The crystal structure of vergasovaite Cu (sub 3) O[Mo,S)O (sub 4) SO (sub 4) ], and its relation to synthetic Cu (sub 3) O[MoO (sub 4) ] (sub 2)

The structure of vergasovaite, space group Pnma, Z = 4, a 7.421, b 6.754, c 13.624 Aa) has been refined to R 3.99 %. The new mineral is isostructural with synthetic Cu (sub 3) O[MoO (sub 4) ] (sub 2) but has the very unusual substitution of S (super 6+) (ionic radius 0.12 Aa) for Mo (super 6+) (ionic radius 0.41 Aa) in tetrahedral coordination. The synthetic and natural structures are both characterized by edge-sharing zigzag chains of Jahn-Teller distorted Cu (super 2+) O (sub 6) octahedra with attached Cu (super 2+) in five-fold pyramidal coordination (4 + 1). The strongly distorted Cu ribbons are connected by S (super 6+) - and Mo (super 6+) -bearing tetrahedra to form a three-dimensional framework. The subsitutional disorder of S and Mo leads to strong static disorder in the entire structure as revealed by relatively high atomic displacement parameters for all atoms. It is suggested that an end-member of the type Cu (sub 3) O[SO (sub 4) ] (sub 2) may not be stable due to the restricted flexibility of the square pyramidal coordination of Cu (super 2+) .