Realgar

Realgar is a mineral consisting of arsenic monosulfide (α-As₄S₄). Its composition in weight is: 70.03% arsenic and 29.97 % sulphur and its molecular weight is 106.9.

Realgar has a red to yellow color and is often found among other arsenic minerals, less often with silver and lead ores. Its crystal structure is monoclinic, showing short prismatic crytals. Under a prolonged exposure to light it disintegrates to a powder which long was thought to be orpiment, but in fact is the polymorph pararealgar. . Realgar when heated forms yellow arsenic, a poisonous oxide used in metallurgy.

The name is from the Arabic, ''rahj al-ġār [powder of the mine].