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

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, 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.

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

• 

  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. Size: 5.5 x 4.0 x 3.8 cm.

• 

  El'brusskiy arsenic mine, Kabardino-Balkarian Republic, Northern Caucasus Region, Russia. Size 5.8 x 3.5 x 1.7 cm.

References

• 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' (Hebrew)
• Orpiment, Colourlex