Nomenclature of the gadolinite supergroup

The newly defined gadolinite supergroup approved by the IMA CNMNC (vote 16-A) includes mineral species that have the general chemical formula A₂MQ₂T₂O₈φ₂ and belong to silicates, phosphates and arsenates. Each site is occupied by: A − Ca, REE (Y and lanthanoids), actinoids, Pb, Mn²⁺, Bi; M − Fe, □ (vacancy), Mg, Mn, Zn, Cu, Al; Q − B, Be, Li; T − Si, P, As, B, Be, S; and φ − O, OH, F. The classification of the gadolinite supergroup is based on the occupancy of A, M, Q, T and φ sites and application of the dominant-valency and dominant-constituent rules. The gadolinite supergroup is divided into two groups defined by prevailing charge occupancy at the T site − Si⁴⁺ in gadolinite group and P⁵⁺ or As⁵⁺ in herderite group. The gadolinite group is divided into the gadolinite and datolite subgroups. The A site is dominantly occupied by divalent cations in the datolite subgroup and by trivalent cations in the gadolinite subgroup. Accordingly, the Q site is dominantly occupied by B³⁺ in the datolite subgroup and by Be²⁺ in the gadolinite subgroup. The herderite group is divided into two subgroups. The herderite subgroup is defined by the dominant divalent cation (usually Ca²⁺) in the A site and Be²⁺ in the Q site, while the M site is vacant. The drugmanite subgroup is defined by the dominance of divalent cations (usually Pb²⁺) in the A site, vacancy in the Q site and the occupation of the M site. Moreover, “bakerite” is discredited as mineral species because it does not meet the conditions of the dominant-constituent rule.