Thermal expansion of anhydrous copper sulfate minerals determined by single crystal X-ray diffraction: chalcocyanite CuSO4, dolerophanite Cu2OSO4 and kamchatkite KCu3O(SO4)2Cl

Abstract

Polythermic single-crystal X-ray studies of chalcocyanite CuSO₄, dolerophanite Cu₂OSO₄, and kamchatkite KCu₃O(SO₄)₂Cl have established their melting points as well as peculiarities of their thermal expansion. Association of oxocentered and sulfate tetrahedra in dolerophanite and kamchatkite leads to the formation of rigid tetrahedral “backbones” only slightly sensitive to thermal variations. Rigid complexes can also be distinguished in the structure of chalcocyanite, if we consider only the system of the shortest and strongest Cu–O and S–O bonds. The anisotropy of the thermal expansion can be explained by either rigid complexes drifting parallel to each other (as in dolerophanite and chalcocyanite), or radial and angular distortions in the polyhedra of alkali cations. The presence of a tetrahedrally coordinated additional oxygen atom in the structure of dolerophanite and kamchatkite leads to an increase in the principal eigenvalues. The demonstrated rigidity of the sulfate tetrahedra in studied anhydrous copper sulfate minerals explains the absence of phase transitions up to the melting temperatures. The variation of chemical composition leads to changes in their thermal decomposition points. Chlorine-containing kamchatkite decomposes at the lowest temperature of 590(5) K, next are chalcocyanite 675(10) K, and dolerophanite 925(10) K.