Minerals with a palmierite-type structure. Part II. Nomenclature and classification of the palmierite supergroup.

The palmierite supergroup, approved by the IMA-CNMNC, includes five mineral species characterised by the general crystal-chemical formula XII M 1 X M 2 2 ( IV T O 4 ) 2 ( Z = 3). On the basis of the crystal-chemical arguments and heterovalent isomorphic substitution scheme M + + T 6+ ↔ M 2+ + T 5+ , the palmierite supergroup can be formally divided into two groups: the palmierite group M 1 2+ M 2 2 + ( T 6+ O 4 ) 2 , and the tuite group M 1 2+ M 2 2 2+ ( T 5+ O 4 ) 2 . Currently, the palmierite group includes palmierite K 2 Pb(SO 4 ) 2 , and kalistrontite K 2 Sr(SO 4 ) 2 , whereas the tuite group combines tuite Ca 3 (PO 4 ) 2 , mazorite Ba 3 (PO 4 ) 2 , and gurimite Ba 3 (VO 4 ) 2 . The isostructural supergroup members crystallise in space group R $\bar{3}$ m (no. 166). The palmierite-type crystal structure is characterised by a sheet arrangement composed of layers formed by M 1O 12 and M 2O 10 polyhedra separated by T O 4 tetrahedra perpendicular to the c axis. The abundance of distinct ions, which may be hosted at the M and T sites ( M = K, Na, Ca, Sr, Ba, Sr, Pb, Rb, Zn, Tl, Cs, Bi, NH 4 and REE; T = Si, P, V, As, S, Se, Mo, Cr and W) implies many possible combinations, resulting in potentially new mineral species. Minerals belonging to the palmierite supergroup are relatively rare and usually form under specific conditions, and their synthetic counterparts play a significant role in various industrial applications.