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.
