realgar

[Middle English, from Medieval Latin, from Catalan, from Arabic rahj al-ġār, powder (of) the mine or cave : rahj, powder + al-, the + ġār, cave.]

A mineral having composition AsS and crystallizing in the monoclinic system. Realgar can occur in short, vertically striated crystals, but more frequently is granular and in crusts. The hardness is 1.5–2 (Mohs scale) and the specific gravity is 3.48. The luster is resinous and the color red to orange. Realgar is found in ores of lead, silver, and gold associated with orpiment and stibnite. It occurs with the silver and lead ores in Hungary, Czechoslovakia, and Germany. Good crystals have come from Binnenthal, Switzerland, and Allchar, Macedonia. In the United States it is found at Manhattan, Nevada; Mercer, Utah; and as deposits from geyser waters in Yellowstone National Park. See also Arsenic; Orpiment; Stibnite.

Environment

In low-temperature veins.

Good crystals fairly common, usually prismatic; also massive.

Orange-red. Luster resinous; hardness 1Ɖ-2; specific gravity 3.4-3.5; streak red-orange; fracture subconchoidal; cleavage perfect side and fair basal. Sectile; translucent to transparent.

Arsenic sulfide (70.1% As, 29.9% S).

Fuses easily, melting to shiny mass, spreading and completely volatilizing with the characteristic arsenic (garlic) odor. Makes red deposit shading to orange and yellow on walls of closed tube, while yellow fumes escape from the end.

It could be mistaken for cinnabar; blowpipe and closed-tube tests would show the differences. Often associated with yellow orpiment.

Not a common mineral, but an important ore of arsenic. Usually in rich veins, as at the Getchell gold mine in Nevada, with calcite and yellow, micaceous orpiment. Like stibnite and cinnabar, it is a late magmatic mineral associated with cooler solutions such as those at hot springs. It is often associated with colemanite, as at Boron, California, and Bigadiç, Turkey. Isolated single 1-in. (2-3 cm) crystals have been found in marble pockets in Carrara, Italy, and in Binnatal, Switzerland. Excellent old-time crystal specimens have come from Transylvania, Romania. Rich masses and well-developed crystal pockets are found in the U.S. at the Getchell gold mine, Nevada, and very fine singles come from the Reward Mine, King Co., Washington. Hunan, China, is now producing the finest realgars of this generation in rich-hued crystals of 2 in. (5 cm) and more.

Realgar is a very unstable mineral, affected by exposure to light. Most museum specimens quickly become disgraceful on the shelf, rimmed with a crumbled orange dust (but still structurally realgar) after a few years of display. Storage in darkness delays the inevitable deterioration, but even this does not indefinitely preserve specimens. Other factors, such as exposure to air or release of pressure, may play a part in their breakup.

The ancient Chinese apparently admired its red hue and cemented small fragments into chunks for carvings, though all these artifacts are now badly affected by time, light, and air. Most good American specimens are obtained from Getchell Mine material by dissolving in hydrochloric acid the calcite from filled veins to expose the arsenic sulfide crystals (realgar and orpiment) lining the walls.

A naturally occurring deposit of arsenic disulfide, used as a pigment and a potential source of poisoning in animals.

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A sample of realgar

Part of realgar's crystal structure

The unit cell of realgar, showing clearly the As₄S₄ molecules it contains

Realgar, α-As₄S₄, is an arsenic sulfide mineral. It was known as Sandarach to Aristotle.^[1] It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment (As₂S₃). It is orange-red in colour, melts at 320 °C, and burns with a bluish flame releasing fumes of arsenic and sulfur. Realgar is soft with a Mohs hardness of 1.5 to 2 and has a specific gravity of 3.5. Its molecular weight is 106.99. Its streak is orange colored. Realgar has a sub-metallic luster.

Its name comes from Arabic rahj al-ġār – 'powder of the mine,' via Catalan, Middle Latin, and Middle English^[2]. It has been proposed that this arose through a misspelling of rahj al-fār ('rat powder', supposedly due to its use as a rodenticide), but this is unlikely given the amount of evidence for the ġ spelling.^[3] In India, realgar is known as manseel, and orpiment as hartal.

It was, along with orpiment, a significant item of trade in the ancient Roman Empire and was used as a pigment and a medicine. It was also used as a medicine in China and "is made up into household ornaments, such as wine pots, wine cups, images, paperweights, and various other kinds of ornaments and charms, to be kept near at hand in use, or worn about the person, with a view of warding off disease."

It is commonly held that after a long period of exposure to light realgar changes form to a yellow powder known as pararealgar (β-As₄S₄). It was once thought that this powder was the yellow sulfide orpiment, but has been recently shown to be a distinct chemical compound.

Realgar, orpiment, and arsenopyrite provide nearly all the world's supply of arsenic as a byproduct of smelting concentrates derived from these ores.

On long exposure to light, realgar disintegrates into a reddish-yellow powder, requiring that specimens be protected from light exposure.

Realgar was also used by firework manufacturers to create the color white in fireworks prior to the availability of powdered metals such as Aluminum, Magnesium and Titanium. It is still used in combination with Potassium chlorate to make a contact explosive known as "red explosive" for some types of torpedoes and other novelty exploding fireworks, as well in the cores of some types of crackling stars.

Other traditional uses include manufacturing shot, printing and dyeing calico, and depilating and tanning hides.

Sources

• 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. 425-426.

External links

• Webmineral.com Mineralogy Database: Realgar
• Mindat.org: Realgar
• Mindat.org: Pararealgar
• American Mineralogist Vol 80, pp 400-403, 1995[1]
• American Mineralogist Vol 20, pp 1266-1274, 1992[2]

References

1. ^ History of Chemistry T. E. Thorpe READ BOOKS, 2007 ISBN 1408603934, 9781408603932
2. ^ Philip Babcock Grove, ed (1993). Webster's Third New International Dictionary. Merriam-Webster, inc.. ISBN 3-8290-5292-8. 
3. ^ "realgar, n.". Oxford English Dictionary Online. Oxford University Press. March 2009. 

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