Formula: AsS
Sulphide of arsenic
Crystal System: Monoclinic
Specific gravity: 3.56 measured, 3.59 calculated
Hardness: 1½
Streak: Orange yellow
Colour: Deep red to orange
Solubility: Slightly soluble in nitric acid

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, arsenolite, other arsenic minerals, calcite, baryte and stibnite. It also occurs as a volcanic sublimation product associated with sulphur, as a deposit in hot springs, and in carbonate sedimentary rocks (HOM, Dana, MOM).
Realgar decomposes to pararealgar with exposure to light, and decays to form orpiment.


At Llallagua, Rafael Bustillo, Potosí, Bolivia, microcrystals of realgar are occasionally seen coating arsenopyrite, and one mass of native arsenic and gypsum shows lamellae of realgar (Minrec 37.2.151).

At Jiepaiyu Mine, Shimen deposit, Shimen county, Changde, Hunan, China, realgar is the principal ore mineral, and calcite is the main gangue mineral (Minrec 38.1.52).

At Bouismas Mine, Zagora Province, Drâa-Tafilalet Region, Morocco, realgar crystals to 2 mm on calcite have been found in a löllingite vein (Minrec 38.5.389).

At the Palomo Mine, Huachocolpa District, Huancavelica Province, Huancavelica, Peru, very dark red crystals realgarto 4 cm on quartz druses have been found (Minrec 42.2.166), partially altered to pararealgar. A few specimens of realgar to 1 cm have been found with tetrahedrite, galena and sphalerite (Minrec 39.2.99).

At Baia Sprie mine, Baia Sprie, Maramureș, Romania, fine crystals realgar to 6 cm on a chalky white matrix with quartz and pyrite. some show mixtures of realgar and stibnite crystals overlying realgar of an earlier generation (Minrec 37.3.243-244).

At White Caps Mine, Manhattan, Manhattan Mining District, Toquima Range, Nye county, Nevada, USA, acicular crystals of picropharmacolite typically occur on a druse of realgar (Minrec 41.6.547)

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