The crystal structure of vergasovaite Cu3O[(Mo,S)O4SO4], and its relation to synthetic Cu3O[MoO4](2)

The crystal structure of vergasovaite, Cu3O[(Mo,S)O4SO4], was determined fr om single-crystal X-ray data (space group Pnma, Z = 4, n = 7.421(2), b = 6. 754(3), c = 13.624(5) Angstrom) and refined to R-1 = 3.99 %. The new minera l is isostructural with synthetic Cu3O[MoO4](2) but exhibits a highly unusu al substitution of S6+ (ionic radius 0.12 Angstrom) for Mo6+ (ionic radius 0.41 Angstrom) in tetrahedral coordination. The synthetic and the natural s tructures are both characterized by edge-sharing zigzag chains of Jahn-Tell er distorted Cu2+O6 octahedra with attached Cu2+ in five-fold pyramidal coo rdination (4+1). The strongly distorted Cu ribbons are connected by S6+ and Mo6+ bearing tetrahedra to form a three-dimensional framework. it is assum ed that the structural strain due to the unusual S6+ <--> Mo6+ Substitution is balanced by development of a different type of Jahn-Teller distortion c ompared to the synthetic end-member variety. The substitutional disorder of S and Mo in the mineral structure leads to strong static disorder in the e ntire structure as revealed by relatively high atomic displacement paramete rs for all atoms. It is suggested that an end-member of the type Cu3O[SO4]( 2) may not be stable due to the restricted flexibility of the square pyrami dal coordination of Cu2+.