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Texasite, a New Mineral: the First Example of a Differentiated Rare-Earth Speciesl

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Texasite, a New Mineral: the First Example of a Differentiated Rare-Earth Speciesl American Mineralogist, Volume 62, pages 1006-1008, 1977 Texasite, a new mineral: the first exampleof a differentiatedrare-earth speciesl Wrr-soNW. Cnoor,III Department of Geology and Mineralogy The Uniuersityof Michigan, Ann Arbor, Michigan48l09 Abstract Texasite,PrrOr(SOn), occurs as a supergenealteration product of primary rare-earth mineralsat the Clear Creek pegmatite,Burnet County, Texas, and at the Rode Ranch pegmatite,Llano County,Texas. Associated minerals include gadolinite, allanite, yttrofluor- ite, bastnaesite,behoite, rowlandite, tengerite, and chlorine-bearingjarosite. Crystals are small(0. I mm), translucent,and apple-greenin color.It is orthorhombic,space group Imm2 or 1222.The unit-cell constants area:4.139(5), b:4.243(5), c: 13.431(t5)A,Z:2.The strongestlines in the X-ray powderpauern are 6.72(3)(002),4.051(7X0t1),3.960(6)(l0l), 3.358(6X004),3.082(10X013),2.238(2)(006),and 1.980(2)(202). These areessentially identical with thoseof syntheticPrrOr(SOn). Texasite is biaxial(-), 2V : 26-31.,with refractive indicesa : 1.862(2),P : 1.917(2),7 : 1.921(2).Density (calc) is 5.769.The name is in honor of the stateof Texas,the locationof the BarringerHill rare-earthpegmatite district. Introduction Burnet County, Texas (38'48'N, 98o20'W). Addi- In 1975 the writer discovered a 5 cm X 5 cm tional specimenswere also found at the Rode Ranch specimen of a supergenealteration crust between pegmatite,Llano County, Texas, some 26 miles west large crystalsof allanite,yttrofluorite, and gadolinite of the type locality. The Clear Creek and Rode at the Clear Creek pegmatite,Burnet County, Texas. Ranch pegmatitesare two of the rare-earthpegma- Associatedminerals in the alteration assemblagein- tites for which the region is noted. Rare-earthminer- clude bastnaesite,behoite, rowlandite, tengerite,and als from this province have beendescribed by Hidden chlorine-bearingjarosite. In the alteration several and Mackintosh(1889), Hidden (1905). Hess (1908), small green crystals of an unknown specieswere Landes (1932), Ehlmann et al. (1964), Ewing and found intimately associatedwith tengeriteand jaro- Ehlmann(1973), and Crook (1976,1977).Large crys- site. Further investigationof this mineral indicated tals up to 300 pounds (Hess, 1908) of gadolinite, that it was a new rare-earth oxysulfate. fergusonite,uraninite, yttrialite, yttrocrasite, zircon The name, texasite,is in honor of the state of (cyrtolite), samarskite,polycrase, and allanite have Texas,the location of the famous Barringer Hill rare- been reported. Alteration by both late pegmatitic earth pegmatite district. The mineral and the name solutionsand by oxidizing surfacewaters is common. have been approved by the Commission on New Secondaryminerals include thorogummite, behoite, Minerals and Mineral Names, IMA. Single speci- bastnaesite,doverite, synchysite,cordylite, tengerite, mens of type material have been deposited in the lanthanite,and rowlandite. collectionsof this department and in the Smith- Texasiteoccurs at the Clear Creek pegmatiteon an sonian Institution. alteration rind between crystals of allanite, yttro- fluorite, and gadolinite. Many of the small cavitiesin Occurrence the alteration material are lined with 0.1-0.4 mm Texasite occurs on one specimencollected at the crystals of texasite,intergrown with tengerite,fluor- Clear Creek pegmatite,7 miles northwest of Burnet, ine-bearing jarosite, and chlorine-bearingjarosite. ' Jarosite shows Contribution No 337from the MineralogicalLaboratory, De- distinctive selectivereplacement of partment ol Geology and Mineralogy, The University of Mich- hydroxyl by both fluorine (as much as 0.92Vo)and igan,Ann Arbor, Michigan48109 chlorine (from 0.08 to 5.3Vo)and apparently repre- I 006 CROOK: TEXASITE sents a new solid-solution series in the alunite- Table L Powder X-rav diffraction data for texasite iarosite familv. svnthFtlc Pr2o2(Sn1,)* mexaslte** Physical properties hkl dobs d".1" 1/r o hk1 dohs dcalc r/7o Texasite occurs as radial stellate groups and as 0o2 6.72 5.775 a02 6.72 6.72 30 ',i^,1". individual bladed crystals up to 0.4 mm long and oLr 4.053 4.049 70 011 4.05t 4.0q5 101 3.960 j.957 60 lot 3.c60 3.955 t^. elongate along the c axis. Crystals of texasitehave oo4 3.360 3.357 6c) co4 3.358 3.358 6. or 3.081 o8r loo 013 3.080 3.08? 10. two equallyabundant morphologies, both tabularto 3 3. but 103 3.043 3.0110 5 103 3.040 3.039 a {010} one type is terminatedby a {011}prism and 110 2.a66 2.965 5 110 2.965 2,e63 ' the other by Indiceswere obtainedby gonio- 112 2.714 2.7r? 10 1r2 2.7A8 2.7rr 1c {101}. 015 2,269 2.268 10 0r5 2.27I 2.270 lC metric measurements.Mohs hardness is approxi- 105 2.255 ) 2qa 5 mately 2t/2,and the crystals are brittle with a well- 0c6 2.23c 2.238 2A fr6 2.2?8 2.278 20 o2a z.tztl 2.r2j 10 f20 2.r2\ 2.r22 10 developed {010} cleavage.Crystals have a vitreous 022 2.425 ?.025 5 422 2.031 2.423 5 luster, perfectlytranslucent and apple-greenin color. 242 r.979 1.979 2A czz 1.980 )..978 20 211 1.843 1.843 5 2rr 1.843 r.843 5 The streak is pale green. 116 7.787 r.286 5 Optically it is biaxial negative,refractive indices for t23 1.7)J2 1.741 5 : r07 NaD area: 1.826(2),0: 1.917(2),7 1.921(2),2V 2r3 1.719 1.718 10 2r3 r.7r9 1.718 IO : 26-31". It is pleochroic:a : colorlessto pale gray- green, B : pale green to greenish-gray,7 : pale * Gulnler canera data (J. l{aschke, Ders. comn., 1976 green. The optic plane is parallel to {010} with the *x CuKo radlatlon: 114.6 nn Gandolfl camera. optic orientation a : Z, b : Y, c : 1. fntensltles v1sual1v estlmated. Due to the combination of small crystal size and the high specificgravity, direct determination of the Chemicalanalysis specific gravity on both natural and synthetic Electron microprobe analysis(Table 2) was usedin PrrOr(SOn)was not possible,Density (calc) is 5.769 the identificationof texasite,due in part to the lack of for texasiteand 5.762(calc) for syntheticPrrOr(SOr). sufficient material for a standard chemical analysis. The specificgravity was calculatedusing a valueof Z The analysiswas conductedwith 150 nA specimen : 2 as determinedfrom partial crystal-structureanal- current and 15 kV excitation voltage. Microprobe ysis.Neither natural nor syntheticmaterial fluoresces standards used for the following elements are: Pr under ultravioletradiation. [PrrO3and syntheticPrrOr(SOr)],La(La"O, and syn- thetic LarOH,rNOr),Ce (CezOs),Nd (Ndros), and S X-ray crystallography (galena).Corrections were made using the EMPADR Unit-cell parameters from Weissenberg photo- Table 2. Chemical analvsesof texasite graphs using CuKa (1.54051A) radiation with esti- mated standard deviations in parenthesesare a = 4.139(5),b : 4.243(5),c : 13.431(15)A, Z : 2, v : 1 2 j 4 235.87A3. Lattice parameterswere obtained by ex- Pr20, Bo.B7 8o.60 80.36 80.47 trapolation as a function of cos2d(0-90') for ft00, 0.08 o. o6 0.11 0k0, and 00/ reflectionsfrom zero-levelWeissenberg n n? photographs. The lattice parametersfor texasiteare CerOj 0.03 in good agreementwith data obtained from Weissen- Nd.o. 0.01 0.03 0.02 berg photographsof syntheticPrrOr(SOn)(c : 4.141, so^ 19.r6 19.50 r9.89 1,9i 53 b : 4.247, c : 13.429A) (Haschke,personal commu- t nication, 1976).The spacegroup is either Imm2 or TOTAL 100.15 100.19 I00.41 100.00 1222(see below). The only extinctionsobserved were those correspondingto the body-centeredlattice, h -t I 2) lllpen liraclr ncomtiltp Rrrtnpt llnrrntv k + I : 2n t l. IndexedX-ray powder data are listed Texas. rn Table l. Indiceswere obtained using latticepara- 3) Rode Raneh pegmatlte, Llano County, meters from the single crystal study. The powder Texas. data are essentiallyidentical to those of synthetic 4 ) Ideal- Pr.o^ ( S0,,) . alI Pr,O,(SOo). 1008 CROOK: TEXASITE VII program (J. Rucklidgeand E. L. Gasparrini, Texasiteis relatedto syntheticPrrOdSOo) by an University of Toronto, written communication, order-disorderrelationship involving a differentori- 1969).Tests for chlorine,fluorine, additional rare- entationof the anionicgroup alongthe c direction. earthelements, and waterwere negative. Texasite is This superstructure,which occursin the synthetic insolublein mineralacids. phase,indicates a reductionin idealsymmetry, al- though the substructuresare identical.The super- Discussion structurereflections are limitedin numberand quite Rare-earthminerals are typicallydifferentiated to weak (Haschke,personal communication, 1976). someextent, expressing a preferencefor eitherthe Many rare-earthoxy- and hydroxysulfatesand ni- lighteror heavierlanthanides. However, even strongly trateshave similar types of superstructures(Peacor, selectiveminerals such as bastnaesite(Ce) and personalcommunication 1976). In each case,the thalenite(Y) commonlycontain tracesof all the rare-earthpolyhedra are translationallyperiodic, rare-earthelements and only approximate their ideal- whereasthe sulfateor nitrategroups have a period- izedchemical formulae. In this regard,texasite is a icity whichis somemultiple of that of the rare-earth notable exception.Not only are the rare-earths polyhedra. stronglypartitioned to the cerium-groupelements, but in all analyzedmaterials, praseodymium accounts Acknowledgments for 99.9 percentof the total rare-earthspresent. The writer thanksDr. John Haschkeof the Departmentlof Chemistry,The of Michigan,for theX-ray Haschke(personal communication, 1976) suggests that University and struc- tural informationof syntheticPrzOdSOr). I also thank Dr. EricJ. a uniquecrystal-chemical situation relative to theco- Esseneand John Geissman for their adviceon electronmicroprobe ordinationgeometry of the praseodymiumion is re- analysis.Special thanks is extendedto Dr. D. R. Peacorand Dr. sponsiblefor the partitioning of the rare-earths. RolandRouse for theircritical reviews of thismanuscriot. Haschkehas synthesized all of the rare-earthoxysul- fates,and hasfound that the praseodymiumoxysul- fateis by far themost stable and easiest to synthesize.
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