Nomenclature of the gadolinite supergroup

Abstract

The newly defined gadolinite supergroup approved by the IMA CNMNC (vote 16-A) includes mineral species that have the general chemical formula A 2 MQ 2 T 2 O 8 ' 2 and belong to silicates, phosphates and arsenates.Each site is occupied by: A À Ca, REE (Y and lanthanoids), actinoids, Pb, Mn 2þ , Bi; M À Fe, □ (vacancy), Mg, Mn, Zn, Cu, Al; Q À B, Be, Li; T À Si, P, As, B, Be, S; and ' À O, OH, F. The classification of the gadolinite supergroup is based on the occupancy of A, M, Q, T and ' 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 À Si 4þ in gadolinite group and P 5þ or As 5þ 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 B 3þ in the datolite subgroup and by Be 2þ in the gadolinite subgroup.The herderite group is divided into two subgroups.The herderite subgroup is defined by the dominant divalent cation (usually Ca 2þ ) in the A site and Be 2þ in the Q site, while the M site is vacant.The drugmanite subgroup is defined by the dominance of divalent cations (usually Pb 2þ ) 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.