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 [a-and b-dimorphite, realgar, b-As 4 S 4 phase, pararealgar, Kutoglu's As 4 S 4 (II) phase, alacranite, uzonite, orthorhombic As 4 S 5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As 4 S n (n ¼ 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, As 2 S 3 , 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.

Crystal-structure properties and the molecular nature of hydrostatically compressed realgar

“…Analytical conditions were 15 kV acceleration voltage, 1 nA beam current and 20 s peak and 10 s background counting time. The electron microprobe analyses from eight different individual measurements reveal almost homogeneous composition As 0.95 S 1.05 with only very small impurities of Sn (up to 0.23 wt %) and Sb (up to 0.86 wt %), which agrees with findings on chemical variations reported by Bonazzi and Bindi (2008). The results of the electron microprobe analysis have been deposited as Table D1.…”

“…Adjacent rods stacked along the c-direction can be interpreted as layers parallel to (010) (Fig. 2), with adjacent layers being related via the n-glide plane (Bonazzi and Bindi 2008).…”

“…Realgar and orpiment are the two most abundant arsenic sulphide minerals, although the As-S system is rather complex with As 4 S (=duranusite), As 4 S 3 (=a-and b-dimorphite), AsS (=realgar, pararealgar and b-As 4 S 4 ), As 4 S 5 (=uzonite), As 8 S 9 (=alacranite) and As 2 S 3 (=orpiment) as established solid phases (Bonazzi and Bindi 2008). The natural occurrence of realgar in low-temperature hydrothermal sulphidic veins suggests that the realgar structure is the stable polymorph for As 4 S 4 at low pressures and temperatures.…”

“…Additionally, realgar has been subject to several studies with respect to its structural degradation on exposure by light, which involve pararealgar (As 4 S 4 with space group P2 1 /c) and speculations on both intermediate non-stoichiometric As 4 S 4?x phases and the possible role of oxygen in the irreversible photo-induced changes (Bonazzi et al 1996(Bonazzi et al , 2006(Bonazzi et al , 2011Bonazzi 2007, 2008). Furthermore, the existence of c-As 4 S 4 and v-As 4 S 4 and also the so-called Kutoglu-phase As 4 S 4 (II) still gives rise to debates on the polymorphs and phase relations (Kutoglu 1976;Bonazzi and Bindi 2008).…”

“…Typical for arsenic sulphides with well-defined crystallographic structure is that they contain molecules composed of covalently bonded As 4 S n (n = 3, 4, 5) cages (Bonazzi and Bindi 2008), that is, As 4 S 3 cages (''dimorphite-type'' molecule), two types of As 4 S 4 cages (''realgar-type'' and ''pararealgar-type'' molecules) and As 4 S 5 cages (''uzonite-type'' molecules). The exceptions are orpiment which has a structure composed of As 2 S 3 infinite layers and the recently discovered polymerised AsS high-pressure high-temperature modification with a layered structure (Brazhkin et al 2011a).…”