Tetrarsenic Trisulphide, As₄S₃

Tetrarsenic Trisulphide (Arsenic Tritetritasulphide), As₄S₃, is obtained by fusing together realgar and an excess of arsenic; after cooling, the product is crushed and extracted with carbon disulphide, or allowed to sublime in a vacuum. Yellow orthorhombic prisms are obtained. This sulphide is also formed in small quantity during the photochemical disintegration of realgar.

The crystals have density 3.60 at 19° C. According to Krenner, the sublimed crystals have axial ratios a:b:c = 0.58787:1:0.88258; the optic axial angle 2H_α = 108°46' and the optical character is positive. The density of this product he gives as 2.60 at 19° C., a much lower value than that above. On warming, the crystals darken to an orange colour. The vapour dissociates at high temperatures, as is shown by the following values of the vapour density: at 792° C., 8.204; at about 1000° C., 6.59; (As₄S₃ = 13.67).

Minute orange-yellow crystals, of density 3.58, corresponding in composition with As₄S₃, were obtained from a fumarole in Italy by Schacci and named by him dimorfina, which later became dimorphin or dimorphite. It was described as occurring in two crystalline forms, α- and β-dimorphite, both rhombic, but the α-form is probably identical with orpiment (Dana). The nature of the β-form has not been elucidated and it may be a definite species.