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A Terrestrial Source of Ureyite

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A Terrestrial Source of Ureyite American Mineralogist, Volume 69, pages 1180-1i,83' 1984 A terrestrial sourceof ureYite CsIu Met Ou YeNc Department of Geography & GeologY University of Hong Kong Pokfulam Road, Hong Kong Abstract Ureyite, a chromium sodium pyroxene of the jadeite group, has been detected in four samples of opaque bright green to dark green Burmesejade. It is a fine-grained fibrous aggregate,intergrown with amphibole, chromite and other minerals.The specificgravity is 3.51-3.52(obs.) and density 3.55 g/cmr (calc.). It has very strong pleochroism(yellow- : green)and is biaxial(+) with 2V = 70" (obs'), a = 1.722,P = 1.734'v l'745' The Mcissbauer spectrum shows that iron is trivalent (fenic) only. Unit cell parame.ters : calculatedfrom X-ray powder diffractometerdata are a : 9.533,b : 8'678,c 5'2654, B : 107.49".Electron microprobe analysesshow that it is complex in chemical composition and rich in Al and Fe. The formula for ureyite can be expressedas (Nao.qrCao.oz)oss(Cro.o Als 17Fes.1sMgs.s3)6.eeSi2 s2O6. Introduction minor amounts of chromite and an amphibole mineral. Most of the ureyite in sample B is in the form of tiny Ureyite is a sodium chromium pyroxene(NaCrSizOd. fibers, between 0.05 and 0.1 mm in length, intergrown It has up to now been known in nature only as a rare with the amphibole mineral. Therefore, it was very diffi- accessoryconstituent of someiron meteorites.It was first cult to separatethe ureyite from the amphibole. described(and namedkosmochlor) by Laspeyres(1897). Sample C is a large sample of emerald green jade It was later investigatedby Frondel and Klein (1965)in weighing about 0.5 kg. It is composedof 50% ureyite, iron meteorites and they gave the mineral the name about 30-40%oyellow amphibole and a small amount of ureyite, after ProfessorH. C. Urey, in place of kosmo- jadeite and chlorite. chlor. Ureyite has been synthesizedin the laboratory by Sample D measures about 40 x 40 x 3 mm. It is several mineralogists. composedof 50-60%ureyite, 30Voamphrbole and a small On the basis of unpublishedstudies D. V. Manson amount of jadeite and albite. The minerals in all of these (1979) suggestedthat the emerald green mineral which sampleshave been identified by microscope examination composedthe Maw-sit-sitjade from Burma might have of thin sectionsbut only sampleA was used for the X-ray the samecomposition as ureyite, but no definiteevidence analysis,the electron-microprobeanalysis and the Moss- was ofered. baueranalysis. Recently, ureyite has been identified in four samplesof opaquedark greento bright green Burmesejade, and a series of mineralogical studies of this terrestrial ureyite Chemistry are now reported. The scanning electron microprobe was used to deter- mine the chemical composition of individual grains of Specimen descriPtion ureyite from the polished thin sections. For the electron The ureyite sampleswere obtained in the jade market microprobe analysis, a JCXA 733WDS with an -operating of Hong Kong. They are opaque, dark green to bright voltageof 15kV and a beamcurent of2 x l0-2 ga were green Burmese jade. All are compact polycrystalline used. The data were corrected by a computer program aggregates,with grain size between0.03 mm and 2 mm. using Bence-Albeefactors. The standardsused in the Sample A is composed mainly of ureyite. A small analyseswere jadeite for Na and Al, orthoclasefor K and amountof chromite was detectedby microscopeexami- Si; glass for Ca, Mg; titanioferrite for Ti; pyrolusite for nation; this was easily separatedusing a Frantz isody- Mn; chromitefor Cr and Fe. namic magnetic separator. The total weight of sample A The chemical compositions of seven grains of ureyite was 2 g. (Table 1) show an increase of chromium content with a SampleB consistedof two pieces,each about 19 x l0 decreaseof iron and aluminium. This indicates that they x 2 mm and each composedofabout 70-80% ureyite and may belong to the NaCrSi2O6-NaFeSizOeand 0003-004x/84/ll l2-r 180$02.00 I lEo YANG: TERRESTRIAL UREYITE I l8l Table l. Chemical composition of Burmese ureyite as determined by the electron-microprobeanalysis (wt.%) Ul-a Ul-b Ul-C Ut-d Ul+ Ul-F UI-8 Ul-av* sto2 54.79 52,58 55.E9 55.50 55.56 55.03 54.33 54.El A1203 4.49 1.JZ q.)) 8.r8 A.LO 2.3L 2.20 4.02 12.92 11.49 L4.64 12.5r L3.20 1r.52 14.30 L2.94 "q0 c$ 0,45 0.31 0.55 0.99 0.55 0.42 0.37 0,54 l{eP 0.35 o.54 0.84 0-58 0.62 0.26 0.27 0.49 F.203 3.57 3.31 3.43 6.$ 3.67 2.68 2.23 3.61 CrzO3 23.50 29.37 20.t6 16.30 22.O5 28.00 26,n 23.67 tho 0.01 0.00 0.06 0,05 0.03 0.00 0.02 o.o2 K20 0.00 0,00 0.05 0.01 0.04 0.00 0.00 0.02 Pz05 0.00 0.00 0.06 0.02 0.L2 0.01 0.06 0.09 rro2 0.02 0.0r 0.03 0.05 0.06 0.04 0.02 0,03 Toral 100.10 99.93 100.28 100.55 100.11 100.27 I00.10 100.t9 Fig. l. Nomenclatureof the solid solution series:NaCrSi2O6 NaFeSi2O6-NaAlSi2O6.a - Ula; b - Ulb; c - Ulc; d - Uld; e - * thts ls the @o val@ of 7 Sra1n€ (a-f) of a sa41e. Ule; f - Ulf; g - Ulg; V - Uav. (Soulce: L€boratory eallEts ln the Acedery of Geological Sclhes of Chln., B€lJlnt, AutBr 1982) Physical properties The mineral occurs both as short prismatic crystals with length up to 2 mm and as aggregatesof tiny fibers NaCrSi2O6-NaAlSi2O6isomorphous series. These series, (Fig. 2). It has perfect cleavagein two directions parallel which may not be continuous(see Abs-Wurmbachand to {ll0} intersectingin 85' or 95', and a pronounced Neuhaus,1976), have not previouslybeen found in natu- parting parallel to {001}. The specific gravity is 3.55 ral terrestrial occurrencesof clinopyroxene group miner- (calculatedfrom the unit cell parameters);3.52 (measured als. by the micro-heavyliquid method)or 3.51(measured by Figure I shows the relationship between aegirine the microhydrostaticmethod). (NaFe3*Si2Oo).Jadeite (NaAlSizOojand ureyire (NaCr Si2O6).As plotted in Figure 1, the resultsfor the seven grains analysedall fall into the chemical field of ureyite. Optical properties The plotted compositionsare more or less on a line which The refractive indices vary with chemical composition. passesthrough the apex (NaCrSi2O) and b lies at Ur66 For monochromaticlight (Na) they are a: l.7l-1.73, F (i.e., 86 mol.VoNaCrSi2O6). According to Abs-Wurm- :1.12-1.74, "y= 1.73-1.75.For sampleA theyare a: bach and Neuhaus (1976), Urs6 is the critical value at 1.722,B : 1.734,y : 1.745.The mineralis strongly which the jadeite-ureyite solid solution has the richest pleochroic, with o yellow, B blue-green, y bright emerald ureyite component at 880"Cand I kbar pressure. Further researchon this point would be worthwhile. The average of the seven microprobe analyses of the ureyite in the Burmesejade gives the following formula: (Nao.qlCao.oJo ss(Cro6eAls 17Fe6 roMgo.o:)o eeSi2 6206. NaCrSi2O6co nstitttes 7OVoof the total quantity. Compared with the chemical composition of ureyite in meteorites this ureyite contains appreciableAl and Fe. Unit cell parameters The X-ray powder ditrraction pattern (Table 2) was obtainedwith a D6C diffractometer, with Cui(a radiation, NaCl as internal standard,2'per minute scanningspeed, an operating voltage of 40 kV and current of 15 mA. The unit cell parameters determined by the least squares methodare a : 9.533(3), = : b 8.678(2),c 5.265e\A and Fig. 2. Scanning electron photomicrograph of terrestrial = 107.49-+0.002". F ureyite (magnification 10000x ). l 182 YANG: TERRESTRIAL UREYITE Table 2. X-ray powder diffraction data for ureyite; d values are Table 3. M<issbauerparameters of ureyite inA 3+ u' (r.$ 4 coordlnatlon) ts. (F 6 c@rdlurlon) d, - , d, . nrl (oDal (cgr. l I.S. q,S. Total area Z r.s. Q.s. Totel area 5 6 5 ,28 6.278 1r0 0.33 r.28 0.37 0.05 0.30 0.34 0.37 0.95 7 4, 35 4,339 020 0.05 0.01 0,00 0.00 0.01 I 3. 388 tqurlty 0.02 I 3.257 3. 283 o2L 1 220 I.S. : Isour shlft m's-l Q.S. : Q@drt4ole sPllttlng l0 2,942 2,95L 22L 3 Llne rldth8 @'s 9 2.857 2. 859 ?11 1 2.696 l,quilty (Source: Laboretory aalysla in Ioetltute of Geology Aca'leda Slnlca' BelJlnS' SePteder 1981) 8 2.508 002 2.441 I 311 j 2 , 1809 11t 2.0849 2.0919 JJI the experimental analysis when the expected standard I 7.9976 lqurlty deviation in the result of the electron microprobe analysis 1 r.9877 1.9915 04r is considered. t L.9434 L,9461 L32 I 1.94r1 1.9098 241 Paragenesis I Iqurtty polycrystalline I 1.7048 150 All the four ureyite-bearingsamples are 4 r.5208 7.5234 aggregatesshowing granular and/or fibrous texture. They I 1.5901 r.5896 contain 50-95% ureyite, with exsolved jadeite in some 1 r,4924 L.4924 r33 cases.The remainder consists of chromite and/or amphi- I 050 bole group minerals, and chlorite. Two habits of ureyite I r .3858 1.3861 15? can be observed.One is a short prismatic form up to 2 I 1.3510 r.x23 243 mm in length, which shows an exsolution texture with (source: Ldotatory eely315 ln x-ray laboratory of Eeulng jadeite.
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