The structure of thorikosite,a naturally occurring member of the bismuth oxyhalide group

Thorikosite, (Pb₃Sb_0.6As_0.4)(O₃0H)Cl₂, is a naturally occurring member of the bismuth oxyhalide group isostructural with LiBi₃O₄Cl₂. The space group isI4/mmm witha = 3.919(1)A?,c = 12.854(5)A?, andZ = 1. A crystal structure analysis showed complete solid solution of Pb²⁺, Sb³⁺, and As³⁺ on the single cation site and large atomic temperature factors indicative of pervasive structural disorder. The latter is due to the structural adjustments necessary to accommodate cations of very different sizes in the same site. Thorikosite is closely related to synthetic tetragonal PbSbO₂Cl through the coupled substitution Sb³⁺O^2? ? Pb²⁺(OH)^?.