The crystal structure of klyuchevskite, K3Cu3(Fe,Al)O2(SO4)4, was determined in space group I2 using the total of 715 independent reflections up to R 0.12. Klyuchevskite is a structural analogue of piypite (caratiite) K4Cu4O2(SO4)4MeCl with the substitution K+ + Cu2+ → (Fe3+,Al) + MeCl. Compared to piypite, the ordering of Cu2+ and Fe3+ cations in the klyuchevskite structure results in the distortion of the unit cell and in lowering the symmetry of the chains to 1.
This brings about a more compact packing of the chains. The chain-like arrangement of the structure determines physical properties of the mineral, its acicular habit, perfect cleavage, strong anisotropy of optical properties and orientation of optical indicatrix.
