Morphotropism in fumarolic mineral-related anhydrous sulfates: novel representatives in A+2M2+(SO4)2 and A+2M2+2(SO4)3 series.

The discovery of numerous endemic anhydrous sulfate minerals in fumaroles of the Tolbachik volcano (Kamchatka, Russia) has revived interest in the whole family of anhydrous sulfates. Herein is reported the crystal structure of Cs2Cu(SO4)2 which adds important data on the `final' contributor with the largest A+ cation to the A2[Cu(SO4)2] morphotropic series (A = Na, K, Rb, Cs), the `initial' structurally characterized representative of this family being saranchinaite Na2Cu(SO4)2. With increasing ionic radius of the alkali metal cation(s), embedded in the [Cu(SO4)2]2- framework, symmetry-breaking transformations occur. Cs2Cu(SO4)2, which is here designated as the ϵ-phase, has a layered structure. Cs2Co2(SO4)3 is a new representative of another morphotropic series of the orthorhombic A2[M2+2(SO4)3] family, being also the first anhydrous Cs-Co sulfate. Structural relationships in A+2M2+(SO4)2 and A+2M2+2(SO4)3 morphotropic series are discussed in detail.