This mineral forms small, prismatic crystals, though only rarely. More frequently, it occurs as chin, foliated masses or in massive or columnar habits. The color is usually a rich lemon-yellow, though it can be brownish yellow. The streak is pale yellow. Orpiment is transparent to translucent. On fresh surfaces, the luster is resinous, but the cleavage surfaces are pearly.
FORMATION This mineral is found in lowtemperature hydrothermal veins, often with stibnite and realgar. Orpiment also forms in the crusts deposited around hot springs.
TESTS This mineral fuses quite easily. When heated, it gives off a very strong smell of garlic, typical for a mineral rich in arsenic.
Orpiment also dissolves in nitric acid, leaving behind traces of yellow sulfur on the liquid surface.
Group: SULFIDES
Composition: As₂S₃
Hardness: 1½–2
SG: 3.49
Cleavage: Perfect
Fracture: Uneven
