Realgar

Realgar, α-As₄S₄, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It was known as Sandarach to Aristotle.[5] 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 streak is orange colored. It is trimorphous with alacranite and pararealgar.[1]

Its name comes from Arabic رهج الغار rahj al-ġār – 'powder of the mine,' via Catalan, Middle Latin, and Middle English[6]. 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.[7]
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.

Occurrence

Realgar most commonly occurs as a low-temperature hydrothermal vein mineral associated with other arsenic and antimony minerals. It also occurs as volcanic sublimations and in hot spring deposits. It occurs in association with orpiment, arsenolite, calcite and barite.[1]

It is found with lead, silver and gold ores in Hungary, Bohemia and Saxony. In the US it occurs notably in Mercur, Utah; Manhattan, Nevada and in the geyser deposits of Yellowstone National Park.[4]

Uses

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

Realgar was also used by firework manufacturers to create the color white in fireworks prior to the availability of powdered metals such as aluminium, 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.

Traditional and historic uses

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."

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

References

1. ^ a b c http://rruff.geo.arizona.edu/doclib/hom/realgar.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-3375.html Mindat.org
3. ^ http://webmineral.com/data/Realgar.shtml Webmineral
4. ^ a b Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 1985, 20th ed., p. 282 ISBN 0-471-80580-7
5. ^ History of Chemistry T. E. Thorpe READ BOOKS, 2007 ISBN 1408603934, 9781408603932
6. ^ Philip Babcock Grove, ed (1993). Webster's Third New International Dictionary. Merriam-Webster, inc.. ISBN 3-8290-5292-8.
7. ^ "realgar, n.". Oxford English Dictionary Online. Oxford University Press. March 2009.

* 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

* Mindat.org: Pararealgar
* American Mineralogist Vol 80, pp 400-403, 1995[1]
* American Mineralogist Vol 20, pp 1266-1274, 1992[2]


List of minerals