Crystal structure and crystal chemistry of perroudite; a mineral from Coppin Pool, Western Australia

Perroudite, from Coppin Pool, Western Australia, having the composition Hg_4.6Ag_4.4S_4.6(Cl_2.0Br_0.8I_1.6), is orthorhombic, a = 17.43, b = 12.24, c = 4.35 Å, space group P2₁2₁2. In the four asymmetric units, 9.2 Hg and 0.8 Ag are in flattened octahedral coordination with respect to S and halogens, and 8 Ag are tetrahedrally coordinated. Chains of face- and edge-shared Hg (predominantly) octahedra wind their way through the structure in a direction parallel to [010]. Central to each chain are continuous -S-Hg-S- linkages formed from the short diagonal (linear) bonds in the individual flattened octahedra. Vertically stacked columns of octahedral chains in the structure form corrugated layers parallel to (100). The Ag tetrahedra act as bridges between the octahedral chains and layers. Hg₅Ag₄S₅(Cl,Br,I)₄ is proposed as the ideal structural formula for the mineral. A range of composition, based on the ideal structure and interpreted from the results of electron-microprobe analysis, is expressed by the general formula Hg_5-xAg_4+xS_5-x(Cl,Br,I)_4+x, -1.4 < x < 1.4.