Ricerc@Sapienza

A new, IMA-approved classification scheme for the spinel-supergroup minerals is here reported. To belong to the spinel supergroup, a mineral must meet two criteria: (i) the ratio of cation to anion sites must be equal to 3:4, typically represented by the general formula AB2X4 where A and B represent cations (including vacancy) and X represents anions; (ii) its structure must comprise a heteropolyhedral framework of four-fold coordination polyhedra (TX4 ) isolated from each other and sharing corners with the neighboring six-fold coordination polyhedra (MX6 ), which, in turn, share six of their twelve X-X edges with nearest-neighbor MX6 . Regardless of space group, the X anions form a cubic close-packing and each X anion is bonded to three M-cations and one T-cation. The fifty-six minerals of the spinel supergroup are divided into three groups on the basis of dominant X anion: O2– (oxyspinel), S2– (thiospinel), and Se2– (selenospinel). Each group is composed of subgroups identified according to the dominant valence and then the dominant constituent (or heterovalent pair of constituents) represented by the letter B in the formula AB2 X4. The oxyspinel group (33 species) can be divided into the spinel subgroup 2-3 (A2+ B3+2 O4) and the ulvöspinel subgroup 4-2 (A4+ B2+2 O4), the thiospinel group (20 species) into the carrollite subgroup 1-3.5 (A1+ B3:5+2 S4 ) and the linnaeite subgroup 2-3 (A2+ B3+2 S4), finally, the selenospinel group (3 species) into the bornhardtite subgroup 2-3 (A2+B3+2Se4 ) and the potential ‘‘tyrrellite subgroup’’ (A1+ B3:5+2 S4, currently composed by only one species). Once the subgroup is established based on the valence of B, then the mineral species is identified by the combination of the dominant A-and B-cations. Moreover, the present nomenclature redefines the ideal formulae of titanomaghemite, cuprorhodsite, malanite, maghemite, filipstadite, tegengrenite, rhodostannite, toyohaite and xingzhongite as well as discredits ‘‘iwakiite’’, ‘‘hydrohetaerolite’’ and ‘‘ferrorhodsite’’.