Realgar

Realgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". 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 color, 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 the Arabic rahj al-ġār (رهج الغار, "powder of the mine"), via Catalan and Medieval Latin, and its earliest record in English is in the 1390s.[5]

Realgar
Realgar crystals, Royal Reward Mine, King County, Washington, US
General
CategorySulfide mineral
Formula
(repeating unit)		As4S4 or AsS
Strunz classification2.FA.15a
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Unit cella = 9.325(3) Å
b = 13.571(5) Å
c = 6.587(3) Å
β = 106.43°; Z = 16
Identification
ColorRed to yellow-orange; in polished section, pale gray, with abundant yellow to red internal reflections
Crystal habitPrismatic striated crystals; more commonly massive, coarse to fine granular, or as incrustations
TwinningContact twins on {100}
CleavageGood on {010}; less so on {101}, {100}, {120}, and {110}
TenacitySectile, slightly brittle
Mohs scale hardness1.5–2
LusterResinous to greasy
StreakRed-orange to red
DiaphaneityTransparent
Specific gravity3.56
Optical propertiesBiaxial (-)
Refractive indexnα = 2.538
nβ = 2.684
nγ = 2.704
Birefringenceδ = 0.166
PleochroismNearly colorless to pale golden yellow
2V angle40°
Dispersionr > v, very strong
Other characteristicsToxic and carcinogenic. Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment.
References[1][2][3][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 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 branded as 'cracker balls', as well in the cores of some types of crackling stars.

Realgar is toxic. It is sometimes used to kill weeds, insects, and rodents,[6] even though more effective arsenic-based agents are available.

Realgar was commonly applied in leather manufacturing to remove the hair from animal pelts. Because realgar is a known carcinogen, and an arsenic poison, and because competitive substitutes are available, it is rarely used today for this purpose.

Historical uses

The ancient Greeks, who called it sandaracha, understood that it was poisonous. It was used to poison rats in medieval Spain and in 16th-century England.[7][8] From this, realgar has also historically been known in English as sandarac.

Realgar was also used by Ancient Greek apothecaries to make a medicine known as "bull's blood".[9] The Greek physician Nicander described a death by bull's blood, which matches the known effects of arsenic poisoning.[9] Bull's blood is the poison that is said to have been used by Themistocles and Midas for suicide.[9]

The Chinese name for realgar is xionghuang 雄黃, literally 'masculine yellow', as opposed to orpiment which was 'feminine yellow'. Its toxicity was also well known to them, and it was sprinkled around houses to repel snakes and insects, as well as being used in Chinese medicine.[10] Realgar is mixed with huangjiu to make realgar wine, which is consumed during the Dragon Boat Festival in order to ward off evil, alluding to its repellent properties. (This practice has become rarer in modern times, with the awareness that realgar is a toxic arsenic compound.)

Realgar was, along with orpiment, a significant item of trade in the ancient Roman Empire and was used as a red paint pigment. Early occurrences of realgar as a red painting pigment are known for works of art from China, India, Central Asia, and Egypt. It was used in European fine-art painting during the Renaissance era, a use which died out by the 18th century.[11] It was also used as medicine. Other traditional uses include manufacturing lead shot, printing and dyeing calico cloth.

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]

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.

  • The unit cell of realgar, showing clearly the As4S4 molecules it contains
  • Cluster of realgar crystals from Getchell Mine, Adam Peak, Potosi District, Humboldt County, Nevada, United States
  • Cherry-red realgar crystals atop a matrix, and a sharp acicular spray of the rare species picropharmacolite (white needles) below
  • Crystals of realgar, quartz, chalcopyrite and galena, from Quiruvilca Mine, La Libertad, Peru

See also

References
  1. Handbook of Mineralogy
  2. Realgar at Mindat.org
  3. Realgar at Webmineral
  4. Klein, Cornelis and Cornelius S. Hurlbut, Manual of Mineralogy, Wiley, 1985, 20th ed., p. 282 ISBN 0-471-80580-7
  5. Philip Babcock Grove, ed. (1993). Webster's Third New International Dictionary. Merriam-Webster, inc. ISBN 3-8290-5292-8.
  6. See e.g. Hazardous Substance Factsheet for Realgar published by New Jersey Department of Environmental Protection (dated April 2008).
  7. archive.org
  8. archive.org (in French)
  9. Arnould, Dominique, "Boire le sang de taureau: la mort de Thémistocle", Revue de philologie, de littérature et d'histoire anciennes, Vol. LXVII, No. 2; 1993, pp. 229–235.
  10. On the toxicity of these medications
  11. Museum of Fine Arts, Boston

Further reading
  • 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.
  • American Mineralogist Vol 80, pp 400–403, 1995
  • American Mineralogist Vol 20, pp 1266–1274, 1992
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