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Realgar Mineral Facts:

Chemical Formula: AsS
On exposure to the air and to light realgar oxidizes, yielding orpiment (As2Ss) and arsenolite (As2Os).

Colors: Bright Red.
The mineral color and streak are both red to orange - red.

Hardness: 1.5 to 2

Density: 3.55

Cleavage: Indistinct cleavage parallel to clinopinacoid.

Crystallography: Monoclinic
Crystals are short and prismatic, typically vertically striated.

Luster:. Resinous luster, Transparent to opaque.

Optics: (Refractive Index)  2.6

Red Spots on above are Realgar

Composition, Structure and Associated Minerals:
Realgar occurs as a bright red incrustation on other substances, as compact, and granular masses and as crystals implanted on other minerals. It is usually associated with the bright yellow orpiment. In thin splinters it is often translucent or transparent, and strongly pleochroic in red and yellow tints, but in masses it is opaque.

Identification and Diagnostics
Realgar is normally characterized chiefly by deep red color and resinous luster. The mineral is easily fusible and easily volatile. Heated on charcoal yields a volatile white sublimate of arsenious oxide with characteristic garlic odor.  Its bright red color and its reaction for sulfur distinguish realgar from all other minerals but cinnabar, the sulfide of mercury. It may easily be distinguished from cinnabar by its softness, its low specific gravity and the arsenic fumes which it yields when heated on charcoal.

Occurrence, Localities and Origins:
Realgar occurs in masses associated with orpiment and in grains scattered through it at all places where the latter mineral is found. It also occurs associated with silver and lead ores in many places. It is found in crystals implanted on quartz and on the walls of cavities in lavas.  In the United States it forms seams in a sandy clay in Iron Co., Utah. Its crystals are found in calcite in San Bernardino and Trinity Counties, California, and with orpiment it is deposited as a powder by the hot water of the Norris Geyser basin in the Yellowstone National Park.

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