Sulphosalt

Any of a large group of minerals, mostly rare species, marked by some of the most complicated atomic and crystal structures known to inorganic chemistry. They conform to the general composition A_mB_nX_p, in which m, n, and p are integers; A is generally lead, silver or copper, or less commonly thallium, iron, manganese, mercury, zinc or vanadium; B may be antimony, arsenic, bismuth, tin, or germanium; and X may be sulfur, tellurium or selenium (from Britannica & Wikipedia). They can have multiple metal sites.

Formerly it was believed that the sulfosalts were salts of complex hypothetical thioantimonic or thioarsenic acids (e.g., HSbS2, H18As4S15, H3AsS3), but X-ray diffraction analyses indicate that the atomic structures of many sulfosalts are based on structural fragments of simpler compounds such as galena (lead sulfide; PbS) blocks and stibnite (antimony trisulfide; Sb2S3) sheets. No encompassing theory has been evolved to rationalize many of these curious compounds. The complexity of many of the structures evidently results from their having crystallized at low temperatures and the consequent high degree of ordering of the metal atoms. Syntheses of such compositions at higher temperature usually result in structures simpler than the complicated low-temperature forms(from Britannica).

Common sulphosalt species include the tetrahedrite group^M2(A₆)^M1(B₄ C₂)^X3(D₄)^S1(Y₁₂)^S2(Z), bournonitePbCuSbS₃, jamesonitePb₄FeSb₆S₁₄, boulangeritePb₅Sb₄S₁₁, pyrargyite and proustiteAg₃AsS₃.