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Nomenclature of Pyroxenes

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Nomenclature of Pyroxenes Canadian Mineralogist Vol. 27, pp. 143-156 (1989) NOMENCLATURE OF PYROXENES SUBCOMMITTEE ON PYROXENES* COMMISSION ON NEW MINERALS AND MINERAL NAMES INTERNATIONAL MINERALOGICAL ASSOCIATION NOBUO MORIMOTO, Chairman Department of Geology and Mineralogy, University of Kyoto, Kyoto 606, Japan ABSTRACT ou de plusieurs elements. Cent-cinq noms utilises anterieu- rement sont abandonnes formellement. This is the final report on the nomenclature of pyrox- enes by the Subcommittee on Pyroxenes established by the (Traduit par la Redaction) Commission on New Minerals and Mineral Names of the International Mineralogical Association. The recommen- Mots-eMs: pyroxene, nomenclature, Association Interna- dations of the Subcommittee as put forward in this report tionale de Mineralogie, rapport final. have been formally accepted by the Commission. Accepted and widely used names have been chemically defined, by INTRODUCTION combining new and conventional methods, to agree as far as possible with the consensus of present use. Twenty names are formally accepted, among which thirteen are used to The subcommittee on pyroxenes has, after a represent the end members of definite chemical composi- thorough evaluation of the group of pyroxene miner- tions. In common binary solid-solution series, species names als, presented its recommendations for a new clas- are given to the two end members by the "50070 rule". sification and nomenclature to the Commission on Adjectival modifiers for pyroxene mineral names are New Minerals and Mineral Names (hereafter ab- defined to indicate unusual amounts of chemical consti- breviated as CNMMN). These recommendations tuents. This report includes a list of 105 previously used have been approved by the Commission by a for- pyroxene names that have been formally discarded by the Commission. mal vote (May 20th, 1987). The classification and nomenclature of the pyrox- Keywords: pyroxenes, nomenclature, International Miner- enes have been largely based on their crystal chemis- alogical Association, final report. try. In practice, the chemical content of the pyrox- ene formvla unit calculated to six oxygen atoms, or SOMMAIRE to four cations (Vieten & Hamm 1978), is essential for the classification. This formula unit corresponds Nous presentons Ie rapport final sur Ie sujet de la nomen- to one quarter of the unit cell for the monoclinic clature de la famiIIe des pyroxenes, resultat des delibera- pyroxenes and to one eighth of the unit cell for the tions d'un sous-comite de la Commission des nouveaux orthorhombic pyroxenes. The basic principle adopt- mineraux et des noms de mineraux de I'Association Inter- ed for amphibole nomenclature (Leake 1978) is to nationale de Mineralogie. Les recommandations du sous- denote the principal stoichiometries by generally well- comite, publiees ici, ont ete formellement acceptees par la established names, with adjectival modifiers to in- Commission. Nous avons defini les noms utilises couram- ment qui ont ete retenus en termes compositionnels pre- dicate the presence of substantial substitutions that cis, en utilisant les methodes conventionnelles et nouvel- are not essential constituents of the end members; les, afm d'atteindre un consensus avec I'utilisation courante. this has been followed as far as possible in the pyrox- Vingt noms d'especes ont formellement ete acceptes, dont ene nomenclature. treize servent comme poles ayant une composition chimi- No new names have been introduced in the pro- que definie. Dans les solutions solides binaires courantes, posed nomenclature. Accepted and widely used nous acceptons la regie du 50% pour attribuer Ie nom de names have been chemically dermed by combining l'espece. Des adjectifs viennent modifier Ie nom d'un new and conventional methods to agree as far as pos- pyroxene pour indiquer un enrichissement anomale d'un sible with the consensus of present use. Two kinds *Subcommittee members: J. Fabries (France), A.K. Fer- of adjectival modifiers are used, one to specify a part guson (Australia), LV. Ginzburg (USSR), M. Ross (USA), of the compositional range shown by a mineral that F.A. Seifert (Germany) and J. Zussman (UK). Non-voting forms a wide solid-solution (e.g., magnesium-rich au- members: K. Aoki (Japan) and G. Gottardi (Italy, gite, iron-rich augite), and the other to specify deceased). elemental substitutions that are not essential consti- 143 144 THE CANADIAN MINERALOGIST tuents (e.g., titanian augite). One hundred and five previously used pyroxene names, mostly synonyms, and obsolete or almost unused names, have been for- mally discredited by the CNMMN. General publi- cations dealing with the pyroxene group, e.g., Roek- Forming Minerals (Deer et al. 1978, denoted DHZ Si4+ 6 LOOO in the Tables), Special Paper 2 (papike 1969) and Al3+ Al3+ Reviews in Mineralogy 7 (Prewitt 1980), both pub- lished by the Mineralogical Society of America, pro- Fe3+ vide references to the voluminous literature. I Ti4+ er3+ CRYSTAL CHEMISTRY OF THE PYROXENES y3+ Pyroxenes are silicates that, in their simplest form, Ti3+ contain single Si03 chains of linked Si04 tetrahedra. Generally, small amounts of Si are replaced by'AI zr4+ and other small cations. The repeat distance e along Sc3+ the chain (Z axis) comprises two tetrahedra and is ,:.'00 approximately 0.52 nm in length. The general chem- Zn2+ I I ical formula (formula unit) for all pyroxenes*! is Mg2+ Mg2+ _ M2MI T206' where M2 refers to cations in a gener- Fe2+ Fe2+ ally distorted octahedral coordination, MI to cations _ in a regular octahedral coordination, and Tto tetra- Mn2+ Mn2+ hedrally coordinated cations. _ Any pyroxene belongs to either the orthorhom- u+ bic or the monoclinic crystal system. There are two ea2+ types of orthorhombic pyroxene: orthopyroxene Na+ (Pbea) and orthopyroxene (Pben)*2. Only the form- er has been found in nature. Monoclinic pyroxenes FIG,I.Flow chart for ideal site-occupancy of cations are called clinopyroxenes. Their space groups are among the T, MI and M2 sites of pyroxenes. Only C2/e, P2/e and P2ln, depending on their chemi- representative cations are included. Arrows indicate cal composition and genetic history. order of filling of sites. Real site-occupancy usually Throughout this report, the standard pyroxene differs slightly from the ideal site-occupancy. formula is used with superscript arabic numerals (e.g., Fe2+) referring to charges, and subscript numerals (e.g., Mg0, to numbers of atoms. To der- '.rABLE1. FOUR COUPLED SUBsn'l'U'rIONS OF PYROXENES IN ive a pyroxene formula from a chemical analysis, the 'l'HE S'.rANDARDFORMULA R2+R2+R~+06 calculation should be based on six oxygen atoms Substitution M2 M1 examples when both Fe2+ and Fe3+ have been determined. In site '.r microprobe analyses only total Fe is determined, and standard R2+ R2+ 2R4+ the option of calculating to four cations should at substitution(1) (R+) (R3+) 2R4+ ~::~;3+ least be permitted if not actually preferred. Yieten Na_cr3+ & Hamm (1978) showed that calculation to four Na_Sc3+ cations will be more reliable for the majority of pyroxenes characterized by electron-microprobe substitution(2) (R+) R6;s(Rg;S) 2R4+ Na_(U4+/2) analysis. Therefore, for electron-microprobe data, AI-AI it is recommended that the components be totalled substitution(3) R2+ (R3+) (R3+)R4+ Fe3+-AI to six oxygen atoms and four cations by adjusting cr3+_AI the ratios Fe2+IFe3+ , Ti4+/Ti3+, etc. The standard pyroxene formula M2MI T206 con- tains two tetrahedral sites. In the allocation of the cations to obtain a pyroxene formula, the following procedure is recommended: 2) Sum MI to 1.000 using all Al3+ and Fe3+ in 1)' Sum T to 2.000 using Si4+, then Al3+, then excess of that used to fill the T sites. If there is Fe3+ . insufficient Al3+ and Fe3+ to sum to 1.000, then add Ti4+, Cr3+, y3+, Ti3+, Zr4+, Sc3+, Zn2+, Mg2+, Fe2+ and, finally, Mn2+, until the sum is *See comments at end of text. 1.000. NOMENCLATURE OF PYROXENES 145 3) Sum M2 using all Mg2+, Fe2+ and Mn2+ in the solid-solution series into ranges with specified excess of that used to fill the Ml sites. Then add compositions and names. Wherever there is a com- Li+, Ca2+ and Na+ so that the sum becomes plete solid-solution series between two end members, 1.000 or close to it. If the sum is far from 1.000, it is customary in mineral nomenclature to use only one must be suspicious about the results of the two names, and the division between them should analysis. A flow chart (Fig. I) gives a diagrammatic represen- tation of the site allocation of the principal cations TABLE 2. ACCEPTED PYROXENE MINERAL NAMES AND THEIR CHEMICAL in pyroxenes. However, because the distribution of SUBDIVISIONS. cations among the MI, M2 and T sites in a given mineral names composition main composition space pyroxene is partly a function of temperature, the as end-member as solid solution group accurate site-occupancy must be determined by struc- I. Mg-Fe pyroxenes ture determination. The site-occupancy given in (En)(1) Figure 1 is called ideal site-occupancy to distinguish 1. enstatite Mg2Si206 (2) it from real occupancy. A method for classifying 2. ferrosilite{Fs) Fe~+Si206 pyroxenes by their ideal site-occupancies has been 3. clinoenstat!te proposed by Bokii & Ginzburg (1985). In the present 4. clinoferrosil,ite classification of pyroxenes, the MI and M2 sites are 5. pigeonita considered together as a single M site in order to avoid the possible difference between the real and II. Mn-Mg pyroxenes ideal site-occupancies. 6. donpeacorite (Mn,Mg)MgSi206 ~ Starting from the most common pyroxene for- 7. kanoite (Ra) (3) MnMgSi206 (Mn,Mg)MgSi206 R,21/£ mula, M2(R2+)MI(R2+)Ti2R4+)06' four coupled substitutions are possible if one assumes more than III. Ca pyroxenes 8. diopside (Di)(4) CaMgSi206 one R4+ in the T site. They are listed in Table I, }Ca(Mg,Fe)Si206 where the elements in parentheses are coupled sub- 9.
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