Recommended nomenclature of epidote-group minerals

Abstract

Epidote-group minerals are monoclinic in symmetry and have topology consistent with space group P2 1 /m and the general formula A 2 M 3 [T 2 O 7 ][TO 4 ](O,F)(OH,O).Zoisite is an orthorhombic polymorph of clinozoisite Ca 2 Al 3 [Si 2 O 7 ][SiO 4 ]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 Ca 2 Al 3 [Si 2 O 7 ][SiO 4 ]O(OH) by homovalent substitutions only.The key cation-and anion-sites are A1 = M 2+ , A2 = M 2+ , M1 = M 3+ , M2 = M 3+ , M3 = M 3+ , 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+ + M3 M 2+ → A2 Ca 2+ + M3 M 3+ .Thus the valences on the key sites are: A1 = M 2+ , A2 = M 3+ , M1 = M 3+ , M2 = M 3+ , M3 = M 2+ , 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+ + M3 M 2+ → A2 Ca 2+ + M3 M 3+ and M1 M 2+ + O4 F -→ M1 M 3+ + O4 O -2 .Thus the valences on the key sites are: A1 = M 2+ , A2 = M 3+ , M1 = M 2+ , M2 = M 3+ , M3 = M 2+ , 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 Fe 3+ , Mn 3+ , Cr 3+ , and V 3+ 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 Fe 2+ and Mn 2+ 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) = piemontite-(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 M 2+ and or M 3+ , not by a single, dominant ion (i.e., Fe 2+ , or Mg, or Al).In addition, the dominant valence on O4: X -or X 2-must be ascertained.[M 2+ ] A2 > 0.50, [M 3+ ] M3 > 0.50 → clinozoisite subgroup, [M 3+ + M 4+ ] A2 > 0.50, [M 2+ ] M3 > 0.50 → allanite subgroup, {[M 2+ ] M3+M1 -[M 3+ + M 4+ ] 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 (M 2+ ) 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.