The American Mineralogist, Volume60, pages919-923. i,975 MINERALOGICAL NOTES SomeNotes on a-Spodumene,LiAlSirO' J. GneHnlr Diuision of Mineralogy, CSIRO, Floreat Park, WesternAustralia, 6014 Abstract Many spodumenesin polished section show a large variety of inclusions, some of them un- usual minerals. The most common appear to be albite, plagioclase,and muscovite. It is sug- gestedthat X-ray reflections thought to indicate a low-symmetry spacegroup for spodumene may be due to such phases in preferred orientations, and that these may also account for a good deal of the variation in spodumene analyses.Spodumene at Ravensthorpe in Western Australia is replaced by massive muscovite in three intergrown orientations related to the original structure; the chemical changes are discussed. Introduction but not in sufficient quantity to make up the original As part of a study of the formation and alteration stoichiometry. of a-spodumene, LiAlSirO", z refinement of the It was of interestto know whether the spodumene crystalstructure of this mineral was made someyears itself could accommodate thesechanges in stoichiom- ago on a fragment of almost ideal compositiont(S32, etry, or whether they were always due to the presence Table l) from a pegmatiteat Mt. Marion in Western of additional phases. Australia. The three-dimensionalstructure analysis resulted in a conventional R of 0.048 in the Composition centrosymmetric space group C2/c, including un- A seriesof full and partial analyseswas carried out observed reflections at half background intensity. on spodumenesand their decomposition products The parameters are in excellent agreementwith those from Mt. Marion and Ravensthorpe,which together obtained by various authors (Clark, Appleman, and with published data on other spodumenes(Deer, Papike, 1968, 1969;Peacor, private communication, Howie, and Zussman, 1963; Simpson, 1952) sug- 1970;Cameron et al,l973) but in commonwith these gested a considerable variability of composition. authors, space-group-forbidden reflections were Selectedanalyses from our seriesare shown in Table observed. l. Optical examination of powdered samplesand of At the Catlin Creek pegmatite, Ravensthorpe, thin sectionsdid not indicate any major degreeof WesternAustralia, spodumenesof variouscolors and heterogeneity,although there was often alteration on differing composition occur. One area is character- partings and cleavages which made it necessaryto ized by "rotten spodumene,"a dark greenmica pseu- hand pick material for analysis. domorph after spodumene. Even in fresh spodumene An attempt was made at the time of the structure sectionedparallel to {001}, mica flakes can be seen analysis to detect differences due to chemical optically aligned parallel to {100}and and it is {110}, differences between Mt. Marion and Ravensthorpe possibleto separatesome mica from the spodumene spodumenes(analyses S32 and S30in Table l). Two- by grinding and heavy-liquid separation. In oth'er dimensionalrefinements showed identical positional regions,alteration producing a white color is some- parameters, so a difference Fourier was calculated times accompaniedby an increasein sodium. Partial from the ikO intensities, 1ss2- Isro.The lithium site chemical analysesshowed.that low lithium figures was characterizedby a weak positive region of charge were usually associatedwith high KrO or high NarO, and the aluminum site by a weak negative region, but 'All bulk analyseswere carried out by Mr. C. E. S Davis, to the effect was very small, and the Fourier was on the whom the author is deeply grateful. whole remarkablyflat. The meaningof this resultde- 9t9 920 J. GRAHAM T,rsle I. Analvses of Soodumene and Muscovite mica was often lamellar, but the plagioclase, which was on a much more massivescale, showed no ob- Spodumene Iquscov ite vious orientation relationships. s30 s36 s20 Polished sections of a number of spodumene si02 64.3 63,8 62.23 63.19 52.5 crystals,including gem-quality hiddenite and kunzite, Ar2o3 26.0 26,7 26.7 27.4 29.2 were examined microscopicallyand by microprobe. Fe2O3 0.57 0.41* 0.19* 0.7* 2.04* Every spodumenecrystal examined contained some FeO 0.43 extensive veining and a variety of the following Mgo 0.01 0.04 0.07 0.01 2.40 CaO 0 .06 0. 09 O.62 0 .34 0.08 widely dispersed micro-inclusions (deduced from semi-quantitativemicroprobe data): albite, various KzO 0 .03 0. 09 0.83 0 . 36 9.9 of plagioclase feldspars, muscovite Na2O 0. 08 0 .26 | .77 0. 90 0.26 compositions (some with extensive Li2o 7.55 7.72 6.06 6.82 0.02 Na with K), cassiterite,siderite Mn,Mg substitution, mangan-apatite,lithiophilite, Hro* n.d. n.d. 0.13 0.05 4.56 silica, pentlandite (l - 2p only), prehnitc, pollucite, HZO n.d. n.d. 0.94 0.59 0.59 new cesiummineral, and Na,Mn,Fe alumino-silicate. 99.03 99.1I 99.54 99.73 101.55 Mica, albite, and plagioclasewere ubiquitous except l',lusc. 0 3 0.9 8.5 3.6 97.4 in good areasof the gemstones.Because of the small Alb. 0.7 2.2 15.0 7.6 2.2 An. 0.3 0.4 3.3 1.7 0.4 size of most of the inclusions, analyseswere only semi-quantitative, and some of the mineral identities Structural ForruIae (Spodunenescorrected for feldspars and nica) are deduced. Errors could be introduced by the fact lithium content is not known. s32 (Lio.gzF"o.or)(AIo.goF"o.ot)siz.ozoo that the in Western s30 Lit.ot(olo.sgF"o.o1)siz.oooo In a sample from McPhee's Range s36 ti, Australias for which an analysis is available .o+A1r. orsit. ggou s38 Lio.gaAlt.ossir.ggoo (Simpson, 1952), a cesium mineral was observed in (Ko.a+Lio.or)(A1r.ooF"o. (A10.61si3.39)010(oH)z forms up to about 20 p across.It s20 roltgo.zq) rounded or dentate also occurred in small veinsin closeassociation with *IotaL Analyst c.E.S, DaDis. f,"o4 as FeZ03, muscovite,from which it could not be distinguished in polished section.The composition was similar to pends on the detailed structural formula assumed, that of pollucite, but with calcium instead of some but it seems clear that some Fe3+ enters the sodium, and d somewhat higher Al content. In aluminum site. This has since been confirmed by polished thin section no grain could be found free of microprobe analysisof hiddenitefrom Spargovillein interferencefrom spodumene,so optical properties Western Australia2 which contains approximalely t/z- could not be determined.Its Si(Li) spectrumis com- I percent Fe in solid solution. The result is fully ex- pared with that of pollucite in Figure l. plained if iron is distributed as shown in Table I for ln one specimen,two generationsof spodumene s32. were observed.No differencein composition could be The variability of bulk composition,and the slight detected in the electron microprobe, but the original effects produced by it in the spodumene structure, spodumenefluoresced red under electron bombard- suggested that spodumene crystals could be ment, while secondary spodumene in major veins heterogeneous.This has beenproposed before to ex- (largely filled with albite) fluorescedblue. plain the commonly observedexcess of silica in the It became evident from a seriesof partial analyses bulk analyses(Deer et al, 1963). of more-or-lessaltered spodumenesthat most of the An optical study of altered material suspendedin a potassium content is associated with muscovite medium of R.I. 1.70 gave no indication of the alteration and most of the sodium and potassium presenceof significant amounts of a secondphase, with plagioclase feldspar. In performing a full although there was local staining at the edges of analysis, it is essentialto determine all elementson a many grains and lamellar defectswere nearly always single sample,as the heterogeneitycan be observed observed.In polished section, however, the sample on a macroscale,even when apparentlyclean regions was obviously heterogeneous,and in fact contained are selected. about l0 percent feldspar and 2 percent mica. The When muscovite and feldspar are allowed for, 3 'Kindly supplied by the Government Chemical Laboratories. Kindly supplied by The Government Chemical Laboratories. NOTES ON a-SPODUMENE 921 2.48 LiAlSirOo + l.74HzO * K+ - KAlr.r?(Alo.6rSis.se)O1o(OH),+ 2.48Li+ + t.57SiO,+ 1.48(OH)-. E E c 0 This reactionis accompaniedby only a 2lz percent E (, volume change,and the transport requirementsare smaller. The metal-deficientoctahedral layer is )E o presumably product o unstable,so that the actual at Ravensthorpehas magnesiumreplacing some of the aluminumand a reducedlayer charge.One could producethe observedmica from ideal spodumeneby the reaction Encrgy KcV 2.37LiAlSirO. + 0.24Mg'z+* 0.85K+* 1.52HrO- FIc. l. Electron microprobe spectra of pollucite (lower) and the calcium-containing mineral from McPhee's Range spodumene. KoruMgo.rnAlr.?6(Alo.6rSis.se)O,o(OH), + 2.37Li+ Calcium and cesium are positively correlated. + l.35SiO,+ r.04(OH)-, structural formulae from analysessuch as those in whichwould be accompanied by a volumeincrease of 2 percent. Table I or in Deer et al (1963)are quite closeto The alterationfrom spodumeneto mica represents proportion theoreticalvalues; in particular, the excessof silica an increasein the of alumina; mentionedin Deer et al is usually considerably the equationsassume that no aluminum is in- troduced.They show reduced.The formulaein the tableare calculated to 6 that the amountof spodumene required oxygens.From the differenceFourier and from the for the transformation, and hence the overall phases, changein the temperaturefactor of the lithium atom changeof volumeof the solid is a function betweensamples, there is some isomorphoussub- of thedetailed composition of theindividual phases. stitutionfor Li in the spodumenestructure, but it is evidentlyvery slight. The structurcsof the two mineralsand their orien- tation relationshipssuggest that the alterationto The Transformationto Muscovite muscovitemay be partly by a topotacticmechanism. A stylized(001) section of spodumeneis shownin A single-crystaldiffraction pattern of the "rotten Figure2a.