Orpiment

Orpiment is a deep-colored, orange-yellow 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. Orpiment takes its name from the Latin auripigmentum (aurum, "gold" + pigmentum, "pigment") because of its deep-yellow color.

Orpiment
General
Category	Sulfide mineral
Formula (repeating unit)	As2S3
Strunz classification	2.FA.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 11.475(5), b = 9.577(4) 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
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]

Historical uses

Bright golden-yellow streak color of orpiment

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 fly poison[4] and to tip arrows with poison.[5] Because of its striking color, it was of interest to alchemists, both in China and the West, searching for a way to make gold. It also has been found in the wall decorations of Tutankhamun's tomb and ancient Egyptian scrolls, and on the walls of the Taj Mahal.[6]

For centuries, orpiment was ground down and used as a pigment in painting and for sealing wax, and was even used in ancient China as a correction fluid.[7] 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,[8][6] 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 is mentioned in the 17th century by Robert Hooke in Micrographia for the manufacture of small shot.[9]

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 (570 °F) to 325 °C (620 °F). Optically, it is biaxial (−) with refractive indices of a = 2.4, b = 2.81, g = 3.02.

Crystal structure

• 
  Orpiment's unit cell
• 
  Orpiment's crystal structure consists of sheets
• 
  The sheets are stacked into layers

Gallery of orpiment specimens

• 
  Orpiment and realgar on a vuggy, quartz matrix, Nishinomaki Mine, Gunma Prefecture, Japan
• 
  Orpiment from La Libertad, Quiruvilca, Peru
• 
  El'brusskiy arsenic mine, Kabardino-Balkarian Republic, Northern Caucasus Region, Russia
• 
  Orpiment crystal from Twin Creeks Mine, Potosi District, Humboldt County, Nevada, United States

See also

• List of inorganic pigments

References

1. Handbook of Mineralogy
2. Mindat.org
3. Webmineral data
4. Miller, George (1826). Popular philosophy: or, The book of nature laid open upon Christian principles, by the ed. of The Cheap magazine.
5. Mesny, William (1899). Mesny's Chinese Miscellany. China Gazette Office.
6. St. Clair, Kassia (2016). The Secret Lives of Colour. London: John Murray. pp. 82–83. ISBN 9781473630819. OCLC 936144129.
8. 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
9. 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.
• 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. 47 – 80

External links

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