Pararealgar

Pararealgar is a polymorph of the mineral realgar (arsenic monosulfide, AsS). It is obtained from realgar by exposure to light. It has a monoclinic crystal structure.

Pararealgar from two localities in British Columbia (Mount Washington on Vancouver Island and the Gray Rock property, Lillooet district), was first identified in 1980 as a new polymorph of AsS. Pararealgar always occurs as an alteration product of realgar. Since it resembles orpiment, pararealgar remained unidentified for a long time. In 1992 it was confirmed that the well-known degradation product of realgar, formed by exposure to light, consists of pararealgar. It is the stable AsS polymorph that is formed under the influence of light, whereas the formation of orpiment (AS2S3) requires a much higher pressure of S than normally present in air.

Reference
P. Bonazzi, S. Menchetti, G. Pratesi, The crystal structure of pararealgar, American Mineralogist, vol. 80, pp. 400-403 (1995) pdf