Realgar from various locations, high-purity synthetic realgar, and synthetic realgar with 2 mol% Sb were subjected to unfiltered sunlight, filtered sunlight, and filtered light from a quanz-rungsten-halogen lamp for various times. Both single-crystal and powder samples were used. All of the realgar transformed to pararealgar at wavelengths between about 500 and 670 nm, but no transformation occurred at wavelengths greater than about 670 nm. The reaction rate decreased at wavelengths above 560 nm and was very slow at wave-lengths greater than 610 nm. The high-temperature polymorph, β, also transformed to pararealgar. The alteration to pararealgar did not proceed directly; an intermediate phase, χ, of unknown crystal structure but having the same stoichiometry as realgar, formed first. The time to form χ increased with increasing wavelength at a constant flux of photons. There was no measurable difference in stoichiometry between realgar and pararealgar; both were slightly hyperstoichiometric. Pararealgar forms on the surface as a thin layer or nodules and then fissures at some critical thickness, causing the well-known degradation ofrealgar exposed to light.
The reaction is reversible at elevated temperatures. In all cases, even below the α-β realgar transformation temperature, χ always formed initially, followed by the formation of β. The α phase then formed from the β phase at a rate dependent upon temperature. The time to transform β (formed during the reverse reaction) was about 1 d at 220 °C and 2 d at 175 °C, whereas transformation of normal β, cooled from above the transformation temperature to either 175 or 220 °C, takes several months.
It is proposed that light breaks As-As bonds, which are weaker than As-S bonds, and that the covalently bonded cage molecules form a new crystal structure in which free As is intercalated. The behavior is compared to the photo-decomposition of orpiment, which has been studied extensively.