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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 . The most common appear to be , 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 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 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 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 , 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 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,, Hro* n.d. n.d. 0.13 0.05 4.56 silica, pentlandite (l - 2p only), prehnitc, , 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 .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, 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 .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. ln it, eachquadrilateral represents an end- spodumene"from Ravensthorpeshowed that it con- on view of a silicate chain (composition sistedof muscovitein three orientations,with the Sios'z-) which runs the lengthof the crystalin the c direction. silicatesheets parallel respectively to {110}and {100} The (100)layering (ll0) of the original spodumene.The pseudomorphous and the cleavageare easily recognized.The mica structuremay relationshipsuggests that the transformationto mas- be stylizedin a similarway (Fig. 2b). sivemuscovite occurred by materialexchange in solu- lf every alternateSios'z- chain of spodumeneis tion, with little or no changein volumeof the solid rotatedthrough 180'about phases.An idealreaction could be written its long axis,and half of the non-bridgingoxygens are converted to hydroxyls which remain in appropriateholes in the structure, 3LiAlSirO6+ 2HzO * K+ - KAlr(AlSis)O,o(OH), the mica sheetscan be built up on the spodumene + 3Li+ + 3SiO,+ 2OH-. (100) provided that the Ml and M2 cationsrotate with thechain. This is a minormode of occurrenceof The silicaand OH- producedcould be combinedin muscovitein "rotten spodumene," and is ac- someform of solublesilicic acid. However, a decrease complishedwith relativelyminor changeof shape(12 in volumeof the solid phasesof about20 percentis percentexpansion along spodumene[00], 9 percent associatedwith this reaction. along[010], and a contractionof I percentparallel to The volume changewould be reducedsufficiently the chainsalong [001]).The Ml cationsare com- by producinga silica-richmuscovite. Assuming a pletelyreplaced by potassiumat somestage during silicacontent equal to thatobserved (S20, Table l) we the process,and silicais lost to providethe necessary maY wnte alumina enrichmentand to compensatefor the 922 J. GRAHAM

(o) -\,r\,.\,t\(b) orientation, but the major orientations must alter- ,r\,r\ nate, giving the massive finely crystalline form )ooooo( observed. w Perhaps the most likely micro-inclusions, especial- ly in gem-quality material, would be small quantities of or exsolved during the crystal- lization of a-spodume-nefrom the high temperature B-spodumenesolid solution. Indeedit would be quite remarkableif suchexsolution did not occur,given the formation of a-spodumeneby a double eutectoid as suggestedby Roy, Roy, and Osborne (1950). The petalite structure consistsof spodumenechains con- densedto form a corrugated sheetsomewhat different from the mica discussedin this paper (Liebau, 1961).

The Space Group Problem AV/-\V+:^> Most structure analysesof spodumenehave been plagued by a number ofspace-group forbidden reflec- tions which suggested a lower symmetry for spodumenes than for most other clinopyroxenes (Clark et al, 1968,1969; Peacor, private communica- tion, 1970).We made many attempts to refine the structure in space groups C2 and Cc, but no matter what displacementswere applied to the atoms before refinement, they always returned to the same centrosymmetricpositions. Only a few of the reflec- FIc. 2. Stylized structures of spodumene (a) and of mica (b) in tions could be interpreted as due to the Renninger the two orientations observed relative to spodumene. The silicate effect, as the geometric criteria were not satisfied. chains of the spodumene are perpendicular to the paper, and are Cameron et al (1973)did not observeany forbidden represented by fiapezia. The mica sheet is made up of similar reflectionsin precessionphotographs of spodumene chains sharing oxygens at the corners of the trapezium. Oxygens (1969) releasedby this sharing are converted to hydroxyls, a few of which from the same sample in which Clark et al had are shown as crossesin (b). Squares are largely aluminum (M2) observedup to ten. As shown in Table 2, a changein and small circles largely Li(Ml).In the mica, the larger circles are radiation or a change in rotation axis alters the reflec- potasslum. tions observed,and they are often associatedwith general radiation streaks which suggeststhat some volume increaseof 2l percentwhich would occur if a may representspots from an adjacentlayer, but we l: I correspondenceexisted between the structuresas could not exclude them with layer screens.Various suggestedby this mechanism. tests,including the sensitiveoptical secondharmonic If every chain is rotated approximately 46" generation (Kurtz and Perry, 1968), indicate clockwise (or anti-clockwise), leaving Ihe M, and M2 centrosymmetry.The observationsdescribed in this cations approximately in their original spodumene paper suggestedthat the reflections could be artifacts positions,mica sheetsparallel to (l l0) in spodumene due to the presenceof a second phase.A b-axis Weis- are produced. This is the major orientation of the muscovite layers. A similar result, which requires rnsls 2 Space-Grouptiltjjln*.tections frdtnMt. Marion somewhat smaller final adjustments,is obtained if pairs of silicatechains (the "I-beam" units of Papike et al, 1973) rotate together about their common conditions kflections Observed aluminum ions. Sincethis transformation is accom- g axis rotation, Cu{o 60r, 1203* panied by a large expansionin the spodumene[l l0] b axis rotation, Cu{d Strong 201; 2o3; 2O5; 2a]-; 2Al; 2OI direction (normal to the mica sheets)and a contrac- WeakOO3r 603; 8Ol; f0,0,1 4O3;403; I0,0,1 b axis rotation bIo l0 ,O, f*; 2O1; 2O1t 203 tion along [110] (parallelto the mica sheets),it is not possibleto build up extensivemica layersin any one * reflectlon trucated. NOTES ON a-SPODUMENE 923 senberg pattern from gem-quality kunzite still Drrn, W. A., R. A Howrp, ruo J. ZussvrN (1963)Rock Forming showed three forbidden reflections.This would re- Minerals, Vols. 2 and 3, Longmans, London. Ecclrrou, quire that any inclusionsgiving rise to the reflections R. A. (1974) Nontronite intergrown with hedeflbergite (abstr.). Int. Crystallogr. Conf. Difraction Studies of Real Atoms must be coherentor finely dispersed; the kunzite did and Real Crystals, Melbourne. Australian Academy of Science, contain visible rutile needles,and the hiddenite was Canberra. not free of inclusions.In any case,the variability of Kunrz, S K., lNo T. T. Pnnnv (1968)A powder techniquefor the the non-spacegroup reflections,and the good refine- evaluation of nonlinear optical materials. J. Appl. Phys. 39, 3798-38l3 ments which have been obtained, leave no obvious LIEBAU, F. (1961) Untersuchungen an schictsilicatendes alternativeto the C2/c symmetry. formeltyps l-(Si,O')" lll. Zur Kristallstruktur von Petalit, LiAlSi{Oro. Acta Crystallogr. 14, 399-406. References Peprxe,J. J., C. T. PREwrrr, S. SusNo, lNn M. Ce,usnoN(1973) CnnrnoN,M., S.SurNo, C. T. Pnrwrrr,,qNo J. J pAprKE(1973) : comparisons of real and ideal structural topologies. High temperature crystal chemistry of six pyroxen es. Am Z. Kristallogr. l3E, 254-273. Mineral. 58, 594-618. Rov, R., D. M. Roy, ANDE. F. OssonN (1950)Compositional and stabitity relationships among Clenr, J R., D. E. Appr-EulN,nNo J. J. perrxr (1968)Bonding in the lithium aluminosilicates: Eucryptite, spodumene, and petalite. eight ordered clinopyroxenes isostructural with diopside. J. Am. Ceram. Soc. 33, Contrib Mineral Petol 20, 8l-85. 152-159 StnapsoN,E. (1952) AND- (1969) Crystal chemicalcharacteriza- S. Minerals of , Vol. III, p. tion ofclinopyroxenes basedon eight new structure refinements. 585, Government Printer, Perth. Pyroxenes and Amphiboles Crystal Chemistry and phase Manuscript receiued, December 17, 1973; accepted Petrology, Mineral. Soc. Am. Spec. Pap.2, 3l-50. for publication, March 20, 1975.