Orpiment

Orpiment

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 As
2S
3. 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 + pigmentum − pigment) because of its deep-yellow color.

Historical uses

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,[4] meant that its use as a pigment ended when cadmium yellows, chromium yellows and organic 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.[5]

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

Physical and optical properties

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

Gallery of orpiment specimens

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
  4. ↑ Fitzhugh, E.W., Orpiment and Realgar, in Artists’ Pigments, A Handbook of Their History and Characteristics, Vol 3: E.W. Fitzhugh (Ed.) Oxford University Press 1997, p. 52
  5. ↑ Hooke, Robert. "Micrographia". Project Gutenberg. Retrieved 24 October 2012.

External links

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