31 Photos
Orpiment
Chemical Formula	As2S3
Species	Sulfides
Crystal System	Monoclinic
Mohs Scale	1-2
Specific Gravity	3.49
Color	Lemon-yellow to golden or brownish yellow
Streak	Pale lemon-yellow
Luster	Pearly
Refractive Index	n = 2.400 n = 2.810 n = 3.020
Diaphaneity	Transparent
Cleavage	PerfectPerfect imperfect
Crystal Habit：Commonly in foliated columnar or fibrous aggregates; may be reniform or botryoidal; also granular or powdery; rarely as prismatic crystals

Orpiment, As₂S₃, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.49. It melts at 300 °C to 325 °C. Optically it is biaxial (?) with refractive indices of a=2.4, b=2.81, g=3.02. Orpiment is an orange to yellow mineral that is found worldwide, and occurs as a sublimation product in volcanic fumaroles, low temperature hydrothermal veins, hot springs and as a byproduct of the decay of another arsenic mineral, realgar. It is often found in association with realgar. It takes its name from the Latin auripigmentum (aurum ? gold + pigmentum ? pigment) because of its deep yellow color.

Historical uses

Orpiment was an important item of trade in the Roman Empire and was used as a medicine in China although it is highly toxic. It was also used as a fly poison and to poison arrows. Because of its striking color, it was also a favourite with alchemists searching for a way to make gold, both in China and the West.

Orpiment was ground, processed and used for centuries as a pigment in painting and for sealing wax, being one of the few clear, bright yellow pigments available to artists up until the 19th century. Orpiment presented problems, however, such as its extreme toxicity and its incompatibility with other common pigments like lead and copper-based substances such as verdigris and azurite. The use of orpiment as a pigment material ended almost entirely with the advent of the cadmium yellows and the various dye-based colors of the 19th century.

Contemporary uses

Orpiment is used in the production of infrared-transmitting glass, oil cloth, linoleum, semiconductors, photoconductors, pigments, and fireworks. Mixed with two parts of slaked lime, orpiment is still very commonly used in rural India as a depilatory. It is also used in the tanning industry to remove hair from hides.

Crystal structure

Gallery of orpiment specimens