A new nomenclature scheme for the alluaudite supergroup

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 P2(1)/n space-group types), and by unit-cell parameters generally around a similar to 12, b similar to 12.5, c similar to 6.5 angstrom, beta similar to 115 degrees. 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 rootnames, in order to differentiate arsenates with identical M(2) contents, but with different M(1) contents. Charge-balanced endmember formulae are given for all minerals of the supergroup, and a method to calculate formulae from the electron-microprobe analyses is given as appendix.