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Epidote-Group Minerals

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Epidote-Group Minerals Eur. J. Mineral. 2006, 18, 551-567 Recommended nomenclature of epidote-group minerals THOMAS ARMBRUSTER1,* (Chairman), PAOLA BONAZZI2 (Vice-chairman), MASAHIDE AKASAKA3, VLADIMIR BERMANEC4, CHRISTIAN CHOPIN5, RETO GIERÉ6,SORAYA HEUSS-ASSBICHLER7, AXEL LIEBSCHER8, SILVIO MENCHETTI2,YUANMING PAN 9 and MARCO PASERO10 1Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestr. 3, CH-3012 Bern, Switzerland 2Dipartimento di Scienze della Terra dell’Università di Firenze, Via La Pira 4, I-50121 Firenze, Italy 3Department of Geoscience, Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690, Japan 4Geoloski odsjek, Mineralosko petrografski zavod PMF-a, University of Zagreb, Horvatovac bb, HR-1000 Zagreb, Croatia 5Laboratoire de Géologie, UMR 8538 du CNRS, Ecole Normale Supérieure, 24 rue Lhomond, F-75005 Paris, France 6Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universität, Albertstrasse 23b, D-79104 Freiburg, Germany 7Department für Geo- und Umweltwissenschaften, Sektion Mineralogie, Petrologie und Geochemie, Ludwig-Maximilians-Universität, Theresienstraße 41/III,D-80333 München, Germany 8Department 4 Chemistry of the Earth, GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 9Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada 10Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy Abstract: Epidote-group minerals are monoclinic in symmetry and have topology consistent with space group P21/m and the general formula A2M3[T2O7][TO4](O,F)(OH,O). Zoisite is an orthorhombic polymorph of clinozoisite Ca2Al3[Si2O7][SiO4]O(OH) and is thus not considered a member of the epidote-group. Epidote-group minerals are divided into three subgroups. (1) Members of the clinozoisite subgroup are derived from the mineral clinozoisite Ca2Al3[Si2O7][SiO4]O(OH) by homovalent substitutions only. The key cation- and anion-sites are A1 = M2+, A2 = M2+, M1 = M3+, M2 = M3+, M3 = M3+, O4 = O2-, O10 = (OH)-. In other words, the dominant valence as listed above must be maintained. (2) Members of the allanite subgroup are REE-rich minerals typified by the eponymous mineral “allanite”. This subgroup may be derived from clinozoisite by homovalent substitutions and one coupled heterovalent substitution of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M3+, M2 = M3+, M3 = M2+, O4 = O2-, O10 = (OH)-. (3) Members of the dollaseite subgroup are REE-rich minerals typified by the eponymous mineral “dollaseite”. This subgroup may be derived from clinozoisite by homovalent substitutions and two coupled heterovalent substitutions of the type A2(REE)3+ + M3M2+ → A2Ca2+ + M3M3+ and M1M2+ + O4F- → M1M3+ + O4O-2. Thus the valences on the key sites are: A1 = M2+, A2 = M3+, M1 = M2+, M2 = M3+, M3 = M2+, O4 = F-, O10 = (OH)-. The key cation-sites M3 and A1 (and, in principle, M2) determine the root name. In both clinozoisite and allanite subgroups no prefix is added to the root name if M1 = Al. The prefixes ferri, mangani, chromo, and vanado indicate dominant Fe3+, Mn3+, Cr3+, and V3+ on M1, respectively. In the dollaseite subgroup no prefix is added to the root name if M1 = Mg. Otherwise a proper prefix must be attached; the prefixes ferro and mangano indicate dominant Fe2+ and Mn2+ at M1, respectively. The dominant cation on A2 (other than Ca) is treated according to the Extended Levinson suffix designation. This simple nomenclature requires renaming of the following approved species: Niigataite (old) = clinozoisite-(Sr) (new), hancockite (old) = epidote-(Pb) (new), tweddillite (old) = manganipiemontite-(Sr) (new). Minor modifications are necessary for the following species: Strontiopiemontite (old) = piemon- tite-(Sr) (new), androsite-(La) (old) = manganiandrosite-(La) (new). Before a mineral name can be assigned, the proper subgroup has to be determined. The determination of a proper subgroup is made by the dominating valence at M3, M1, and A2 expressed as M2+ and or M3+, not by a single, dominant ion (i.e., Fe2+, or Mg, or Al). In addition, the dominant valence on O4: X- or X2- must be 2+ 3+ 3+ 4+ 2+ ascertained. [M ]A2 > 0.50, [M ]M3 > 0.50 → clinozoisite subgroup, [M + M ]A2 > 0.50, [M ]M3 > 0.50 → allanite subgroup, 2+ 3+ 4+ - {[M ]M3+M1 – [M + M ]A2 } > 0.50 and [X ]O4 > 0.5 → dollaseite subgroup. Coupled heterovalent substitutions in epidote-group minerals require a special application of the so-called 50 % rule in solid-solution series. (1) Clinozoisite subgroup: The dominant trivalent cation on M3 determines the name, whereas the A2 cation appearing in the suffix has to be selected from among the diva- lent cations. (2) Allanite and dollaseite subgroups: For the sites involved in the charge compensation of a heterovalent substitution in A2 and O4 (i.e. M3 in the allanite subgroup; M3 and M1 in the dollaseite subgroup), identification of the relevant end-member formula must take into account the dominant divalent charge-compensating octahedral cation (M2+) and not the dominant cation in these sites. Formal guidelines and examples are provided in order to determine a mineral “working name” from electron-microprobe analyt- ical data. Key-words: Nomenclature, epidote-group minerals, clinozoisite, allanite, dollaseite. *E-mail: [email protected] 0935-1221/06/0018-0551 $ 7.65 DOI: 10.1127/0935-1221/2006/0018-0551 © 2006 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart 552 T. Armbruster et al. 1. Introduction uses analytical data from the literature to obtain a simplified end-member formula corresponding to a mineral name. 1.1. Some obvious problems There are two major criticisms to his approach: (1) Analytical data from the literature that had been obtained on Deer et al. (1986) concluded in their Rock-forming metamict materials were used, which do not fulfill the crite- Minerals: “There is no universally accepted nomenclature of rion that a mineral be crystalline; (2) Standard guidelines for the monoclinic Fe–Al members of the epidote group. Some mineral nomenclature (Nickel & Grice, 1998) were uncriti- earlier names, e.g. fouquéite for compositions containing up cally applied, resulting in end-member formulae [e.g., 1 3+ to about 10 mol. % of the Ca2Fe3Si3O12(OH) component, CaLaAl2V [Si2O7][SiO4]O(OH)] that are not charge- have fallen into disuse and clinozoisite is now used to balanced and thus meaningless. describe those members of the series that are optically posi- tive, corresponding with approximately 15 mol.% 1.2. Objectives Ca2Fe3Si3O12(OH). For the more iron-rich, optically nega- tive members, the name pistacite is used by some authors; The Commission on New Minerals and Mineral Names the majority, however, describe those members with between (CNMMN) of the International Mineralogical Association 15 and 33 mol.% Ca2Fe3Si3O12(OH) as epidote. This (IMA) established at the beginning of 2003 the common use of the name is preferred in spite of its use to Subcommittee on Epidote-Group Mineral Nomenclature. designate the group as a whole. The optical distinction of This subcommittee defined following aims: (1) members of the clinozoisite-epidote series around the 15 Development of a consistent nomenclature system to mini- mol.% composition is difficult, and in the absence of a mize proliferation of unrelated mineral names; (2) to explain chemical analysis is generally based on color and appropriate use of existing and new names within this pleochroism. Where the composition is known it is usually mineral group by defining simple rules for nomenclature expressed in terms of the theoretical pistacite end-member, based on chemistry and ion (cation and anion) order; (3) to 3+ 3+ ps = 100 × Fe /(Fe + Al).” provide a simple but powerful scheme to derive a “working The nomenclature problems discussed by Deer et al. name” of an epidote-group mineral based on electron- (1986) were only partly solved after publication of the microprobe analytical data. criteria for new mineral names (Nickel & Mandarino, 1987). Finally, this report supplies an Appendix with specifi- Are these “rules” only applicable to new minerals (the corre- cally selected chemical data of epidote-group minerals sponding paragraph was entitled “criteria for new mineral representing either borderline cases, or incomplete or erro- names”)? What about the “grandfathered” mineral names neous analytical data, or data originating from partly epidote and clinozoisite described in the past and generally metamict minerals. These examples have been chosen to believed to represent valid species names? How are the demonstrate naming of epidote-group minerals in non-trivial boundaries between epidote and clinozoisite defined? cases. Of the 22 “piemontite” analytical data reported by Deer It is not the intent of this nomenclature recommendation et al. (1986) only seven meet the minimum criterion of Fe < to provide a complete view of chemical variations reported Mn > 0.5 required to satisfy the rule given by Nickel & for epidote-group minerals. For additional references on this Mandarino (1987): “At least one major structural site is subject we refer to Liebscher & Franz (2004). occupied by a different chemical component.” Ercit (2002), in his article “The mess that is allanite”, asks the provocative question: What is “allanite”? He states 2. Historical synopsis that the name “allanite” is often incorrectly used to describe any REE-bearing epidote. Investigators were additionally 2.1. Epidote and clinozoisite confused by introduction of new mineral names for REE- bearing epidote-group minerals such as dissakisite-(Ce), Epidote, a monoclinic mineral with the idealized formula 3+ dollaseite-(Ce), khristovite-(Ce), and androsite-(La). Until Ca2Al2Fe [Si2O7][SiO4]O(OH), was named by Haüy (1801). The 1988, the mineralogy of REE-rich epidote-group minerals type locality is Bourg d’Oisans, Dauphiné, France. The name was very simple because there was only the one official root comes from the Greek epi over plus dotós given (verbid of didóni), name allanite with the end-member formula i.e., given besides, “increased”. A related noun is epidosis, i.e.
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