Orpiment
| Orpiment | |
|---|---|
 Orpiment | |
| General | |
| Category | Sulfide mineral |
| Formula (repeating unit) | As2S3 |
| Strunz classification | 02.FA.30 |
| Crystal system | Monoclinic Prismatic |
| Space group | 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 |
| 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 mineral, a form of arsenic sulphide that occurs worldwide. 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 and dye-based colors were introduced during the 19th Century.
Orpiment is mentioned by Robert Hooke in Micrographia for the manufacture of small shot in the 17th century. Hooke transcribed an account by Sir Robert Moray to the Royal Society:
- Take Lead out of the Pig what quantity you please, melt it down, stir and clear it with an iron Ladle, gathering together the blackish parts that swim at top like scum, and when you see the colour of the clear Lead to be greenish, but no sooner, strew upon it Auripigmentum powdered according to the quantity of Lead, about as much as will lye upon a half Crown piece will serve for eighteen or twenty pound weight of some sorts of Lead; others will require more, or less.[4]
Auripigmentum here is the Latin for Orpiment.[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.
Technical Properties
Orpiment, As2S3, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.49. 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
- ^ http://rruff.geo.arizona.edu/doclib/hom/orpiment.pdf Handbook of Mineralogy
- ^ http://www.mindat.org/min-3021.html Mindat.org
- ^ http://webmineral.com/data/Orpiment.shtml Webmineral data
- ^ Hooke, Robert. "Micrographia". Project Gutenberg. Retrieved 24 October 2012.
- ^ "Wiktionary".
- 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' Template:He icon
