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Recommended nomenclature of epidote-group minerals

¹ Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestr. 3, CH-3012 Bern, Switzerland
² Dipartimento di Scienze della Terra dell'Università di Firenze, Via La Pira 4,I-50121 Firenze, Italy
³ Department of Geoscience, Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690, Japan
⁴ Geoloski odsjek, Mineralosko petrografski zavod PMF-a, University of Zagreb, Horvatovac bb, HR-1000 Zagreb, Croatia
⁵ Laboratoire de Géologie, UMR 8538 du CNRS, Ecole Normale Supérieure, 24 rue Lhomond, F-75005 Paris, France
⁶ Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universität, Albertstrasse 23b, D-79104 Freiburg, Germany
⁷ Department für Geo- und Umweltwissenschaften, Sektion Mineralogie, Petrologie und Geochemie, Ludwig-Maximilians-Universität, Theresienstraße 41/III,D-80333 München, Germany
⁸ Department 4 Chemistry of the Earth, GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
⁹ Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
¹⁰ Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53,I-56126 Pisa, Italy

^* E-mail: Thomas.Armbruster{at}krist.unibe.ch

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 = O^2–, 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⁺ + ^M3M^{2+ A2}Ca²⁺ + ^M3M³⁺. Thus the valences on the key sites are: A1 = M²⁺, A2 = M³⁺, M1 = M³⁺, M2 = M³⁺, M3 = M²⁺, O4 = O^2–, 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⁺ + ^M3M^{2+ A2}Ca²⁺ + ^M3M³⁺ and ^M1M²⁺ + ^O4F^{– M1}M³⁺ + ^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 X^2– must be ascertained. [M²⁺]_A2 > 0.50, [M³⁺]_M3 > 0.50 clinozoisite subgroup, [M³⁺ + M⁴⁺]_A2 > 0.50, [M²⁺]_M3 > 0.50 allanite subgroup, {[M²⁺]_M3+M1 – [M³⁺ + M⁴⁺]_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 divalent 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 analytical data.

Key-words: Nomenclature, epidote-group minerals, clinozoisite, allanite, dollaseite.

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