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Orpiment Mineral Facts:

Chemical Formula: As₂S₃; contains 61% Arsenic by weight

Colors: Yellow to yellow-orange.
Its streak is always of some lighter shade than that of the mineral.

Hardness: 1.5 to 2.0

Density: 3.4 to 3.5

Cleavage: Very perfect cleavage parallel to clinopinacoid.

Crystallography: Monoclinic
Usually occurs foliated or massive. The laminae are flexible, but not elastic. The mineral is sectile and can be cut. 

Luster:. Resinous luster, pearly on cleavage face.  Translucent.

Optics: (Refractive Index):  Opaque

Composition, Structure and Associated Minerals:
Orpiment, though more abundant than realgar, is not a common mineral. It is usually found in foliated or columnar masses with a bright yellow color. It shades into orange when the mineral is impure through the admixture of realgar.  In small pieces, orpiment is translucent and possesses an orange and greenish yellow pleochroism.

Identification and Diagnostics
Characterized by its yellow color, perfect cleavage and foliated structure. Crystals small and rarely distinct. The chemical properties of orpiment are the same as those described for realgar, except that the sublimate in the closed tube is yellow instead of red. Distinguished from sulfur by it's perfect cleavage.

Occurrence, Localities and Origins:
The origin of orpiment is similar to that of realgar.  Becasue of this, it is usually associated usually with that other arsenic sulfide, realgar. It occurs in veins associated with various ores, found in various places in Hungary; in Kurdistan; in Peru, etc. Occurs at Mercur, Utah. Deposited from geyser waters in the Yellowstone Park. Orpiment occurs in the same forms and in the same places as does realgar. Small specks of it occur on arsenical iron at Edenville, N. Y. It is also found in the deposits of Steamboat Springs, Nevada. Together with realgar, it is associated with a number of the sediment hosted gold ores of Nevada. It is also formed by the oxidation of this mineral.  Still, it is an important ore of arsenic. Many of the industrial uses of arsenic have been eliminated because of its toxic nature, but some high tech applications remain.

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