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

Abstract

AbstractThe palmierite supergroup, approved by the IMA-CNMNC, includes five mineral species characterised by the general crystal-chemical formulaXIIM1XM22(IVTO4)2(Z= 3). On the basis of the crystal-chemical arguments and heterovalent isomorphic substitution schemeM++T6+↔M2++T5+, the palmierite supergroup can be formally divided into two groups: the palmierite groupM12+M22+(T6+O4)2, and the tuite groupM12+M222+(T5+O4)2. Currently, the palmierite group includes palmierite K2Pb(SO4)2, and kalistrontite K2Sr(SO4)2, whereas the tuite group combines tuite Ca3(PO4)2, mazorite Ba3(PO4)2, and gurimite Ba3(VO4)2. The isostructural supergroup members crystallise in space groupR$\bar{3}$m(no. 166). The palmierite-type crystal structure is characterised by a sheet arrangement composed of layers formed byM1O12andM2O10polyhedra separated byTO4tetrahedra perpendicular to thecaxis. The abundance of distinct ions, which may be hosted at theMandTsites (M= K, Na, Ca, Sr, Ba, Sr, Pb, Rb, Zn, Tl, Cs, Bi, NH4and 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.