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Nomenclature of Pyroxenes

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Nomenclature of Pyroxenes American Mineralogist, Volume 73, pages I I 2 3- I I j 3' I 988 Nomenclatureof pyroxenes Subcommittee on Pyroxenes Commission on New Minerals and Mineral Names International Mineralogical Association N. Momvroroo Chairman Department of Geology and Mineralogy, Kyoto University, Kyoto 606' Japan SubcommitteeMembers J. F.LsRrns(France), A. K. FnncusoN (Australia)I. V. GrNzruRG(USSR), M. Ross (U.S.A.), F. A. Srmnnr (Germany),J. ZussMAN (U.K.) NonvotingMembers K. Aoxr (Japan),G. Gorr.l'nor (Italy) Ansrucr This is the final report on the nomenclature of pyroxenes by the Subcommittee on Pyroxenesestablished by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The recommendations of the Subcommittee as put forward in this report have been formally acceptedby the Commission. Accepted and widely used names have been chemically defined, by combining new and conventional methods, to agreeas far as possiblewith the consensusof presentuse. Twenty names are formally accepted,among which thirteen are usedto representthe end membersof definite chemical compositions. In common binary solid-solution series,species names are given to the two end members by the "500/oruIe." Adjectival modifiers for pyroxene mineral names are defined to indicate unusual amounts of chemical constituents.This report in- cludesa list of 105 previously used pyroxene names that have been formally discardedby the Commission. INrnooucrroN present use. Two kinds of adjectival modifiers are used: one to specify a part of the compositional range shown The Subcommittee on Pyroxeneshas, after a thorough by a mineral that forms a wide solid solution [e.g.,mag- evaluation ofthe group ofpyroxene minerals, presented nesium-rich (or Mg-rich) augiteand iron-rich (or Fe-rich) its recommendationsfor a new classificationand nomen- augitel; the other to specifo elemental substitutions that clature to the Comrnission on New Minerals and Mineral are not essentialconstituents (e.g., titanian augite). The Names (hereafter abbreviated as CNMMN). These rec- CNMMN has formally discredited 105 previously used ommendations have been approved by the Commission pyroxene names-mostly synonyms, obsolete or almost by a formal vote (May 20,1987). unused, or recommendedfor rejection. The classification and nomenclature of the pyroxenes General publications dealing with the pyroxene group have been largely based on their crystal chemistry. In include Rock-Forming Minerals (Deer et al., 1978),Min- practice the chemical content of the pyroxene formula eralogical Society of America Special Paper 2 (Papike, unit calculated to six oxygens,or to four cations (Vieten 1969) and MSA Revkws in Mineralogy, volurne 7 (Prew- and Hamm, 1978),is essentialfor the classification.This itt, 1980), which provide referencesto the voluminous formula unit correspondsto one-quarter of the unit cell literature. for the monoclinic pyroxenes and to one-eighth of the unit cell for the orthorhombic pyroxenes.The basic prin- Cnvsr.lr, CHEMISTRYoF THE PYRoxENES ciple adopted for amphibole nomenclature (kake and Pyroxenes are silicates that, in their simplest form, Winchell, 1978) is to denote principal stoichiometriesby contain singleSiO, chains oflinked SiOotetrahedra. Gen- generally well-establishednames, with adjectival modi- erally, small amounts of Si are replacedby Al and other fiers to indicate the presenceof substantial substitutions small cations. The repeat along the chain (c axis) com- that are not essential constituents of the end members; prises two tetrahedra and is approximately 0.52 nm in this principle has been followed as far as possible in the length. The general chemical formula (formula unit) for pyroxene nomenclature. all pyroxenes'is M2MlTrOu, where M2 refers to cations No new names have been introduced in the proposed nomenclature. Accepted and widely used names have been chemically defined by combining new and conventional I In omphacite-P2/n,the Ml and M2 sitesare further divided methods to agreeas far as possiblewith the consensusof into Mla and Mlb (for Ml) and M2a and M2b (for M2). 0003{04v88/09 I 0-l I 23$02.00 tt23 n24 MORIMOTO:NOMENCLATURE OF PYROXENES gens and four cations by adjusting the ratios Fe2*/Fe3*, Ti4+/Ti3+,etc. The standard pyroxene formula M2MlTrO6 contains two tetrahedral sites. In the allocation of the cations to obtain a pyroxene formula, the following procedure is recommended: -3+ 1. Sum T to 2.000 using Sia*,then Al3*, and then Fe3t. !e 2. Sum Ml to 1.000using all Al3* and Fe3*in excess Ti4+ of that used to fill the T sites. If there is insufficient Al3* 1+ Cr- and Fe3*to sum to 1.000,then add Tia*, Cr3*,V3+, Ti3+, ..3+ Zra*, Sc3', Zn2*, Mg2', Fe2*, and finally Mn,* until the ?+ sum is 1.000. Ti-' M2 A+ 3. Sum using all Mg2*, Fe2*,and Mn2t in excessof zt that used to fill the Ml sites. Then add Li*. Ca2*. and Na* so that the sum becomes 1.000 or close to it. If the zn2+ sum is far from 1.000, one must be suspiciousabout the results of the analysis. tg2* - tf2* 2+' A flow chart (Fig. l) gives a diagrammatic representa- Fe2* - Fe- tion ofthe site allocation ofthe principal cations in py- l,t 12* - Itr2+ roxenes. However, because the distribution of cations I,i- among the Ml, M2, and T sites in a given pyroxene is partly a function of temperature, the accurate site occu- + pancy must be determined by structure determination. Na' The site occupancygiven in Figure I is called ideal site Fig. l. Flowchart for idealsite occupancy of cationsbetween occupancyto distinguish it from real occupancy.A meth- the T, Ml, and M2 sitesof pyroxenes.Only representativecat- od for classifuingpyroxenes by their ideal site occupan- ions areincluded. Arrows indicate order offilling ofsites.Real cies has been proposedby Bokij and Ginzburg (1985). In site occupancyis usually slightly different from the ideal site the present classifrcationof pyroxenes, the Ml and M2 occupancy. sites are considered together as a single M site in order to avoid the difference between the real and ideal site occupancles. in a generally distorted octahedral coordination. Ml to Starting from the most common pyroxene formula, cations in a regular octahedral coordination, and T to M2(R'z-)MI (R2*)Tr(2R4*)O6,four coupled substitutions tetrahedrally coordinated cations. are possible if one assumesmore than one Ra* in the T Any pyroxene belongs to either the orthorhombic or site. They are listed in Table l, where the elements in the monoclinic crystal system.There are two orthorhom- parenthesesare coupled substitutions. bic pyroxene types: (Pbca) orthopyroxene and orthopy- Substitution I encompassesthe end members jadeite roxene (Pbcn).,Only the former has been found in nature. (NaAlSirOu), aegirines (NaFe3*SirOu),kosmochlora Monoclinic pyroxenes are called clinopyroxenes. Their (NaCr3*SirO.,Ko), and jervisite (NaScSirOu,Je). Substi- space groups are C2/c, P2,/c, and P2/n, depending on tution 2 results in components such as NaFefrjTiftsi2o6, their chemical composition petrogenetic and history. but is less important than the other substitutions. Throughout this report, the standardpyroxene formula In substitution 3, the Al-Al couple is often referred to is usedwith superscriptedarabic numerals (e.g., Fe2*)re- as "Tschermak's component"; CaAlAlSiOu,in particular, ferring to chargesand subscripted numerals (e.g., MgJ is called "calcium Tschermak's component." Substitu- referring to numbers of atoms. tion in esseneite,sCaFe3*AlSiO., is obtained by this type In order to derive a pyroxene formula from a chemical of substitution. This substitution is also important in analysis, the calculation should be based on six oxygen atoms when Fe2* and Fe3* are both determined. In mi- croprobe analyses,only total Fe is determined, and the 3"Aegirine" is usedin preferenceto "acmite" in this report. option of calculating to four cations should at least be "Aegirine"is in commonusage in theliterature and is consistent permitted if not actually preferred. Vieten and Hamm with the almostuniversal use of "aegirine-augite"for minerals (1978) have shown that calculation to four cations will of intermediatecompositions, though "acmite" haspriority by years(Dana, practice be more reliable for microprobe analysesof the majority 14 1892).Common in experimentalpe- pyroxenes. trologyhas been to usethe abbreviationAc for NaFe3*SirO6;Ae of Therefore, for microprobe analyses,it is shouldnow be usedinstead. recommendedthat the componentsbe totaled to six oxy- 4The CNMMN, IMA, hasrecently voted in favorof thename "kosmochlor"instead of "ureyite" for the pyroxeneofgeneral- izedcomposition NaCrSirO.. 2 Orthopyroxene(Pbcn) is stahleonly at elevatedtemperatures 5Esseneite is a new pyroxenewith the compositionCa- for a limited composition near MgSiOr. Fe3*AlSiOuGable 2, no. l3). MORIMOTO:NOMENCLATURE OF PYROXENES tI25 TeeLe1. Fourcoupled substitutions. of pyroxenesin the stan- namesare given for the end-member componentsof sub- dardchemical formula R2+R2+R4+06 stitutions 2 and 4. Substitutionsite M1 Examples MrNBul NAMES oF THE PYRoXENES Standardoccupancy R2* R2* 2R4* Twenty (20) mineral namesand their grouping Substitution1 (R) (R*) 2R4* Na-Al The pyroxenesform extensive solid solutions by var- Na-Fe3+ Na-C13* ious types of ionic substitutions, some of which are de- Na-Sc3* scribed above. To cope with the problem of pyroxene Substitution2 (R.) R63(R33) 2R4* Na-Oi4*/2) nomenclature, it is necessaryto subdivide the solid-so- Substitution3 R2* (R"t, (Rr+)Re AI-AI Fe+-Al lution seriesinto rangeswith specifiedcompositions and
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