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Structural and Chemical Variations in Pyroxenes

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Structural and Chemical Variations in Pyroxenes AmericanMineralogist, Volume 66, pages I-50, 1981 Structural and chemicalvariations in pyroxenes MenytrrBN CeurnoN Board of Earth Sciences,University of California Santa Cruz, California 95064 AND J. J. PEPMN Department of Earth and Space Sciences State University of New York StonyBrook, New York 11794 Abstract Within the last 15 years, approximately 80 high-quality, three-dimensionalstructure re- finementsof terrestrial,lunar, and meteoritic pyroxeneswere published.The majority of the refinementsinvolve A/c cl;nopyroxenesand Pbca orthopyroxenes,but P2r/c structuresare alsowell represented.Few data are available onthe Pbcnpyroxenes that constitutethe fourth major structuretype. The topology of the four spacegroups can be describedwith idealized models composedof tetrahedral-octahedral-tetrahedral"I-beam" units that lie parallel to [001].The different symmetriesare a result of different stackingsequences of the octahedral layersand,/or of symmetrically-distincttetrahedral chains in adjacentlayers. In all of the py- roxenesrefined, the Ml cation site is coordinated by six oxygens arranged in a regular pseudo-octahedralconfiguration. The M2 site is irregularly coordinated by six, seven,or eight oxygens.The M2 coordination dependsupon the size of the cation occupying the site: higher coordinationnumbers are usually associatedwith larger cations.The maximum devia- tion of the tetrahedralchains from an extendedconfiguration (O3-O3-O3 : 180") occursin the B chainsof severalPDca structures where O3-O3-O3 = 136".The A tetrahedralchains in most F2t/c structuresare S-rotated,but all other chains are O-rotated. Studiesof pyroxene structuresat elevatedtemperatures and pressuresrevealed that the cation polyhedra expand and compressdi-fferentially. The high temperaturestudies documented a Y2r/c S C2/c tran- sition in the Fe-Mg pyroxenes,and showedthat the temperatureof the transition decreases with increasingferrosilite content. In addition, thesestudies provided further insight into the miscibility betweenthe high-calcium and low-calcium pyroxenesand producedcell parame- ter data that are basic to geothermometrystudies involving exsolution lamellae. Site occu- pancy refinementsconfirmed the preferenceof the Fe2* for the larger,more distortedM2 site in the Fe-Mg pyroxenes.Cations in synthetic pyroxenesshow a preferenceof Mn > Zn > Fe2*> Co > Mg for the M2 site. T-O distancesin the Pbca orthopyroxenesindicate that Al concentratesin the TB tetrahedron. Examination of the chemistryof 175naturally-occurring pyroxenes from a variety of lith- ologiesconfirms complete solid solution betweendiopside and hedenbergiteand extensive solid solution betweenenstatite and ferrosilite under crustal P-Tconditions. In this limited setof samplesthe number of Ca atomsper formula unit doesnot exceedL0, which is consis- tent with its occurrenceonly in the M2 site.The range in total Al between0 and 1.0is smaller than expected,and the maximum amount of rvAl substitutionis 557aof the T site occupancy. Na, when presentas a jadeite or acmite component,is responsiblefor the highestnon-quadri- lateral contentsof the pyroxenesexamined. The most important substitutionalcouples in ter- restrial Fe-Mg pyroxenesand augites are trFe3*-ttAl and rvAl-vrAl. Detailed statistical analysisof 12fi) high-quality pyroxeneanalyses from I I planetary basalt suitesrevealed that the vITi-I"Al coupleis one of the two most important couplesfor essentiallyall suitesconsid- ered. Fe3* is important in all of the terrestrialsuites, but is virtually absentin the lunar and M3-004X/8r/0102-0001$02.00 r CAMERONAND PAPIKE: PYROXENES meteoriticsuites, reflecting the lower oxygenfugacities that obtained on the moon and mete- orite parent bodies. Transmissionelectron microscopy(TEM) studiesdocumented the presenceof anti-phase domainsin pigeoniteand omphaciteand also elucidatedthe growth and developmentof ex- solution lamellae as wedge-shapedprecipitates. In addition, TEM studiesof the texturesof exsolutionlamellae contributed significantly to the understandingof the mechanisms(spin- odal decompositionvs. nucleation and growth) by which exsolution proceeds. Featureswithin individual pyroxenecrystals potentially useful as geothermometersinclude the Fe2*-Mg intracrystalline distribution, the orientation of exsolution lamellae relative to (001)and (100) ofthe host phase,and differential changesin the unit-cell parametersof the host and lamellar phasesduring cooling. Attempts to usethe sizeof anti-phasedomains as an indicator of cooling rate are of limited use at present. Introduction or those covered in a cursory manner (e.g., micro- Since the late 1960's,pyroxenes have receivedin- structures),the interestedreader is referredto Deet et . creasingattention from both mineralogistsand pe- al. (1978)or to the 1980MSA Reviewsin Mineralogy, trologists.The initial upswing in researchcoincided Volume7: Pyroxenes(Prewitt, 1980)prepared by the with the introduction of sophisticated'automated Mineralogical Societyof America. equipment into many laboratories,and it was given impetusby studiesof lunar rocks,basalts sampled by Chemicalclassification and nomenclature the Deep SeaDrilling Project,and the newly discov- The general formula for pyroxene can be ex- ered meteorites in Antarctica. Although previous pressedas XYZrOu, where X representsNa, Ca, workers recognizedthe importance of pyroxenesas Mn2*, Fe'*, Mg, and Li in the distorted 6- to 8- petrogeneticindicators, it is only in some of the re- coordinatedM2 site; Y representsMn2*, Fe2*, Mg, cent researchthat their usefulnessin providing infor- Fe'*, Al, Cr, and Ti in the octahedralMl site; and Z mation on .fo, conditions, temperatureand pressure representsSi and Al in the tetrahedral site. Chro- of crystallization,and cooling rates was more fully mium usually occurs as Cf* and titanium as Tio*, documented.Chemical studies using the electronmi- but under the reducing conditions that obtained on croproberevealed complex zoning trends (e.g.,Bence the moon and meteoritesCf* and Ti'* may occur. and Papike, 1972) and,elucidated the substitutional The cationsmentioned above are the most common couplescharacteristic of variousgeologic associations onesin the rock-forming pyroxenes;however, others (e.9., Schweitzeret al., 1979; Papike and White, do occur in trace amountsor as major constituentsin 1979).In addition, scoresof X-ray refinements of syntheticpyroxenes. both end-member and disordered pyroxenes pro- Although pyroxene nomenclature has been dis- vided excellentdetailed data on structural variations cussedfor many years and no generalconsensus ex- as a function of composition,temperature, and pres- ists,we believethat the schemeproposed by Deer et sure. al. (1978)(hereafter referred to as DHZ) is satisfac- Despitethe voluminous amount of data published tory for most purposes.We use a slight variation of on pyroxenesin the last 15 years,there are few com- their classificationcombined with the method of Pa- prehensivereviews of their chemical or structural pike et al. (1974).The major chemical subdivisions, variations(e.9., Appleman et al., 1966;Smith, 1969; which are basedon occupancyof the M2 site, are in- Zussman,1968; G. M. Brown, 1972;Morimoto, dicated in Table l. For a discussionof frequently- 1974).The main objectivesof this paper are: (l) to used pyroxene names (e.g., the varieties of ortho- provide a concisesummary of recenttrends in pyrox- pyroxene-bronzite, hypersthene,etc., or varietiesof ene research,(2) to discussthe fimits of structural calcium pyroxenes-fassaite, titanaugite, etc.) the and chemicalvariations in pyroxenes,and (3) to in- readeris referredtoDHZ.In addition, certain pyrox- terpret the observedchemical variations in terms of enes such as enstatite and ferrosilite have several crystal chemicalconsiderations. [n preparing this re- polymorphs.Multiple spacegroups listed after each view, we systematicallyexamined, compiled, and entry in Table I indicate the diferent polymorphs; plotted much of the data (exclusiveof abstracts)pub- for example, enstatite has three polymorphs with lished in recentpapers; however, for a more detailed symmetriesPbca, Y2r/c, and Pbcn. Structural details discussionoftopics not covered(e.9., phase relations) are given below, but for the P-Z synthesisconditions CAMERON AND PAPIKE: PYROXENES and relationshipsamong the various polymorphs the Table l. Major chemicalsubdivisions of pyroxenes(after Deer el readeris referredto Papike and Cameron (1976),Iij- al., 1978) ima and Buseck (1975), Buseck and Iijima (1975), Smith (1969),and Burnham(1965). Magnesium - Iron Pyroxenes l\lg2Si2O5 .Dhr: D?. /r Phrn*+ The systemproposed by Papike et al. (1974) di- Enstatite "e-r 1-t,:r : videspyroxenes into two chemicalgroups designated Ferrosilite Fe22*si2o5 Pbca,P21lc (MB,Fe2+)2si2o6 Pbca "Quad" and "Others." The pyroxenes(Fig. l) Orthopyroxene Quad Pi8eonite (|!1g,Fe2+,ca)2sizo5 P2tlc, C2lc belong to the well-known pyroxene quadrilateral with the end-membersdiopside, CaMgSirOu,heden- Calcium Pyroxenes bergite, CaFe'*SirOu,enstatite, MgrSirO., and fer- AuBire 16u,p2+*11p2+,pl+,ri4*)(si,nt)zog c2lc czlc rosilite, Fe'*SirO.. The Others chemical group in- Diopsrde CaMgSi205 HedenberSite caFe2 +Si2o6 C2lc cludespyroxenes with significantamounts of cations Johannsenite Cal\,lnSi205 C2/c other than Mg, Fe'*, and Ca. Figure I designates one-phasefields for augite, orthopyroxene,and pi- 3, Calcium - Sodium Pyroxenes Omphao te (ca,Na)(R2+,Al)si206 C2lc, P2/n, P2 geonite.When only chemical data are available, we 2 *, +)Si Aegi rine-A ugi te
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