A refinement of the structure of dioptase, Cu₆[Si₆O₁₈].6H₂O Available to Purchase

The crystal structure of dioptase, Cu₆[Si_eO₁₈]-6H₂0 (a = 14.566, с = 7.778 A, R3), was refined to R = 0.039 using 969 non-zero reflections. The structure consists of puckered trigonal rings of six water molecules with an ice-like configuration sandwiched between similarly puckered trigonal rings of six silicate tetrahedra bonded together laterally and vertically by Cu atoms. The water molecules are hydrogen-bonded to one another and to the bridging oxygens of adjacent [Si₆O₁₈] rings, resulting in O-H distances in the water molecule of 1.1 and 0.9 A. The effective aperture diameter of the [001] channel through the rings is only 2.0 A, preventing zeolitic transfer of the water molecules.

The Si-О bond lengths [Si-O(br) = 1.645, 1.646 A, Si-O(nbr) = 1.617, 1.600 A] can be rationalized in terms of the valence angles within and between the silicate tetrahedra in the [Si_eO₁₈] ring and a Mulliken bond overlap population analysis. The Cu atom is coordinated by four oxygens at 1.95–1.98 A in nearly square-planar array, with two water molecules at 2.65 and 2.50 A forming a tetragonally-distorted octahedron. Bond overlap populations suggest that the nonzeolitic water molecules are not bonded to the copper atom, although the empirical equation of Brown and Wu (1976) gives bond-strength sums around the Cu²⁺ ion of 1.81 for 4-coordination and 1.99 for 6-coordination.