The crystal structure of dioptase, Cu6[SieO18]-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 [SieO18] 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.

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First page of A refinement of the structure of dioptase, Cu<sub>6</sub>[Si<sub>6</sub>O<sub>18</sub>].6H<sub>2</sub>O
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