The crystal structure of wallisite1, PbTlCuAs2S5, the Cu analogue of hatchite, PbTlAgAs2S5

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Abstract
Wallisite crystallizes in the space group P[unk] with a″ = 9.215 ± 0.01, b″ = 8.524 ± 0.01, c″ = 7.980 ± 0.01 Å, α″ = 55°59′ ± 6′, β″ = 62°30′ ± 6′, γ″ = 69°24′ ± 6′ and a cell content of 2 [PbTlCuAs2S5]; dx = 5.71 g cm−3. The reduced cell is a = 8.983 ± 0.01, b = 7.761 ± 0.01, c = 7.980 ± 0.01Å, α = 65°33′ ± 6′, β = 65°30′ ± 6′, γ = 73°55′ ± 6′. The structure was determined from [001] and [010] Patterson projections by assuming the y parameters of the isomorphous hatchite structure. It consists of Cu2As4S10 double chains b″, which are joined by the Pb,Tl atoms. Each constituent single chain corresponds to CuAs2S7. Each double chain is composed by Cu2S6 double tetrahedra and AS2S5 pyramid groups. The Pb,Tl(1) position has an eightfold coordination of S and consists mainly of Pb atoms, whereas the Tl,Pb(2) position has only two nearest S neighbours and consists mainly of Tl atoms. The complete (001) cleavage is well explained by the structure (R = 16.4%) which is isomorphous with hatchite PbTlAgAs2S5.

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