A crystallographic review of arsenic sulfides: Effects of chemical variations and changes induced by exposure to light.

Abstract. Crystal data for natural and synthetic arsenic sulfides are reported and discussed. Most of them [alpha- and beta-dimorphite, realgar, beta-As4S4 phase, pararealgar, Kutoglu's As4S4(II) phase, alacranite, uzonite, orthorhombic As4S5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As4Sn (n = 3, 4 and 5) molecules held together by weak interactions of van der Waals character. Their structures are compared in terms of molecular packing and molecular parameters. The layered structural arrangement of orpiment, As2S3, is described and the effects of the incorporation of Se replacing for S is discussed. The structures of wakabayashilite and getchellite, which contain mixed (As,Sb) coordination polyhedra, are also described to outline the geometric effects of the Sb ? As substitution. The results of recent studies dealing with the effects of the exposure of realgar or other arsenic sulfides to visible light are reported and discussed. Their interest in the study of arsenical pigments and their preservation in artwork is outlined with some examples of application.