Orpiment

Orpiment
Orpiment
General
Category	Sulfide mineral
Chemical formula	As2S3
Strunz classification	02.FA.30
Crystal symmetry	Monoclinic 2/m
Unit cell	a = 11.475(5) Å, b = 9.577(4) Å, c = 4.256(2) Å, β = 90.45(5)°; Z=4
Identification
Color	Lemon-yellow to golden or brownish yellow
Crystal habit	Commonly in foliated columnar or fibrous aggregates; may be reniform or botryoidal; also granular or powdery; rarely as prismatic crystals
Crystal system	Monoclinic Prismatic
Twinning	On {100}
Cleavage	Perfect on {010}, imperfect on {100};
Tenacity	Sectile
Mohs scale hardness	1.5 - 2
Luster	Resinous, pearly on cleavage surface
Streak	Pale lemon-yellow
Diaphaneity	Transparent
Specific gravity	3.49
Optical properties	Biaxial (-)
Refractive index	nα = 2.400 nβ = 2.810 nγ = 3.020
Birefringence	δ = 0.620
Pleochroism	In reflected light, strong, white to pale gray with reddish tint; in transmitted light, Y = yellow, Z = greenish yellow
2V angle	Measured: 30° to 76°, Calculated: 62°
Dispersion	r > v, strong
References	[1][2][3]

Orpiment, As₂S₃, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.46. 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 crystals, Humboldt Co., Nevada, USA

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

It is presently used in the production of infrared-transmitting glass, oil cloth, linoleum; in semiconductors and photoconductors, as a pigment and in 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

• 

  orpiment's unit cell
• 

  orpiment's crystal structure consists of sheets
• 

  the sheets are stacked into layers

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/orpiment.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-3021.html Mindat.org
3. ^ http://webmineral.com/data/Orpiment.shtml Webmineral data

• The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals. 11th Edition. Ed. Susan Budavari. Merck & Co., Inc., N.J., U.S.A. 1989.
• William Mesny. Mesny’s Chinese Miscellany. A Text Book of Notes on China and the Chinese. Shanghai. Vol. III, (1899), p. 251; Vol. IV, (1905), pp.26.

External links

• Webexhibits "Pigments Through the Ages: Orpiment"
• Babylonian Talmud Tractate Chullin see Rashi 'haZarnich' (Hebrew)