Orpiment

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Orpiment
Orpiment mineral.jpg
Orpiment
General
Category Sulfide mineral
Formula
(repeating unit)
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 is a deep orange-yellow colored arsenic sulfide mineral with formula As2S3. It is found in volcanic fumaroles, low temperature hydrothermal veins, and hot springs and is formed both by sublimation and as a byproduct of the decay of another arsenic mineral, realgar. It takes its name from the Latin auripigmentum (aurum − gold + pigmentumpigment) because of its deep-yellow color.

Historical uses[edit]

Orpiment was traded in the Roman Empire and was used as a medicine in China even though it is very toxic. It has been used as a fly poison and to tip arrows with poison. Because of its striking color, it was of interest to alchemists, both in China and the West, searching for a way to make gold.

For centuries, orpiment was ground down and used as a pigment in painting and for sealing wax. It was one of the few clear, bright-yellow pigments available to artists until the 19th Century. However, its extreme toxicity and incompatibility with other common pigments, including lead and copper-based substances such as verdigris and azurite, meant that its use as a pigment ended when cadmium yellows and dye-based colors were introduced during the 19th Century.

Orpiment, as the Latin Auripigmentum, is mentioned by Robert Hooke in Micrographia for the manufacture of small shot in the 17th century.[4]

Contemporary uses[edit]

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 commonly used in rural India as a depilatory. It is used in the tanning industry to remove hair from hides.

Physical and optical properties[edit]

Orpiment 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.

Crystal structure[edit]

Gallery of orpiment specimens[edit]

References[edit]

  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
  4. ^ Hooke, Robert. "Micrographia". Project Gutenberg. Retrieved 24 October 2012. 
  • 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[edit]