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Formula: As2S3 with oxidation states As3+2S2-3
Sulphide of arsenic
Crystal System: Monoclinic
Specific gravity: 3.49 measured, 3.48 calculated
Hardness: 1½ to 2
Streak: Light yellow
Colour: Lemon yellow to orange yellow
Solubility: Slightly soluble in nitric acid
Common impurities: Hg,Ge,Sb
Environments:
Fumeroles and hot spring deposits
Hydrothermal environments
Orpiment is a rare mineral, usually associated with realgar and formed
under similar conditions.
It occurs in epithermal (low temperature) hydrothermal
silver and lead ore veins; it is also found
in hot springs and fumaroles.
Common associated minerals include arsenic,
baryte, calcite,
gypsum, realgar,
stibnite and cinnabar
(HOM).
Localities
At the Siglo Veinte mine, Llallagua, Rafael Bustillo, Potosí, Bolivia, orange-yellow orpiment coating
quartz and originating from weathered
arsenopyrite has been found
(Min Rec 37-2.145).
At the Jiepaiyu mine, Shimen deposit, Shimen county, Changde, Hunan, China, druses of short acicular crystals of
orpiment, sometimes coating calcite crystals, and crystals of
orpiment to 10cm long and 1 cm in diameter have been found. Calcite
is the only associated mineral; orpiment can occur as inclusions in
calcite or as large crystals associated with
calcite
(Min Rec 38-1.49-52).
At the Palomo mine, Huachocolpa District, Huancavelica Province, Huancavelica, Peru, greenish yellow orpiment
crusts to about 3 mm cover pyrite,
sphalerite and chalcopyrite.
Orpiment also forms orange crusts on druses of realgar crystals, with
some of the orpiment crusts partially covered by thin crusts of tiny white
baryte crystals (Min Rec 42-2.166). Orpiment
pseudomorphs after realgar are
a feature of this mine, in sizes up to 4 cm
(Min Rec 39-2.97-98).
At the Quiruvilca mine, Quiruvilca District, Santiago de Chuco Province, La Libertad, Peru, orpiment occurs as
orange crusts with colourless baryte crystals to 5 cm
(Min Rec 42-2.141).
Alteration
Orpiment is an alteration product of other arsenic
minerals, especially realgar
(Webmin).
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