An insight into the inverse transformation of realgar altered by light

The light-induced alteration of realgar and b-As4S4 is a well-known phenomenon but still displays interesting aspects not completely explained. In the transformation from realgar to pararealgar the molecule As4S4 undergoes a structural modification, and ever since the initial studies two important issues have been highlighted: the influence of the oxygen and the reversibility of the process. The present study focuses the attention on the question if realgar, when altered by means of the light and when annealed, might behave as b-As4S4 does. These results display that the phenomenon is more complex. The alteration of realgar with the presence of the air yields pararealgar along with arsenolite, a small quantity of uzonite and amorphous material, and when the air is not present pararealgar is the only product. In the first case, when annealing the products of alteration at 220 °C, alacranite, b-As4S4, realgar, and little amorphous material occur along with arsenolite. In the second case at first b-As4S4 crystallizes, then it turns into realgar, but this process yields orpiment and amorphous material as it moves forward, showing that from the products of alteration of realgar it is not possible to obtain the starting material. Examination of the stoichiometry of the products let to infer that the amorphous material occurring in the two cases has very different content of arsenic and sulfur.