Realgar

sulphide

orpiment

Formula: AsS sulphide
Specific gravity: 3.5 to 3.6
Hardness: 1Æ
Streak: Orange yellow
Colour: Deep red to orange
Solubility: Slightly soluble in nitric acid
Environments:

Sedimentary environments
Volcanic sublimates and hot spring deposits
Hydrothermal environments (typical)

Realgar is a common epithermal (low temperature) hydrothermal vein mineral in veins of lead, silver and gold ores associated with orpiment, other arsenic minerals and stibnite. It also occurs as a volcanic sublimation product associated with sulphur, as a deposit in hot springs, and in carbonate sedimentary rocks.

Realgar decomposes to pararealgar with exposure to orange light, and decays to form orpiment.

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