Original paper
A new nomenclature scheme for the alluaudite supergroup
Hatert, Frédéric
European Journal of Mineralogy Volume 31 Number 4 (2019), p. 807 - 822
66 references
published: Dec 12, 2019
published online: Jul 8, 2019
manuscript accepted: May 29, 2019
manuscript revision received: May 29, 2019
manuscript received: Mar 28, 2019
DOI: 10.1127/ejm/2019/0031-2874
Open Access (paper may be downloaded free of charge)
Abstract
The alluaudite supergroup contains sixteen Na–Mn–Fe-bearing phosphates occurring in granitic pegmatites, as well as nineteen Na-bearing arsenates found in various geological environments; these minerals are characterized by a monoclinic symmetry (generally C2/c or P21/n space-group types), and by unit-cell parameters generally around a ~ 12, b ~ 12.5, c ~ 6.5 Å, β ~ 115°. The simplified structural formula of alluaudite-type minerals is A(2)’A(1)M(1)M(2)2(TO4)3, and the new nomenclature scheme proposed herein is mainly based on the contents of the M(1) and M(2) octahedral sites. For phosphates, the root name is defined on the basis of the M(2) content, allowing a valency-imposed double-site occupancy for that site: M2(Fe3+Fe3+) = alluaudites s.l., M2(Fe2+Fe3+) = hagendorfites s.l., and M2(Mn2+Fe3+) = varulites s.l. The root-name is used without prefix if the M(1) site is occupied by Mn, but the prefix “ferro-” is added when M(1) is predominantly occupied by Fe2+. For arsenates, the nomenclature is more complex, and numerous root-names are already defined in the literature. We consequently recommend preferentially adding prefixes to these root-names, in order to differentiate arsenates with identical M(2) contents, but with different M(1) contents. Charge-balanced end-member formulae are given for all minerals of the supergroup, and a method to calculate formulae from the electron-microprobe analyses is given as appendix.
Keywords
alluaudite • phosphate • arsenate • wyllieite • nomenclature