 r0.09 10.15 r0, 14 10.05

0. 004 0.005 0 .003 0.00s 0.004 10.003 !0.001 !0.003 10.00I !0.003

0.001 0.009 0.0I 0.005 0.0r r0.002 r0.01 J0.02 r0 ,001 10.009

0. 70 0.65 0 .43 0.5r 0.57 10.09 r0.I7 !0. 13 !0.06 i0. 14

0 ,008 0.01 0. 05 0.02 0.03 !0.09 r0 .02 i0. 04 !0.01 lo.02

0 .005 0.006 0. 005 0. 006 0.008 !0.002 10,003 !0.003 r0.002 !0,004

Ta r.77 r.74 I .65 r .63 1.63 t0. 06 !0 .07 r0. 07 !0.04 !0. l5

Na 0,71 0.71 0.58 0.60 0.50 !0. I2 !0. 16 !0,12 !0.r2 !0. 10

No. Cases 61 51 )o 18 30

Although the varlables B1, Ba and Th were significant, they are excluded because their values are near the mlnlmun detectlon lioit for the electron nicroDrobe, 582 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

Table 6. Classificationsummaries for the groupingsof lithologic units and unaltered/altered, using the set of most discriminating variables in each case q) E€ -o Number of obeelvatlons o classified into: o 4 E- A. B. C. D. E. TotaIs 0€ quarlz-lath EE A. spodumene zoDe 58 1I01 oE B, Cleavelandite unit 123E00l )l Eo ^.E + C. Microcline-spodumene zone 105500 o "-Ea D. L€pldolite-cleavelandlte subunit 0 0 0 18 0 l8 o E. Cleavelendite unit (in Beayl zone) 000030 c f,o 30 o o 216 C o Number of observations o c c^c6cc clessifled into' Fc unaltered altered totals e o "t- ' unaltered 161 II 172 E c cce ct altered 4 40 ()o c b-c 216 q)

a-2 c distribution. For the microlite data this is only approximately c true, as many of the variables deviate from normality within -4-2024 $oups by positive skewness.Fortunately, for mild deviations of this kind, the test statisticsremain robust (Tabachnickand Fidell, First CononicolVorioble 1983). We will not belabor the credibility of this and other as- Fig. I 0. Plot of the secondversus the first canonical variable (although sumptions they could be discussedat length), in that for the grouping by lithologic unit: quartz-lath spodumenezone our goal is description rather than inferenceor classification. (A), cleavelanditeunit (B), microcline-spodumenezone, (C), le- As rnultivariant procedurescan be adverselyaffected by out- pidolite-cleavelanditesubunit @), and cleavelanditeunit (in the liers, the analyseswere examined for outliers, and a singleanalysis beryl zone) (E). The asterisksmark the gxoup means. with an unreasonablylow total was removed. For the remaining 216 analyses,a stepwisediscriminant procedurewas applied to the groupings of lithologic units to determine the most signifi- To follow the previous analysis using the groupings by lith- cantly discriminating variables. Of the 20 variables examined, ologic unit, the 216 analyseswere regrouped according to the I 4 weresignificant at the 290level; they are (in order ofdecreasing labels unaltered and. altered, which were determined primarily F-value): U, Fe, Ti, Bi, Ca, Ce, Pb, F, Mn, Ba, Sb, Th, Ta, and from SEM observations.The data basewas not of sufrcient size Na (Table 5). However, the Bi, Ba, and Th values are near the to divide the analyseswithin eachlithologic unit. From the new minimum detection limit of the electron microprobe, so these grouping, the overall chemical changes ascribed to primary hy- are not meaningful.The classificationsummary in Table 6 allows drothermal alteration can be identified. For the stepwise discrim- the evaluation of the successof the discriminant analysis. In inant procedure, 5 of the 20 variables were significant at the 2o/o general, all of the microlite analyses are grouped correcfly ac- level, and they are (in order of decreasingF-value) Mn, Ti, Ca, cording to their a priori labels,except for those from the cleave- Ba, and the A-site total (Table 7). The Mn, Ti, and Ca are higher landite unit. A substantialnumber (230lo)of thosemicrolite anal- in the alteredsamples, whereas the Ba and A-site totals are lower. yses were misclassified into the quartz-lath spodumene zone. The decreasein Ba, contrary to typical increasesduring alteration This indicates that the cleavelanditeunit has chemical charac- and weathering,cannot be ascribedany physical significanceas teristics that overlap with those of the quartz-lath spodumene the Ba values are nearly all at the minimum detection limits for zone,which is not surprising,as the cleavelanditeunit is, in part, the electron microprobe. Those microlites identified as being a replacementofthe quartz-lath spodumenezone. Becausethe afected by hydrothermal alteration are restricted almost exclu- overall groupingsare correct, microlites from different lithologic sively to t}te microcline-spodumenezone and cleavelanditeunit units can be easily distinguishedon the basis oftheir chemistry. (in the beryl zone). Consequently,increased Mn content in the Linear combinationsof the most discriminating variableswere altered samplesmay not be geochemicallysignificant, because combined into four canonical variables. which are uncorrelated the microcline-spodumenezone microlites simply have the high- and have the highestpossible multiple correlationswith the lith- estMn contents.The increaseof Ca and decreaseofA-site cations ologic units. Figure l0 displays a plot of the analysesand their is consistentwith compositional changesestablished for altera- group means for the first and second canonical variables. The tion effectsin AB'O. Nb-Ta-Ti oxides (Ewing, 1975). The ctas- first canonical variable provides the greatest separation between sifrcation summary fiable 6) is grossly correct, but not as suc- the quartz-lath spodumenezone and the microcline-spodumene cessfirl as for the lithologic units. Not surprisingly, alteration zone, and thesetwo lithologic units are in turn separatedto the effectsare lesseasily recognizedthan the lithologic unit containing $eatest extent by the second canonical variable from the lepid- the sample. Again, the single canonical variable in this case was olite-

Table 7. Means and standard deviations of the most discriminating chemical variables by the labels unaltered/altered for the Harding pegmatite microlites varlable unalte!ed ol

0.01 0.07 10.01 10.03 = E T1 0,02 0.08 10.02 t0.06

Ca I .04 l. I7 !0.09 r0.09 o E A-totaI r. 83 1.93 i0. 08 r0.10 c No. cases 172

Although the varlable Ba uas slgnlflcant' it waa excluded because the values are near the mlnlmum lihlt for the electron mlcroprobe.

Chemical variations between lithologic units can be summarized as follows: (l) In primary units, the earliest microlites approach NaCaTa, ,Nbo.rO.F within the qvafiz- Fig. I l. Plot of theMn contentversus the canonical variable lath spodumene zone. (2) Subsequentcrystallization of for the groupingby unaltered(solid circles)and altered(open the microcline-spodumenecore unit produced microlites circles).The asterisksmark the groupmeans. For a singleca- with 0.,H.7 Na, 0.14.2 U, 0.G4.1 Pb, 0.2-0.4 Nb, 0.0- nonical variable,the data needonly be plotted as a histogram; 0. I Ti, and 0.14.6 F atoms per formula unit. Theseare we choseto plot Mn becauseit is the most significantlydiscrim' the highest U and lowest Na and F contents observed. inating variable. Primary alteration produced up to 1.3 Ca and 0.1 Mn (dpa) atoms per formula unit in microlites from this unit. (3) mg) and the total number of displacementsper atom Microlites from the cleavelanditecore unit are similar in have beencalculated. It is more common to calculatedose composition to those of the quartz-lath spodumenezone in units of alphas per cubic meter, but this requires a (the except for 0.0-0. I U and 0.4-0.8 F atoms per formula measuredvalue for the density ofeach sample density unit. (4) Where the cleavelanditeunit replaced the beryl decreaseswith increasingdose). The dose was calculated zone, microlites contain up to 0.03 Fe, 0.2 Ti, and 0.6 following the method of Holland and Gottfried (1955) 238U 235U'. Nb atoms per formula unit. These are the highest values based on the decay schemesfor and Because of Fe, Ti, and Nb observedin this study. the Th concentration in the microlites is low, no consid- eration was given to alpha events associatedwith the de- RlorlrroN EFFEcrs (lrnrlrnrrcrrzlrroN) cayof 232Th.An ageof 1300m.y. B.P.was used (Brookins Becauseofthe wide variations in UO, content (lessthan et al., 1979).With eachalpha-decay event, a recoil nucleus 0.1 wto/oto nearly l0 wto/o),the microlites of the Harding (0.07 MeD and an alpha particle (4 MeV) is produced. pegmatite present a rare opportunity to study the effects The range of the alpha particle is up to approximately of alpha-recoil damage on the pyrochlore structure-type l0 000 nm, but its energy is dissipated mainly by elec- as a function of increasingdose. These observations have tronic excitation, ionization events, and at most several important practical applications, as the pyrochlore struc- hundred atomic displacements.The recoil nucleus has a ture-type is a common constituent of polyphase,crystal- range of l0 nm, but becauseof its greater mass, it pro- line, nuclear wasteforms that have beenproposed for the duces up to several thousand atomic displacementsde- long-termisolation of actinides(Ringwood, 1982, 1985; pending on the material. The number of displacements Doschet al., 1984;Morgan et al., I 984).Observations on per atom is calculated assuming 1500 atomic displace- the transition from the fully crystallineto X-ray diffraction ments per alpha-decayevent (Weber et al., 1982;Eyal and amorphous state for the natural microlites over long pe- Fleischer, 1985).One must caution that in the correlation riods of time (1300 m.y. B.P.) can be comparedto syn- of changesin structural or physical properties with either thetic pyrochlore structure-typesthat have been doped dose or dpa for natural samples, "anomalous" samples with '?4Cmor 23tPuand observedto levels of equal dose may result from thermal events that have caused an- (fully X-ray and electron diffraction amorphous) over pe- nealing and/or recrystallization. Alternatively, the cal- riods of severalyears (Wald and Oflermann, 1982;Clinard culated dose and dpa may be in error due to alteration et al., 1984a,1984b; Weber et al., 1985).We presenthere that has removed or added U or Th to the sample. The preliminary observationson the transition from the crys- specialvalue of this extensiveset of microlite data is that talline to metamict state for the Harding pegmatite mi- there are enough data available to clearly identifu such crolites. "anomalous" samples. For eachof the microlites analyzed,a total dose(alphas/ The calculated dosesfor the Harding microlites range 584 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

Table 8. Summary of calculateddoses (alphas/mg) and displacementsper atom (dpa) for selectedmicrolites from the Harding pegmatite [,"",.:

Sanple No. U03(wt.Z) alphas/ng

lil,l ,l^,il 141 <0.05 .rol4 <1 10.429(l) tr, r49 0.8 3 x 1016 (l il.",,.", 3 10.428 ) ,1 .,1 I50 I.8 7 x 10-- -|-1!9f r--.-[]f -i-*____Jt*.^*,_ 9 r0.4r8(2) 288 3,2 t x tO17 17 r0.389(4) 'zaa(t7) ^a r53 8.9 4 x IolT -t53(49) Estinated sEandard errors (1n parentheses) refer to the last dig1t. 32 52 2e Fig. 12. PowderX-ray-diftaction scansfor Harding peg- 1000.C in air for l0 h. Although it is not possible to matitemicrolites. Numbers in parenthesesare calculated values correlatethe changein unit-cell volume directly with the of the displacement/atom.Data for thesesamples are listedin degree of damage (owing to compositional variations), Table8. there is a slight decreasein the unit-cell volume for the annealedsamples. Based on differential scanningcalorim- from < l0'a to 4.5 x 1017alphas/mg with equivalentdpa etry and differential thermal analysis,there is a dehydra- valuesof < 0. I to 6 I . The maximum dosesare well beyond tion endotherm at low temperature(150 to 300'C) and a the dosesnormally required to causea material to become recrystallization exotherm in the range of 450 to 600"C X-ray or electron diffraction amorphous and well beyond (Chakoumakos,1978; Clinard, pers.comm.). doses commonly attained in doped materials. Experi- Details of the transition can be followed by the use of mentson doped, syntheticsamples commonly reachdoses electron microscopy (Figs. 13, l4). The electron micro- of 10'z5alphas/m3, which is roughly equivalent to l0r5 graphsand their accompanyingdiffraction patterns show alphas/mg.It is interesting to consider the differencebe- the atomic structureat dosesof (a) l0'a, O) l0'u, and (c) tweenthe doserate for the natural and synthetic samples. 10'7 alphas/mg. At the intermediate dose (Fig. 14b), the As an example, zirconolite (a pyrochlore structure-type lattice fringe image (two-beam image using the I I I re- derivative) doped with 238Puwhich has reacheda dose of flection) shows that most of the microlite still retains its 1025alphas/m3 experiences a damagerate of l0-8 dpals crystallinity, but with distinct patches or aggregatesof (Clinard et al., 1984b). The microlites of the Harding aperiodic (no fringes) material. The electron-diffraction pegmatite which have reacheddoses of 4 x l0'? alphas/ pattern is distinct, but with the added presenceofa faint mg have experienceda damagerate of l0-15 dpals, a rate broad diffraction halo causedby the aperiodic areas.At fully seven orders of magnitude slower than that of the the highest dose (Fig. l4c), there is no evidencefor crys- synthetic samples. tallinity, and we seeonly a multiple, broad-halo type dif- SelectedX-ray powder diffractograms for unannealed fraction pattern that is characteristicof an aperiodic struc- microlites are illustrated in Figure 12,and the correspond- ture. The equivalent d-spacingfor the innermost halo is ing dose, displacement,and cell-parameter data for the 0.298 nm. The interpretation of theseelectron-diffraction samplesare summarizedin Table 8. The microlites retain halos is discussedby Ewing and Headley (1983). Thus, good X-ray difraction crystallinity up to doses of l0ra the progressionof structural modifications for natural mi- alphas/mg.In the rangeof 10'6alphas/mg there is a marked crolites with increasingalpha doseis one ofisolated defect decreasein the intensity of the diffraction maxima, until aggregates(i.e., individual alpha-recoil tracks) up through at a dose of l0I7 alphas,/mg,there are no diffraction max- dosesof l0'a alphas/mg with no detectableeffect on the ima. The decreasein X-ray intensity over a short range material's ability to diffract X-rays or electrons,continued of the total doseis typical ofthe damagein-growth process damageand overlap ofthese aggregatesyielding coexisting and is seen in natural zircons (Holland and Gottfried. regionsof amorphous material and remaining crystalline 1955),zircons doped with 238Pu(Weber, pers. comm.), domains in the range of l0t5 to 1016alphas/mg, with a ,aaCm GdrTirO, doped with (Wald and Oflermann, 1982), final saturation value reachedat > 101?alphas/mg. A sim- and CaZrTirO, doped with 244Cmand 23sPu(Clinard et ilar transition is observed over approximately the same al., 1984a, 1984b).Although unit-cell refinementsfor the range ofdoses for natural zirconolites and synthetic zir- microlites having various levels of damagehave been made conolites that have been doped with 23EPu(Ewing and (Table 8), there is no systematicevidence for the expan- Headley,1983;Clinard et al., 1984a,1984b). This suggests sion of the unit cell with increasingdamage as described that there is no correlation betweendamage and doserate for the synthetic samplesin other studies. The observed evenwhen the doserate varies by as much as sevenorders decreasein a correlateswith decreasingNa (ro : 0.118 ofmagnitude. For the Harding pegmatitemicrolites, there nm) and Ta (r" :0.064 nm) and increasingamounts of is no evidencefor significantannealing during the damage lJo*(rn : 0.100nm), Uu* (ro : 0.086nm), Ca (ro : 0.112 process. nm), and Ti (r" :0.061 nm) in the five samples(Table The X-ray and electron-ditrraction data do not allow 8). The microlite samplescan be annealedby heating at one to explicitly describethe atomic structure of the final, LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE 585

Fig. 13. Two-dimensionallattice imageof crystallinemi croliteno. I 47 in the( 1I 0) plane.(a) Principal lattice dimensions aregiven for the [001](1.0 nm), U 101(0.7 nm), and I t l] (0.6 Fig. 14. Lattice-fringeimages (two-beam image using the I 1l nm) directions.Arrows denote dislocations within the band of reflection)and electron-diffractionpatterns for microlitesillus- lowercontrast. (b) Enlargedregion shown in (a)is interpretedin tratingthe effectofincreasing alpha-event dose: (a) no. 147'(b) termsof the BrO. frameworkstructure of microlite. no. 150,and (c) no. 153.A summaryof thedata for eachsample is givenin Table8. damage-saturatedstate. It is important to understandthe structureofthis aperiodic statein order to evaluatewheth- to have begun relatively late, after the formation of the er further structural modifications may be expectedwith beryl and quartz zones, and continued throughout the increasingdose. Structural information obtained from X- formation of the core zones and subsolidusreplacement ray absorption spectroscopyfor Ti and U (Greegoret al., units. Microlite is most abundant in the quartz-lath spod- I 985a, I 985b) indicatesthe persistenceof TiOu octahedra umeneand microcline-spodumene("spotted rock") zones. and, tentatively, uranyl groups in metamict pyrochlore In both zonesthe microlite occurs as small subhedral to group minerals. euhedral crystals commonly zoned from light cores to dark rims and never more than 5 mm in size. Suprlr,lnv AND coNcLUSroNS Over 200 chemical analysesof microlite determined by Microlite, ranging from crystalline to metamict, is a electron microprobe are reported, and they are consistent principal accessoryin severallithologic units of the Hard- with the accepted structural formula, A2-.B2X6Yr-,O' ing pegmatite, Taos County, New Mexico. From the se- pHrO, whereprincipally A : Ca, Na, U, Mn; B : Ta, Nb, quence of lithologic units within the pegmatite, crystal- Ti; X: O; and Y: F, OH, O. Typical ranges(weight lization of microlite from the pegmatitemagma is inferred percent)for major constituentsare 60-78 TarOr, 2.5-12 586 LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE

Nb2O5,0.0-3.2 TiOr, 0.0-ll UOr; 8.0-13CaO, 2.0-5.5 alteration and is contemporaneous and on-going with sec- NarO, 1.5-3.5 F, and 0.1-4.0 HrO. Primary (hydrother- ondary (weathering) alteration. mal) alteration resultsin an increase(in atoms per formula unit) in Mn of 0.05-0.10; Ca, 0.204.40; and O, 0.10- AcxNowlnocMENTS 0.40. The HrO content remains essentiallyconstant. Sec- Art Montgomery in donating the Harding pegmatite property ondary (weathering)alteration results in decreases(in at- to the University of New Mexico has preserveda unique outdoor oms per formula unit) in Na of up to 0.7; Ca,0.5; and F, geologiclaboratory for researchand teaching.His generousgift 0.5. Low analytical totals in the probe results for altered and continued support has stimulated renewed interest in this areassuggest total HrO contentsof 8-l 3 wt0/0.The zoning classiclocality. R. C. Ewing is grateful for the many discussions petrogenesis in the individual crystals corresponds to three general he shared with Dick Jahns on the mineralogy and of the Harding pegmatite. Dick strongly supported the University types:(l) Nb, Na, and F decreaseand Ta, Ca, U, Ti, and of New Mexico's and Art Montgomery's efforts to preservethe HrO increasefrom core to rim; (2) cores are rich in Nb, Harding pegmatiteas a classiclocality for teachingand research. Ta, Na, and F and rims are relatively rich in U, Ti, and This paper only follows Dick's lead. We thank EugeneFoord, HrO; (3) Ta, Na, and F decreaseand Nb, Ca,tJ, Ti, and Lynn Boatner, Dick Jahns,and Paul Hlava for providing micro- HrO increasefrom core to rim. The compositional vari- probe standards,Ian Mackinnon for assistancewith the rrv, ations between grains and the zoning relations are used Elaine Faust-Stevensfor drafting the figures,and Paula Stout for to identify the principal substitution schemesin the mi- assistancewith data and word processing.The authorsare grateful crolite structure (Table 4). Major substitution schemes for the thorough reviews by G. J. McCarthy and E. R. Vance. include (l) ACaYO- rNaYF and (2) AEYD + ANayF (zon- The electron-microprobeanalyses, scanning electron microscopy, and transmission electron microscopy ing patterns)and (3) ACaYO- AlyD (primary alteration). were completed in the Electron Microbeam Analysis Facilitv in the Department of One of the most interesting correlations is the inverse Geology and Institute of Meteoritics at the University of New relationship between U and F. On the basis of the as- Mexico supported in part by the Center for High Technology sumption that the U is hexavalent, the data suggestthat Materials. the uranium is in the cubic A site in the pyrochlore struc- This work was supportedby the U.S. Department of Energy, ture type with much the same coordination geometry as Office of Basic Energy Sciences under grant #DE-FG04- that found in the uranyl group. This is an important ob- 84ER45099. servation, as the uranyl group has been tentatively identified as a molecular unit in fully metamict pyrochlore RnrnnnNcns group minerals (basedon extendedX-ray absorption fine- Aleshin, Eugene,and Roy, Rustum. (1962) Crystal chemistry of structure spectroscopy). pyrochlore. Arnerican Ceramic SocietyJournal, 4 5, | 8-25. Aldrich, L.T., Wetherill, G.W., Davis, G.L., and Tilton, Multivariant analysesof the chemical variations G.R. in the (1958) Radioactive agesof micas from granitic rocks by Rb- microlites grouped according to lithologic unit demon- Sr and K-Ar methods. American Geophysical Union Trans- stratethat unique chemistriescharacterize each group. Of actions,39, 1124-1134. 20 variablesexamined, I 4 were significantat th e 2o/olevel; Bonshtedt-Kupletskaya,E.M. (1966) Systematizationof the pyrochlore-microlite mineral group. (in Zapiski VsesoinznoeMiner- they are order of decreasingF-value): U, Fe, Ti, Bi, alogicheskoeObshechestvo, 95, 134-144 (In Russian). Ca, Ce, Pb, F, Mn, Ba, Sb, Th, Ta, and Na. In the single Borodin, L.S., and Nazarenko,I.I. (1957) Chemical composition caseof substantialmisclassifi cation (23o/o of the microlites of pyrochlore and diadochic substitution in the ArBrX, mol- in the cleavelandite unit were misclassified into the quartz- ecule. Geokhimiya, 4, 386-400 (transl. Geochemistry Inter- national, 4, 33U349, 1957\. lath spodumenezone), the similarity is expected,as the Brookins,D.G., Chakoumakos,B.C., Cook, C.W., Ewing,R.C., cleavelanditeunit is, in part, a replacementof the quaftz- Landis, G.P., and Register, M.E. (1979) The Harding peg- lath spodumenezone. matite: Summary of recentresearch. In R.V. Ingersolland L.A. Radiation efects (metamictization) are clearly evident Woodward, Eds. New Mexico Geological SocietyGuidebook, 30th Field Conference,Santa Fe Country, 12'7-133. in the microlites. Becauseof the wide variations - in UO3 Cernf, Petr. (1982) Anatomy and classificationofgranitic peg- content (less than 0. I wto/oto nearly l0 wt0/0),the mi- matites. In P. Cernf, Ed. Granitic pegmatitesin scienceand crolites exhibit the full rangeof structural periodicity from industry, 1-40. Mineralogical Association of Canada Short completely crystalline (< I dpa), partially crystalline (up Course Handbook 8. (1984) to 10-20 dpa), to completely X-ray and electron diffrac- C-ernf,Petr, and Burt, D.M. Paragenesis,crystallochem- ical characteristics, geochemical tion amorphous (>20 and evolution of micas in dpa). Basedon X-ray and electron- granitic pegmatites.In S.W. Bailey, Ed. Micaq257-298. Min- diffraction analysis, the progressivestructural modifica- eralogicalSociety of America Rewiewsin Mineralogy, 13. tion of natural microlite with increasingalpha doseis one Chakoumakos, B.C. (1978) Microlite, the Harding pegmatite, of isolated defect aggregates(i.e., individual alpha-recoil Taos County, New Mexico. B.S. thesis, University of New Mexico, Albuquerque. tracks) up through dosesof l0ra alphas/mg with no effect - (1984) Systematicsofthe pyrochlore structure type, ideal on the material's ability to diffract X-rays or electrons, A,B2X6Y.Journal of Solid StateChemistry,53, 12U129. continued damageand overlap ofthese defect aggregates Chakoumakos,B.C., and Ewing, R.C. (1985) Crystal chemical yielding coexisting regions of amorphous and crystalline constraintson the formation of actinide pyrochlores.In C.M. domains at l0'5 to l0'6 alphas/mg,with a final saturation Jantzen,J.A. Stone, and R.C. Ewing, Eds. Scientific basis for nuclear waste managementVIII, Materials ResearchSociety value received at greater doses than l0r7 alphas/mg. The Symposia,Proceedings, 44, 641-646. Materials ResearchSo- accumulated radiation damage postdates hydrothermal ciety, Pittsburgh. LUMPKIN ET AL.: MICROLITE FROM HARDING PEGMATITE 587

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