Richardsite – a member of the stannite group?

A new chalcogenide mineral, Richardsite, was proposed in literature with a composition Zn2CuGaS4 and crystallizing in the tetragonal stannite-type structure [1].

This compound can be viewed as a mixed crystal in the solid solution 2(ZnS) – CuGaS2. The composition of 2(ZnS)x(CuGaS2)1-x with x=0.5 results in the Richardsite chemical composition. We have studied this solid solution series concerning the crystal structure of mixed crystals using powder samples synthesized by solid state reaction of the elements. Additional single crystals, aiming for a composition of Richardsite, have been grown by chemical vapour transport (CVT). The chemical composition of the synthesized material has been determined by WDX spectroscopy as well as EDX-analysis in the STEM.

Both end members are non-isotype: CuGaS2 crystallizes in the chalcopyrite and ZnS in the zinc-blende structure. Rietveld analysis of X-ray diffraction data and TEM investigations revealed that the 2(ZnS)x(CuGaS2)1-x system forms a solid solution series but with a broad miscibility gap (0.2

We could not prove the existence of Richardsite, Zn2CuGaS4, as a new member of the stannite group. Our findings show that Zn2CuGaS4 crystallizes not in the tetragonal stannite structure but in the cubic zinc-blende structure.

[1] L. Bindi, J. A. Jaszczak, Minerals 10 (2020) 467