Chemistry and radiation effects of thorite-group minerals from the ...

American Mineralogist, Volume 73, pages 1405-1419' 1988

Chemistry and radiation effects of thorite-group minerals from the Harding pegmatite,Taos County, New Mexico GnnconvR. Luvrprrr'q,B. C. CrHrouvr,mos Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131' U.S.A.

Ansrn-lcr Dark brown and yellow thorites, with compositions extending the rangeof natural solid solution for the zircon structure type, occur as 0.5- to 2-cm grains in the quartz zone, cleavelanditeunit (replacingthe beryl zone),quartz-lath spodumenezone, and microclinespodumenezone of the Harding pegmatite, Taos County, New Mexico. Electron-microplobe analyses(wto/o)give the following for thorites from the cleavelandite:unitl.2.9-19.6 F , o r , 6 . 4 - t 6 . 5 V 2 O s ,0 . 2 - 1 . 2A s r O , , 2 . 6 - 7 . 7S i o r , 4 1 - 5 7 T h o r , 0 . M . 9 U O r , O ' l - 2 ' 9 0.04.7 BirO3,5.8-8.4 CaO,0.l-0.6 PbO, and 0'+l'7 F' In conY,O3,0.1-6.5 REE2O3, trast, yellow thorites from the quartz zone contain (wto/o)17-20 SiOr, 36-65 ThOr, 3-20 uor, 0.4-0.8 Alro3, 0.3-0.6 Y2o3, 0.24.6 REE2O3'0.6-1.0 Biro3' 0.8-1.3 CaO, 3-17 pbO, and 0.0-0.9 F. Low totals of the microprobe analysesprobably indicate the presence of 7-12 wto/oHrO, in agreementwith rcn. Infrared spectra confirm the presenceof moIecularwater and are consistentwith the presenceof mixed types of hydroxyl species.Both varieties of thorite are partially recrystallized.In the brown variety, the degreeof crystallinity correlateswith increasingsubstitution of P, V, and Ca for Si and Th. Dark brown thorites from the quartz-lath spodumenezone have an averagecomposrtlon are fully metamict, and show only slight alteration. Thorites near (ThorrU00rPb00s)Sioo, from a partially albitized area of the microcline-spodumenezone show slight to moderate alteration characterizedby enrichment in P, V, As, and Ca. Compositions (wto/o)are 0.27.0 P2Oi,0.2-6.0V,O,,0. l-l'8 AsrOr,7.8-19.4SiOr,0.3-2'6ZrO2,6l-75 ThOr,0'7-3'5 UOr, 0.1-0.9 Alror, 0.1-0.4 REErO3,1.94.5 CaO, 0.1-0.5 PbO, and 0'249 F' Altered areasare partially recrystallized. Calculatedalpha-eventdoses(4-12 x 1017a/mg) are greaterthan the damagesaturation dose (-1 x 10'6 a/mg), but substantial crystallinity is present. The energy barrier to recrystallization of alpha-decay{amaged thorite enriched in P and V is inferred to be lower than for ideal thorite or U-rich microlites from the microcline-spodumenezone. Thorite from the quartz, quartz-lath spodumene,and microcline-spodumenezonescrystallized as a primary magmitic phase close in composition to ideal thorite, but enriched in U. Partial alteration of primary thorite in the microcline-spodumenezone is related to partial replacement of this unit by cleavelandite. Brown thorites also crystallized during subsolidusreplacementof the beryl zone by cleavelandite.The hydrothermal fluid present during albitization of the beryl zone and microcline-spodumenezone was enriched in Ca, V, p, As, Y, REEs, and Bi. U was depleted by the earlier formation of microlite in the quartz-lath spodumeneand microcline-spodumenezones.Remaining U was preferentially incorporated by microlite in the cleavelandite unit. Thorite concentrated Y, REEs, and Bi relative to microlite.

unidentified ochre-colored mineral. These samples apINrnonucrroN pear to be similar to those describedby Hirschi (1928)' In 192g, Hirschi published a brief description of an paragenesisis not exactly the sameas noted unnamed mineral containing 32.35 wto/oTh and 0. 185 although the description. in the original pegmatite, New Taos County, Harding wto/oU from the During field investigations we have found additional Mexico. Discussion and a partial translation of Hirschi's description appearedin Northrop (1959), but no name samples of dark brown, brown, and yellow material in was assignedor further description made. To our knowl- the beryl zone, quarlzzone, quattz-lath spodumenezone, edge,none of thesesampleswas preserved.Recently,Ar- and microcline-spodumene zone. Preliminary work thur Montgomery collected and donated to us two sam- showedthat all of the samplespossessthe zircon structure ples from the Harding pegmatitethat contained an (I4r/amd,Z:4)withlatticeconstantssimilartoideal 1405 0003404x/88/lI 12-1405$02.00

1406

LUMPKIN AND CHAKOUMAKOS: THORITE-GROUP MINERALS

thorite (Lumpkin and Chakoumakos,1986). Chemical compositions of the samplesare complex, ranging from U-PLrich thorite to varieties in which the components CaTh(VOo), or CaTh(POo),predominate. Furthermore, rare-earth elements (REEs), Bi, and F were detected by electron-microprobeanalysis. Other specieswith the zircon structure in which B : V are wakefieldite, YVO. (Hogarth and Miles, 1969; Miles et al., l97l), and dreyerite, BiVOo (Dreyer and Tillmanns, l98l). The only naturally occurringorthophosphate with the zircon structure is xenotime, YPO.. Brabantite, near-cnd member CaTh(POo)r,has the monazite structure as do the synthetic Sr and Pb analogues(Rose, 1980).Natural ThSiO4 occurs as the polymorphs thorite and huttonite, which has the monazite structure (Pabst and Hutton, I 9 5 I ; Taylor and Ewing, I 978; Speer,I 982). Previous work has shown that thorite can contain (wto/o) up to 20 UO,, l5 REE,O3,6.3 Y,O3,4.8 2rO,,5.8 CaO, 4.0 P,O5,and 2.1 AsrO, (Frondel, 1953; Robinson and Abbey, 1957; Staatzet al., 1976;Speer,1982).For most thorites, analy.tical data have been obtained by wetchemical methods on impure material, hindering the calculation of empirical formulas. To complicate matters, water is usually present at 5-16 wt0/0.As in zircon, the problem arisesas to whether the water occurs as OH or H,O (cf. Speer,1982). The major naturally occurring ABO. structure types include monazite, zircon, fergusonite, and scheelite.At given values of temprature and pressure,stability fields of the different structure types can be delineatedin terms of the ionic radii of the A and B cations (Muller and Roy, 1974;Aldred, 1984).In nature,monazite,huttonite, and xenotime are invariably crystalline, whereaszircon, thorite, and fergusonite often occur in the metamict state, amorphous to x-ray and electron diffraction (Palacheet al., 1944; Pabst, 1952; Frondel, 1958; Mitchell, 1973; Ewing, 1975; Ewing and Haaker, 1980). Such observations suggestthat in addition to annealing kinetics, both structureand composition affectthe ability of a particular phaseto sustain alpha-recoil damage(Ewing and Haaker, 1980). Thorite appears to be particularly susceptibleto low-temperature alteration and annealing to secondary, hydrated thorite (Frondel, 1953; Robinson and Abbey, 1957). An important aspect of this work involves the study ofradiation effectsin thorite using X-ray diffraction and transmission electron microscopy. Results are applicable to the study of long-term stability relations in crystalline nuclear waste forms and to the geochemistry of radiogenic elementscommonly used for age dating. Furthermore, detailed chemical data allow for an interpretation of the behavior of rare elementsduring pegmatite emplacement.

counts. Data were reduced following the empirical alpha-factor approach developedby Benceand Albee (1968) and Albee and Ray (1970). Standardsfor data reduction were ThSiOo Gh, Sil, UO, (U), YPO4 (Y), REEPO4@a, Ce, Nd, Sm, Gd, Dy, Er, Yb), ZrSiO,(Zr), NaNbO, (Na), apatite (Ca, P, F), vanadinite (Pb, V, C1),mimetite (As), olivine (Mg, Fe), stibiotantalire(Sb,Bi), manganotantalite(Mn), and anorrhite (Al). Infrared spectroscopy Infrared spectrawere recorded with a Nicolet model 6000 rr-rn spectrometer.Data were collectedover the range400-4000 cm t at room temperature.Sampleswerein the form of pressedpellets consisting of a finely ground mixture of -300 mg of KBr and -0.5 mg of thorite. Thermogravimetricanalysis WeightJoss measurementswere made on powdered aliquots of two samplesusing a DuPont model 951 rcl and model 990 recorder. Samplesweighing 1G-20 mg were heated in air from room temperature (-25 'C) to 1000 oC at a rate of l0 'Clmin. The instrument was calibrated using calcium oxalate as a standard. Transmissionelectron rnicroscopy Crushedgrains of sample HT5 were depositedonto a holey C film supportedby a 200 mesh Cu grid. Thin edgesofgrains were examined with a rnor 2oooFxtransmission electron microscope operatedat 200 kV. Electron-diffractionpatternswere calibrated using a polycrystallineAu film. High-resolution imageswere taken at 500 000 x using axial illumination and a I 00-pm objective lens aperture. Images were calibrated using a partially graphitized carbon-black standard. X-ray diffraction analysis Powdered aliquots of three samples were deposited on polished quartz (low background)slidesand scannedfrom 5 to 85" 2d using a scrNrAGautomated diffractometer.Sampleswere analyzed with graphite-monochromatizedCu radiation at 40 kV and 30 mA. The instrument was calibrated using BaF, (a : 6.198 A) and Si (a : 5.430825A). t attice constantswere refined using the least-squaresprogram by Appleman and Evans (l 973). All three sampleswere reanalyzedafter annealingat 1000 "C.

Pnrnocn-lpHy

Thorite was found in the quartz zone, quartz-lath spodumene zone, microcline-spodumene zone, and in cleavelandite replacement masses in the beryl zone of the Harding pegmatite in the western end of the main quarry. Detailed descriptions of these units can be found in Jahns and Ewing (1976, 1977) and Lumpkin et al. (1986). The thorium mineral described by Hirschi (1928) was reported to occur in replacement zones in spodumene, presumably within the quartz-lath spodumene zone. The mineral was described as dark brown to ochre-yellow masses or tetragonal prisms associated with albite, muscovite, ExpnnrnnrurAl DETAILS and quartz (Northrop, 1959). Most of our samples (HTl, HT2, HT4, HT5) were Electron-microprobe analysis Chemicalanalyses wereobtainedusinga reolz:: Superprobe found in the clevelandite unit where it replaces the beryl operatedat 15 kV and 20 nA and usinga 10-pmbeam.Each zone adjacent to the hanging-wall contact. Thorite occurs elementwascountedfor a maximumof 20 s or until a standard as anhedral to euhedral crystals 0.5-8.0 mm in maximum deviationof 0.50/o wasreachedon the accumulated numberof dimension. The color ranges from dark brown to yellow-

LUMPKIN AND CHAKOUMAKOS: THORITE.GROUP MINERALS

r40'7

/t

7i

Fig. l. Photomicrographsof a brown thorite crystal (3 mm) from a cleavelanditemass in the beryl zone. (A) View with planepolarized light shows subhedral outline and mottled texture. (B) Viewed with crossedpolarizers, some areaswithin the crystal are ofanisotropic crystals. isotropic whereasothers appear as fine-grainedaggregates

brown. Many crystalshave reddish-brown to pale yellow rims. Crystal morphology is simple, consisting of the prism, pinacoid, and pyramid forms. The prism and pinacoid tend to dominate, giving the appearanceof"cubic" morphology. Associated minerals are bladed albite, columnar lithian muscovite, qrtartz,and minor microcline. Mottled blue-greenand tan apatite crystalsup to 7 cm in length are present in a few specimens. In thin section (Fig. l) the thorite grains range from pale yellow to nearly opaque.Inclusions and fracture fillings of orange to red-brown iron oxides are common in many of the crystals.Observation with crossedpolarizers reveals that the thorite contains both isotropic and microcrystallinematerial. Both the ratio of isotropic to crystalline thorite and the size of the crystalline domains are highly variable from grain to grain. The amount of isotropic material is approximately 0-50 vol0/0.In grains free of isotropic material the crystalline domains approach a maximum size of 200 pm. Crystal rims are occasionally more crystallinethan their cores.SampleP20.l, mistaken for microlite by Chakoumakos (1978) is also from a cleavelanditemass in the beryl zone in the westernmost beryl adit. Sample P20.1 exhibits a distinctive texture resembling the recrystallization spherulites observed in metamict polycrase, an orthorhombic Nb-Ta-Ti ABrO6 oxide (Ewing,1974). A few samples(HT3) occur in the quartz zone as anhedral, tan to yellow massesup to 2 cm in diameter. One samplehad a thin brown rim of glassymaterial surround-

ing a pale yellow, powdery core. One 2-cm massin quartz showeda pronouncedsmoky aureolewith radial fractures extending outward into the host quartz. These thorite massesare enclosedwithin massive qvatlz, often associated with lithian muscovite. Other specimenswere found in the quartz-lath spodumene zone (HT7) and microcline-spodumene zone (HT6) as B Zl Th U REES Bi Ca Pb >A F

o H.o

3.86 16.2 0.83 4.81 0.04 53.1 0.62 0.09 1.89 2.62 0.00 674 0.36 0.34 1.08 92.68 0.45 92.23

2.96 15.8 0.60 5.14 0.07 528 0.88 0.07 174 2.76 0.09 6.86 0.11 0.34 1.09 91.32 0.46 90.86

0.166 3.834

0j27 0.522 0.01 6 0.257 0.003 0.92s 0.001 0.599 0.010 0.045 0.050 0 000 0 365 0.003 1.073 0.183 3.817

1.25

1.50

0.159 0.522 0.021 0.233 0.006 0.941 0.001 0.586 0.007 0.049 0.047 0.000 0.3s0 0.006 1.046

10.1 5.22 6.25 9.86 10.4 12.1 14.2 131 0.49 0.67 0.77 0.51 4.08 3.42 4.23 3.38 0.09 0.07 0.14 0.02 49.1 510 50.4 51.0 0.20 0.57 0.69 0.21 0 11 0.07 0.06 0.06 2.59 1.79 2.36 2.89 2.73 2.70 2.65 3.02 0.00 0.04 o.12 0.00 7.48 8.24 7.12 7.34 0.02 0.17 0.09 0.09 0.27 0.27 0.17 0.14 1.30 1.45 1. 6 0 1.O7 89.22 89.40 90 14 91.58 0.55 0.61 0.67 0.45 88.67 88.79 89.47 91.13 Structuralformulasbasedon O + F : 0 412 0.220 0.262 0.393 0.331 0.376 0.467 0.427 0.013 0.018 0.020 0.012 0.165 0 160 0.211 0.203 0.004 0.006 0.004 0.003 0.945 0.927 0.920 0.915 0.003 0.001 0.000 0.001 0.538 0.578 0 560 0.543 0.001 0.006 0.007 0.003 0.065 0.071 0.048 0.059 0.048 0.048 0.053 0.045 0.000 0.002 0.000 0.000 0.374 0.425 0.380 0.383 0.003 0.003 0.002 0.002 1.081 1.067 1.041 1.063 0.228 3.772

0.248 3.752

1.85

170

0.158 3.842 1.38

0.197 3.803 1.80

19.5 6.41 0.27 2.72 0.02 42.9 0.05 0.08 2.63 6.45 0.00 8.26 0.13 0.19 1.14 90.82 0.48 90.34 4.00 0.692 0.177 0.006 0.115 0.003 0.993 0.000 0.409 0.000 0.059 0.099 0.000 0.370 0.003 0.940 0.151 3.849

5.57 13.0 0.51 5.29 0.13 53.3 0.32 0.05 't.02 3.05 0.00 6.49 0.11 0.47 0.97 91.38 0.41 90.97

1.34

5.51 10.0 0.62 6.59 0.20 55.8 0.26 0.05 0.76 3.01 0.00 5.84 0.04 0.22 1.15 91.78 0.48 91.30

5.57 13.7 o.52 5.41 0.11 53.0 o.17 0.06 0.79 3.27 0.02 7.27 0.37 0.28 1.19 91.84 0,50 91.34 0.230 0.440 0.013 0.263 0.004 0.950 0.003 0.583 0.001 0.019 0.058 0.002 0.376 0.003 1.045

0.149 3.851

0.245 0.334 0,017 0.33til 0.004 0.933 0.004 0.638 0.003 0.020 0.054 0,018 0.314 0.003 1.054 0j82 3.818

1.49

1.44

1.40

0.235 0.428 0.013 0.263 0.004 0.943 0.003 0.601 0.003 0.025 0.055 0.014 0.343 0.006 1.050

0.181 3.819

5.34 11.5 0.76 6.87 0.10 55.2 0.31 0.05 0.52 3.28 0.01 6.66 0.24 0.24 1.13 92.20 0.47 91.73 0.223 0.372 0.020 0.338 0.004 0.957 0.003 0.613 0.003 0.014 0.058 0.000 0.346 0.003 1.040 0.176 3.824 1.33

- Analyses range from rim to core (center column) to rim in the order given.

analysis of coffinite from the Woodrow mine, Grants County, New Mexico revealed0.64 wto/oVrO, along with 2.18 wto/oP,O, and 2.34 wto/oCaO (Kim, 1978).Speer (1982) listed F and Cl as questionableminor constituents. Our electron-microprobe analyses provide conclusive evidence for the presenceof F in amounts approaching 1.6 wto/o(Tablesl-3). Microprobe analysesof thorites from the quartz-lath spodumenezone (HT7) and microcline-spodumenezone (HT6) are listed in Table 4. Sample HT7 most closely approachesideal thorite in composition, containing moderate amounts of UO, (4.0-8.6 wto/o)and PbO (2.54.8 wto/o),and negligiblewater. Only occasionalspotsshowed as much as 0.4 wto/oAlrO3, 1.8 wto/oCaO, and 0.4 wto/o F, possibly ow'ing to incipient alteration. In contrasl, sample HT6 from the microcline-spodumenezone contains relatively unaltered areas close to end member ThSiOoin composition and moderately altered areasenriched in P, V, As, and Ca (Table 4). Analytical totals indicate HrO contents of 0-l0o/o wto/0,clustering near 45 wt0/ofor altered areas.Alteration of sample HT6 ap-

pears to be related to partial albitization of the microcline-spodumenezone. Stoichiomefiy Structural formulas of the Harding thorites were calculatedon the basisof 4.00 O + F anions.Water contents are estimatedby diference and calculatedas HrO. Values of)A and 2B allow for further interpretation ofthe role of water in the structure. A-site cations. Thorites from the cleavelandite unit contain 0.4M.65 Th, 0.284.43 Ca, 0.004.10 REEs, 0.0M.08 Y, and 0.00-0.06 Bi atoms per formula unit. Concentrations of Zr, IJ, and Pb are below 0.01 atoms per formula unit. In contrast, yellow thorites from the quarlz zorrecontain 0.464.79 Th, 0.05-0.22 U, 0.050.22Pb, and 0.05-0.08 Ca atoms per formula unit (see Tables l-3). The relative variation of A-site cations gouped according to valence is depicted in Figure 2. Samplesfrom the cleavelanditeunit contain the greatest amounts of divalent and trivalent cations, approaching 45 molo/oA2* and 20 molo/oA3* cations in sample P20.1.

1410


TABLE3. Microprobeanalysesand formulasof thorite samplesHT3, HT4, and HTs HT4

HT3 PrOu V,O. AsrO.

sio, ZrO, Tho, UO, Al,o3 Y.o" REE,Os Bi,o3 FeO PbO F Subtotal LessO=F Total

0.15 0.09 0.24 17.5 052 628 455 0.76 033 026 0.64 124 004 384 0.61 93.57 026 93.31

018 0.08 0.26 18.5 UJC

61.4 4.75 0.69 054 o21 0.94 099 033 463 0.89 94 95 037 94 58

0.19 0.04 028 180 060 565 / .oc

0.73 056 0.21 0.72 090 019 /.oc

080 95.02 034 94.68

0.14 010 024 17.8 o.42 J/.O

18.8 0.47 0.51 0.54 0.79 o.82 0.18 16.4 0.05 94.87 0.02 94.85

517 14.4 0.61 4.96 o.23 53.9 0.19 o.12 0.'t4 0.10 4.O7 7.27 0 00 0 57 0.96 92.69 0.40 91.29

5.01 12.4 0.86 s.77 0.25 55.2 0.18 0.06 0.10 0.49 4.54 6.70 0.00 0.28 0.72 92.56 0.30 92.26

5.45 10.8 1 18 6 34 0.27 54.9 0.24 0.11 0.12 0.14 4.58 6.31 0j2 0.25 0 95 91.76 0 40 91.36

7.44 10.8 0.26 5.43 0.16 527 0.19 0.05 1.23 2.88 2.77 654 0.10 0.46 0.78 91.79 0.33 91.46

6.42 10.4 055 656 0.12 az,o 029 0 08 1.27 3.53 3.18 5.97 0.06 0.24 147 92.74 0.62 92.12

5.11 10.2 046 726 010 c5 / 0.27 0.00 0.81 3.19 0 62 5.28 0.00 0.35 094 90 30 0 39 89.91

Structuralformulas based on O + F:4.00 P AS 5l AI )B Zl Th

U Y REEs Bi Pb 2A F HrO

0.006 0.002 0 006 n oRo

0 048 1 021 0015 0.784 0.053 0 009 0 004 0 009 0.072 0.0s6 1 002 0 105 3.895 120

0.007 0.002 0 007 0.983 o.042 1.041 0.014 0.743 0.0s4 0015 0 004 0013 0.058 0.067 0.968 0.150 3.850 1.08

0.008 0 001 0 008 0 967 0.046 1.030 0.016 0.692 0.086 0.018 0.004 0.011 0.052 0.113 0.992 0 137 J.OOJ

0.93

0.007 0.003 0.007 0.976 0.029 1.022 0.012 0.470 0.2t5 0.015 0.011 0.011 0.050 o.214 0.988 0.010 3.990 0.88

0.214 0.466 0.015 0.243 0.008 0.946 0 006 0.600 0.001 0.004 0.002 0.051 0.379 0.006 1.049 0.150 3.850 1.24

0 216 0418 0.023 0.295 0.004 0.956 0.006 0.641 0.001 0.002 0.007 0.059 0 365 0 003 1.084 0.117 3.883 1.30

0.233 0360 0 032 0 320 0.008 0.953 0.006 0.628 0 003 0.002 0.002 0.058 0.338 0.003 | 040

0.311

0 152 3.848

0.122 3.878 1.38

1.45

0.3s3 0.007 0.269 0.002 0.942 0.004 0.591 0.001 0.031 0.051 0.033 0.344 0.006 1.061

0 266 0.337 0.014 0322 0.004 0 943 0.003 0 583 0.003 0.031 0.062 0.039 0.311 0 003 1.035 0.226 3.774 1.27

0.221 0.345 0.01 2 0.372 0.000 0.950 0.003 0.647 0.003 0.021 0.059 0.008 0.289 0.006 1.036 0.151 3.849 1.71

Nofe: SamoleHT3 is from the ouarlz zone: HT4 an HT5 are from the cleavelandite unit.

Sample HT3 from the quartz zone contains only 15-30 molo/oA2+ and l-3 mol0/0A3+ cations. Individual ,A,3* cations are plotted in Figure 3. SampleHT3 has Y as the dominant A3* cation, but contains significantamounts of REEsand Bi. Among samplesfrom the cleavelanditeunit, HT4 is Bi rich, whereasmost other samplesare enriched in the REEs. Relative abundancesof the REEs (normalized to Ce : 1.0) are summarized in Figure 4. The light REEs are dominant in all specimens, of which five showed Cedominant patterns and three showed La-dominant patterns. LalCe ratios range widely from 0.4 to 1.4. Low REE contentsof samplesHT3, HT4, HT6, and HT7 precluded determination of complete REE distributions; however, Ce was the major REE found in all four specimens. Prior to this work, Staatz et al. (1976) atalyzed four thorites from the Seeriepegmatite,JeffersonCounty, Colorado, and found all four to be Yb dominant. Staatz et al. (1976) also made a literature survey ofthorite analyses, finding five Ce-dominant, two Nd-dominant, and two Dy-dominant. Sample HT7 from the quartz-lath spodumene zone

closely approaches ABO. stoichiometry. The average compositionof the A site is 0.87 Th, 0.08 U, and 0.05 Pb atoms per formula unit. Maximum values of 0. l0 Ca and 0.02 Mn atoms per formula unit occur in small areas of the specimen and may be related to incipient alteration. Partial alteration of sample HT6 from the microcline-spodumene zone produced a wide range in the number of A-site cations: 0.73-0.87 Th, 0.01-0.04 U, and 0. l0-O.26 Ca atoms per formula unit. The amounts of Y, REEs, Mn, and Pb are generallybelow 0.01 atoms per formula unit (Table 4). B-site cations. Brown thorites from the cleavelandite unit have complex B-site compositions involving 0.120.70 P, 0.17-0.53V, 0.1l-O.38Si, and 0.01-0.04As atoms per formula unit. Al contents are generally below 0.01 atoms per formula unit. Yellow thorite HT3 has a relatively simple B-site compositionof 0.95-0.99 Si and 0.02-0.06 Al atoms per formula unit. Amounts of P, V, and As are below 0.01 atoms per formula unit (Tables l3). Except for small amounts of Al (A F HrO

0.31 0.28 0.21 1 90 032 746 175 o.27 0.00 0.33 0.00 2.06 007 0.46 u./o 100.42 0.32 100.10 0 013 0.009 0.006 0.964 0.016 1.008 0 008 0.862 0.020 0.000 0.006 0.000 0.112 0.006 1. 0 1 4 o.121 3.879

0.38 0.12 0.19 1 93 0.48 ov.4 2.21 0.62 0.00 0.11 000 265 0.06 0.28 0.3 96.03 0.10 95.93 0017 0 004 0 005 0.995 0.038 1.059 0.012 0815 0 025 0.000 0.002 0.000 0.146 0 004 1 004 0 042 3.958 U.bJ

7.00 4.24 2.29 6.00 4.60 4.12 0.55 1.80 0.75 7.81 11.8 11.5 1.57 2.60 0.54 65.7 61.2 67.8 1.67 1.84 0.76 0.23 o.17 0.87 0.04 0.00 0.00 0.25 0.01 0.18 0.02 0.00 0 00 3 81 | .YO 4.42 277 0.03 0.06 0.04 0.02 0.19 0.21 021 0.07 0.60 0.85 o.75 0.83 95.37 95.71 95 55 90.49 0.21 U.JO 0.32 0.35 95.16 95.19 90.17 95.36 Structuralformulasbasedon O + F : 4,00 0.300 0.184 0.088 0.103 0.201 0.161 0.140 0.005 0.015 0.020 0 004 0.050 0.396 0.909 0.626 0.590 0.014 0.028 0.011 0.053 0.926 1.034 0.951 0.987 0.039 0.026 0.067 0.014 0 758 0.739 0792 0.788 0.019 0.021 0.039 0.009 0.000 0.001 0.000 0.000 0.00s 0.003 0.000 0.003 0.000 0.000 0.000 0.000 0.207 0.152 0.106 0.251 0.003 0.001 0.003 0.003 1032 1.067 0985 0.965 0.100 0.139 0.131 0.137 3.900 3.863 3 861 3.869 0.79 1.73 0.77 0.77 2.03 014 0.15 17.7 1.03 67.4 3.41 0.47 0.00 0.19 0.00

0.00 0.07 0.04 18.3 003 69.5 6.67 0.06 0.07 0.37

0.00 0.04 0.02 18.7 0-o2 70.1 6.71 0.05 0.09 0.41

0.00 0.04 0.03 18.6 0.00 70.2 5 60 0 06 0.10 0.55

0.00 0 02 0.03 18.6 0.00 69.3 6.18 0.36 0.00 0.11

0.06 000 3.61 0.19 98.97 0.08 98.89

0.01 0.02 3.99 0.06 100.22 0.03 100.19

0.02 0.01 4 01 0.10 99.32 0.04 99.28

1.70 0.00 1.34 0.33 97.97 0.14 97.83 0.000 0.001 0.001 0.992 0.022 1.016

0.001 0.861 0.081 0.003 0.008

0.000 0.001 0.001 1.014 0.004 1.020 0.000 0.868 0.067 0.003 0.010

0.003 0.053 1.013

0.000 0.057 1.011

0.001 0.058 1.007

0.097 0.019 1033

0.033 3.967

0.010 3.990

0.017 3.983

0.057 3.943

0.000 0.002 0.001 1.003 0.004 1.010

0.000 0.001 0.000 1.008 0.003 1.012

0.001 0.866 0.081 0.002 0.007

0.000 0.842 0.073 0.000 0.002

zone,HT7 is from the quartz-lathspodumenezone. Note. SampleHT6 is from the microcline-spodumene

0.31 P, 0.00-0.21V, 0.00-0.05As, 0.39-1.00Si, and 0.01-0.05 Al atoms per formula unit, reflectingthe partially altered nature of the specimen(Table 4). Relative amounts of Si, P, and V are plotted in Figure 5. Except for samplesHT3, HT7, and unaltered areasof HT6, the B-site compositions of the Harding thorites are unique. Most data points of samplesfrom the cleavelandite unit have V as the major B-site cation (up to 60 molTo). A few analyseshave Si as the dominant B-site cation (up to 50 molo/o).The most interestingresults were found for sampleP20.1. Data points for P20.1 scatteralong a line of constant V/Si : 2.0 over a range of 10-70 molo/oP. Most data points, however, clusterjust inside the P-dominant composition field near42 molo/oP, 40 molo/oV, and 18 molo/oSi. Alteration of sample HT6 from the microcline-spodumenezone is revealed by the trend of data points that lie within the Si-dominant field along a line of constantP/V: 1.0 (Fig. 5). A literature survey of thorite and xenotime analyses indicates maximum substitution of 20 molo/oSi in xenotime and l5 molo/oP in thorite (Palacheet al., 1944;Robinson and Abbey, 1957; Frondel, 1958; Heinrich, 1963; Amli, 1975; Staatzet al., 1976).V was not determined

in theseanalyses,but is probably very low in abundance. Basedon X-ray fluorescenceanalysesthat showed l1 wt0/o Y and 3 wto/oV in thorite ("thorogummite") from the Evans-Loupegmatite,Quebec,Miles et al. (1971) suggestedthe substitution of - l0 molo/oV for Si. They also inferred the possible substitution of - l0 molo/oP for Si, presumably to balanceexcessamounts of Y. Miles et al. (1971) also described wakefieldite from the Evans-Lou pegmatite,but did not obtain a good analysisof the mineral becauseof impurities. In addition to SiOr, several weight percent of ThO, and UO, may in fact be present in wakefieldite (Miles et al., 197l, p. 401 and their Table 5). Using the unit-cell volumes of natural wakefieldite, synthetic YVO4, ThSiOo,and USiOo, we estimate a maximum possible substitution of -20 molo/oSi for V in wakefieldite. Anions and water. F contents of samples from the cleavelanditeunit rangefrom 0.07 to 0.25 atoms per formula unit. Sample HT3 from the quartz zone contains 0.01-0.16 F atoms per formula unit. Partially altered thorite HT6 from the microcline-spodumenezone contains 0.03-0.15 F atoms per formula unit. Relatively unaltered material (HT7) from the quartz-lath spodumene

t4l2


Th+U

A-Site

Y+ REE+Bi

Co+Pb

Fig.2. Ternary plot ofA-site cations grouped according to valence state. Samplesfrom the cleavelanditeunit cluster near 35 mo10/0 A2+ and l0 molo/o,A.3+cations, approaching45 molo/oA'?+and 20 mo10/o A3* cations in sampleP20.1. Samplesfrom primary lithologic units are dominated by 44+ cations.

zone contains only 0.00-0.06 F atoms per formula unit. Oxygen contents of all samplesrange from 3.75 to 4.00 atoms per formula unit. Water contents of the samples wereestimatedby differenceand calculatedas HrO. Samples from the cleavelandite unit contain 1.2-1.9 fJ2O moleculesper formula unit. Lower water contentsof 0.81.6 HrO moleculesper formula unit are inferred for sample HT3. Sample HT6 has an inferred water content of 0.0-1.8 HrO moleculesper formula unit, with higherwater contents associatedwith altered areas of the specimen. However, most analysesof altered material contain 0.60.8 HrO moleculesper formula unit. The role ofwater in the structure can be inferred from the HrO content and values of )A and )B by plotting on a ternary diagram (Mumpton and Roy, 196l). Figure 6 shows that data for thorites HT3 and HT6 scatter along the ABO.-HrO join. Data for samples from the cleavelandite unit scatterjust below the ABO.-H,O join, indicating that there are B-site vacancies and possible OH groups due to the substitution of 4H* for Sio*. The magnitude of this substitution must be small, becausemean valuesof )A and )B are 1.05and 0.95, respectively.Al-

though marginal, this departure from perfect stoichiometry appearsto be real. Based on counting statistics,the 1ostandarddeviations on 2A and 2B are both near +0.01 atoms per formula unit. Normalizing to )A : 1.00, we get >B : 0.90 and 0.10 B-site vacanciesby diference. The maximum amount of H* required to fill all B-site vacanciesis 0.4 + 0. I atoms per formula unit. The large standarddeviation arisesfrom uncertaintiesin estimating the total HrO content by diference. As discussedbelow, infrared spectroscopyconfirms the presenceof molecular water and is consistent with the presenceof hydroxyl specres. Infrared spectroscopy The rn spectrum (Fig. 7) of yellow thorite HT3 shows an absorption at 1630 cm-' that is identified as the H-O-H bending mode of molecular water. The O-H stretchingmodesof molecular water and hydroxyl species are indicated by the band at 3400 cm r, whose large breadth is evidence of molecular water and hydroxyl speciesthat are not in a single well-defined site or of a consistenttype (Aines and Rossman,1986).


x P20.1 OH T 5 A HT4 ^ HT3 . HT2 OH T l

r413

A-Site Trivolent Cotions AnAA

A

F"

o-rt

"#1

Bi

REE

Fig. 3. Ternary plot of the trivalent cations Y, REEs, and Bi (molVo).Note the gap betweenBi-poor and Bi-rich compositions.

The n spectrum of brown thorite HT5 (Fig. 7) is similar to that of sample HT3, but with additional absorption bands at 1540 and 1350 cm-'. The broad band at 3200 cm ' appearsto be a compositeofthe 3400 cm ' band due to O-H stretching in HrO and OH, plus an additional broad band at lower wave numbers from (POo;:-.The doubleband at I 540 and I 350 cm-' is characteristic of the carbonateanion, (COr)'-, suggestingthe presenceofan additional carbonatephaseas an alteration product of thorite. However, no carbonate phaseshave been identified by optical microscopy, X-ray diffraction analysis,or transmission electron microscopy.

grains removed from sample HT3. Figure 8 shows that U and Pb scatter near the l:l correlation line. Linearregressionanalysis of the data yielded a slope of 0.952 and a correlation coefficient of 0.973. Unfortunately, the powdery nature of the specimenhas made it difficult to prepare an adequatepolished section. As a result, we have not analyzedenoughofthe sampleto establishany spatial zoning pattern. Substitution schemes The major substitutions indicated by core-to-rim zoning in samplesHTl, HT2,HT4, and HT5 are ACaBV,oThBSi2and ACaBP,- AThBSiz(Tables 14, Figs. 2-6). In sample HT4 the substitution is predominantly of the first type as indicated by the trend ofdata points in Figure

Intracrystalline zoning Core-to-rim variations in composition of most thorite grains are minor. When pronounced zoning is observed, coresare richer in Si and Th, and rims tend to be enriched Harding in V, P, and Ca (i.e., sampleHTl, Table l). In general, thorites absolute concentrations of Y, REEs, and Bi are also higher in crystal rims. On a relative basis,the core-to-rim trend ct 1.O ofA3* cationsproceedsfrom REEsto Y to Bi alongcurved o paths (Fig. 3). UJ uJ The most spectacularzoning was observed in sample ct .6 P20.1. The zoning pattern is not concentric;compositional variations occur betweentwo opposite sidesof the grain. One side of the grain is rich in V, Si, and Th and has higher amounts of the minor elementsAs and U. The opposite side is rich in P with increasedamounts of Y, La Ce Nd Sm Gd Dy Er Yb REEs,and Ca. B-site cations follow the V/Si : 2.00 trend in Figure F B-site vacancies reach maxishown 5. and Fig. 4. REE distributions (atom ratios) of Harding thorites mum values near the middle of the grain and have min- normalized to Ce : 1.00. Individual data points are shown only imum values near the V-rich and P-rich edges. for LalCe. Minimum and maximum REE/Ce values are plotted Compositional variations were observed in several for Nd-Yb.

t4t4


HT3,7 x P201

1 g

o Ht5

^ ^ o o

B-Site

HT4 HT3 HT2 HT1

= 2

v HT6

t:

4000

3180 2360 1540 W o v e n u m b e r(sc m - r )

7?O

Fig.7. rr-n patternsfor yellowthoriteHT3 andbrownthorite HT5.

/zz'!z-zz'u

Fig. 5. Ternary plot of B-site cations Si, P, and V (molo/o). Data for sample P20.1 scatter along a line of constant V/Si : 2.0 from 10 to 70 mol0/0,clusteringnear 42 molo/oP. Most other data points for samplesfrom the cleavelandite unit lie within the V-dominant composition field, extending just into the Sidominant field. Samplesfrom primary lithologic units contain Si as the major B-site cation. 5. In samples HTl, HT2, and HT5 the two substitutions operate in roughly equal proportions. This is also true of the alteration pattern in specimen HT6. A general substitution scheme for these four samples is ACaBVBP r AThBSi2.The unusual zoning pattern in sample P20.1 is consistent with the maior substitutions BP - BV and

AREEBP- AThBSi.As shown in Figure 5, P increases along a line of constant V/Si : 2.0, such that a generalized substitution scheme can be written as AREEBP,AThBSiBV2. Obviously, minor substitutions like AYBP + AThBSi,AYBV + AThBSi,and ABiBV - AThBSialso play a role in the crystal chemistry of these samples. Approximate l: I variation of U and Pb in sampleHT3 provides evidence for the substitution APbAU6+- AThz (Fig. 8). Most of the U in this sample is probably present as the U6+ ion. A possibleminor substitution accounting for the occurrenceof Al and F in HT3 can be expressed as BAIXF- BSixO.In general,the substitution of F for O servesto charge-balanceexcesslow-valencecations in the structure, preventing instability due to underbonding at the anion site. Figure 9 presentsa summary of Harding thorite compositions expressedin terms of idealized end members. Samplesfrom the cleavelanditeunit contain 50-75 mol0/o

r HT'l, HT2, HT4, HTs oHt.' o = Averoge

c a o O.? f

E / ( = o *=O ""o-oo,o r

973

E

o o,l f

Hzo

A(oH)a

rA

Fig.6. Ternary diagramof 2A,28, and HrO (molo/o). Data for samplesfrom primary lithologic units scatteralong the ABO.HrO join. Samplesfrom the cleavelanditereplacementunit scatter just below the ABO.-HrO join, consistentwith a small amount of H* at the B site.

o.0 oo

o1 P b o t o m s / f o r m u l ou n i t

o3

Fig. 8. A plot of U vs. Pb (molo/o)for sampleHT3. Data points scatternear the I : I U:Pb line, well off the calculated 1300-m.y. U-Th-Pb line. The averagecomposition (open circle) does plot near the 1300-m.y. referenceline.

LUMPKIN AND CHAKOUMAKOS: THORITE.GROUP MINERALS

( T h ,U ) S i O c

Thsi04 o HTl H T z , H T 4 ,H T 5 o HT3 V HT6

A3+85+O4

tcoTh(85+o+)z

Fig. 9. Harding thorite compositions plotted in terms of idealized end members (molVo).Samplesfrom the cleavelanditeunit are dominated by the component CaTh(B5+On)r,where B : P and V. A sample (HT3) from the quartz zone is near