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New Data on Yttrocrasite from the Clear Creek Pegmatite, Burnet

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New Data on Yttrocrasite from the Clear Creek Pegmatite, Burnet American Mineralogist, Volume 62, pages 1009-10ll, 1977 Newdata on yttrocrasite from the Clear Creek pegmatite, Burnet County, Texas Wrr-soNW. Cnoor, III Departmentof Geologyand Mineralogy The Uniuersityof Michigan A nn A rbor, Michigan49 I 09 Abstract X-ray diffraction data for non-metamictyttrocrasite from the Clear Creek pegmatite, BurnetCounty, Texas,shows it to be orthorhombic,space group Pbcn, a = 12.862(7),b : 4.810(2),c : 4.571(3)A, V = 282.79As. The mineralis biaxial(-) witha = 2.131(2),4= 2.137(2),y : 2.A2(2),2V,: 60-70'.The strongestdiffraction lines are 2.147(100X600), 6.455(90X200),4.331(90X110), and 3.121(30)(310).The calculatedformula, based on ABr(O,OH)6. is (Yo..orREo.1a1Tho.116l.JoorrMno oorCao ,0nPb' o0r)(Ti1.636Feo o"2Wo ourTao oor)j(Or.ous OHor*).X-ray, optical, and chemicalanalysis suggests that ytirocrasiteis the titanium-rich 'nd-member of the euxenitegroup of rare-earthmultiple oxides. Introduction fergusonite and cyrtolite intergrown within biotite plates.Three distinct crystalswere found, megascop- Yttrocrasite, an oxide of titanium, thorium, and ically distinguishablefrom the other rare-earthspe- the yttrium-group rare earthswas discoveredby Hid- by their color (green-brown) and their shape den and Warren in 1906. The original description cies (orthorhombic-dipyramidal). included an incompletewet chemical analysisand a few physical properties.The type locality, a granite Physical properties pegmatitetwo miles eastof the famous BarringerHill rare-earthpegmatite, was inundated during the con- Yttrocrasite occurs as prismatic crystalswith ob- structionof Lake Buchananin 1936.Additional data servedforms {100},{010}, and {001}.Fracture is un- on yttrocrasite have not been obtained becauseof even to subconchoidal,and there is no observable the combination of scarcityof material and its char- cleavage.The color is green-brown and the crystals acteristicmetamict state. are typically opaque but are transparentin thin frag- ments. A dull exterior alteration coating is common. Occurrence The streak is pale-brown.to greenish-grayand the The pegmatitesof the Llano Uplift region of cen- hardnessis 5t/z-6. The specific gravity (21'C) was tral Texasare well known assources of a largevariety determined by repeatedmeasurements using a Ber- of primary and secondary rare-earth minerals. In man balanceto be 5.315(5)on unalteredmaterial, May 1976the author discoveredthree crude crystals 4.624(5)-5.149(5)on metamict specimens,and 4.26\5) of yttrocrasite at the Clear Creek pegmatite,Burnet on alteration coatings. The habit is prismatic, the County, Texas (Crook, 1977). Associated minerals morphology being similar to polycrase,a member of include gadolinite,behoite, rowlandite, tengerite, ytt- the euxenite group. Although a tabular habit has rialite, allanite, samarskite,fergusonite, bastnaesite, been reported for specimens from the Southern cyrtolite (zircon), uraninite, thorite, "gummite", Pacific Silica Quarry, Nuevo, California (Murdoch thorogummite, and texasite. and Webb, 1956),it was not observedon those from Yttrocrasite is found in perthitic blocky microcline Clear Creek. and biotite associatedwith hematite staining within The mineralis biaxial(-) with a : 2.131(2),0: the inner intermediatezone of the pegmatite.Small 2.137(2), t : 2.142(2), 2V, : 60-70" (Na light). areaswithin the perthite are vuggy and are filled with Pleochroismis X : colorless,amber to palebrown, I smoky quattz, hematite, biotite, and fergusonite. : brown, Z : brown, green-brownto opaque with Clear Creek yttrocrasite is clearly associatedwith absorptionX> Y> Z. I 009 l0l0 CROOK: YTTROCRASITE Table L X-ray powder data for yttrocrasite total cations, consistent with the euxenite group, ABr(O,OH).. There is considerablesubstitution in all hk1 -obs -ca1c sites,and the chemicalformula of the analyzedmate- rial is (Yo..orLao.oo3Ceo 200 6.455 6.431 90 orrProoorNdo.ooorSmo.orrGdo.oorn 110 4.331 4.310 90 Tbs.e6lDlo.orsH06o62Ere.e2, Tmo oorYbo.oouLuo.oorTho.r* 3r0 3.r2J, 3.126 30 IJo.ornM no.oorCA0. (Tir.rr. 2II 2.954 2.944 20 ro4Pbo.006) Feo.orrWo.o.2T ao. oo, ) o02 2.401 2.403 20 (O5.b8oH..eor). Thus the formula is ideally AB, : : r02 2.369 2.363 20 (O,OH). with A Y, Th, Ca, U, RE; B Ti, Fe3+, 510 2.24L 2.24L 30 and W. Both metamict and non-metamictmaterial 600 2.147 2.t44 100 420 r. 865 1.863 30 were found to have essentiallythe samecomposition. o22 r. 658 1.656 10 912 l. 189 1.186 5 11,10 l. L32 1.133 10 Table 2. Microprobe analysis yttrocraslrc(l) 4l-4 1.094 1.093 10 of and samarskite (2), Burnet County, Texas CuKc radiation, Phillips-Norelco dlffractooeter. Intensities visually estinated. Ca0 7.71 5.08 X-ray crystallography Mco 0.08 Single crystalswere studied using oscillatingcrys- Mn0 0.14 tal, Weissenberg,and precessionmethods. Unit-cell Na20 0.10 parametersfrom a least squaresrefinement are a : Kzo 0.05 12.862(7),b : 4.810(2),c : 4.571(3)4, aib..c = F"zo3 7.63 10.88 2.674:1:0.950,V = 282.79A3. Extinctionsobserved Ti02 49.56 2.t7 were:{0kl}, k:2n+ 1;lh\ll,l:2nt- l:lhk}l,h+ k A1-0. 0.15 = 2n l- 1. The spacegroup Pbcn is proposed.Powder w0. 1.79 data from a Phillips diffractometer (Table I ) were si02 0.93 indexed using refined lattice parameters from the Pb0 0.38 t.62 single-crystalstudy. Ta.0, 0.06 6.04 The spacegroup, Pbcn, is consistentwith members Nb^0_ 0.02 4r.29 of the rare-earthmultiple oxides, and Strunz(1970) ThO2 I.12 1.95 has tentativelyclassified yttrocrasite as a member of Uso8 2.36 16.26 the euxenitegroup. All three of the cell parameters ZrO2 0.21 are consistentlyshorter than euxenite and do not RE^0. 29.05 1.2.53 match any of thosereported for group species.How- Hzo 5.07 ever, axial ratios are similar, with those of yttrocra- sitebeing almost identical to samarskite(2.75:l:0.949). TOTAL 100.89 98.70 Y^0- 79.92 7.69 LaZO 0.14 0.02 Chemical analysis 3 C"203 0.63 0.t8 An electron microprobe analysis (Table 2), con- Pr^0- 0.13 trace ZJ ducted with 150 nA specimencurrent and 15 kV Nd-0. 0.34 0.19 excitationvoltage, helpedidentify yttrocrasite.Water St2o3 0.88 0.63 was measuredby weight loss on ignition. The iron is Gd203 2.09 0.66 assumedto be present as Fe3+, becausethis is the Tb.0. 0.07 0.26 valence of iron reported in the originally-described DYzo 2.55 0.64 yttrocrasitematerial (Hidden and Warren, 1906). s Ho20 0.10 0.13 Correctionswere made using Rucklidge'sEMPADR 3 E 1.63 0.68 VII program (J. Rucklidge and E. L. Gasparrini, 2o3 Tnro, 0.14 0.05 Department of Geology, University of Toronto, writ- Yb.0- 0.26 ten communication,1969). 1.36 Lu^0. 0.77 0.04 Electron microprobe analyseswere normalizedto 3 CROOK: YTTROCRASITE l0l I Discussion chargedistribution for the I polyhedra.Ideally the I Thesedata suggestthat yttrocrasiteis the titanium- sitesare eight-coordinated, but variationin thel-site and rich, niobium-, tantalum-poor end-memberof the ionic radii causesdistortion of the octahedra, (Y3+ are more stablein six-fold coor- euxenitegroup of rare-earthmultiple oxides.Optical smallercations ) and physical propertiesare generallyconsistent with dination.Thus theI siteis x-coordinated, the coordi- other members of the series.Metamict yttrocrasite nation numberdepending on the elementspresent. differsfrom non-metamictmaterial only by its lower Assuming a fersmite-like structure, the zig-zag joined specificgravity, isotropism(n : 2.14),well-developed chains of 8-site octahedraare by common conchoidal fracture, and alteration coatings.Chem- corners to form two-dimensionaldouble layers. (RE) ical composition is apparently identical for both Theseare interlinked by eight-coordinatedlarge (Alexandrov, The cell pa- states.X-ray data show yttrocrasite to have space cations 1960). shortened group Pbcn, consistent with the euxenite group. rametersin yttrocrasitemust be due to considerable (Ti) However, all three lattice parametersare significantly corrugation of the zig-zagchains ofB octahedra, smallerthan any of thosereported for euxenite,poly- resulting in an overall contraction of the structure. crase,samarskite, fersmite, etc. The reduction in cell References size is proportional for all three axes, and the axial Alexandrov, V B. (1960)The crystal structureof fersmite.Doki. ratio of yttrocrasiteis very similar to all membersof Akad Nauk. USSR.132,669-672. the AB,(O,OH). series. Crook, W W., lll (1977)The Clear Creek pegmatite:a rare-earth Considerationof the ionic radii of substitutingele- pegmatitein Burnet County, Texas.Mineral. Record,8, 88-90. mentsin eitherthe A or B sitealone does not account Graham, J. and M R. Thornber (1974) The crystalchemistry of for the significantlysmaller cell size in yttrocrasite. complex niobium and tantalum oxides. I. Structural classifica- tion of MOz phases Am. Mineral., 59, 1026-1039. Members of the euxenitegroup crystallizein a colum- Hidden, W E. and C. H Warren (1906) On yttrocrasite,a new bite-like layer structure in which the I cations occur yttrium-thorium-uranium-titanile.Am. J. Sci.,22, 5t5-519. every third layer. This layering is parallel to closest- Murdoch, J and R. W Webb (1956)Minerals of California.Calif. packed planesof zig-zagchains of B octahedra(Gra- Bur Mines, Bull , 173, 351. ham and Thornber, 1974).Slight adjustments in the Strunz, H. (1970) Mineralogische Tabellen Akademische Ver- lagsgesellschaft,Leipzig oxygenatoms changethe coordination number of the I ions to eight. However, all membersof the series are characterizedby extensivesubstitution in the 14 Manuscript receiued, July 30, 1976, accepted site, which results in variable bond lengths and for publication, May 24, 1977.
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