Polyhalite and its analogous triple salts

Polyhalite (K2SO4 center dot MgSO4 center dot 2CaSO(4) center dot 2H(2)O) and analogue triple salts, where Mg2+ is substituted by Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn2+, have been synthesized. The salts were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction.Diffraction patterns and Raman spectra resemble those of natural polyhalite, except K2SO4 center dot CuSO4 center dot 2CaSO(4) center dot 2H(2)O. The latter corresponds to the mineral leightonite, which is structurally different. For polyhalite analogues the cell parameters of the triclinic unit cell have been determined from the powder diffraction patterns. The length of the unit cell vectors varies regularly with the ionic radius of the substituted ion M2+ and is explained by changes in the extension of the coordination octahedron of M (2+). Thereby increasing distances of the coordinated water molecules at M2+ parallel with decreasing dehydration temperatures of the corresponding polyhalite.