Abstract
The crystal structure of As-schwatzite Cu6(Cu5.26Hg0.75)(As2.83Sb1.17)S13 (Aktash deposit, Altai mountains) is refined. Tetrahedrally shaped dark-gray single crystals of the mineral belong to the cubic crystal system: I4¯3m space group, a = 10.2890(1) A, V = 1089.2(1) A3, d = 4.99 g/cm3, Z = 2 for the composition Cu11.26Hg0.75As2.83Sb1.17S13, R = 0.0177. The structure is based on the sphalerite-like framework comprising identically oriented (Cu,Hg)S4 tetrahedra ((Cu,Hg)-S 2.3452(8) A) and (As,Sb)S3 pyramids ((As,Sb)-S 2.311(1) A) sharing their vertices. The centers of [Cu6] octahedra in the (000) and (1/2 1/2 1/2) positions coinciding with the centers of the “cluster” anionic vacancies [□]4 are occupied by the so-called “thirteenth” sulfur atom. Quantum chemical calculations of the electron density are carried out for the [As4S13Cu6]6 fragment. The calculation results confirm the presence of strain in the [As4S13Cu6]6 moiety, which exists due to the support of the surrounding symmetric framework including the external sulfur atoms of the fragment. The possibility of inclusion of mercury into the framework, which is much richer in arsenic than in antimony, is demonstrated. High stability of the framework determines significant compression of the S-centered [SCu6] octahedron in its interstices, bringing together copper atoms to 3.145(1) A and shortening the Cu-S distances to 2.224(1) A
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