| Realgar | |
|---|---|
Realgar crystals |
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| General | |
| Category | Sulfide mineral |
| Chemical formula | As4S4 or AsS |
| Strunz classification | 02.FA.15a |
| Crystal symmetry | Monoclinic 2/m |
| Unit cell | a = 9.325(3) Å, b = 13.571(5) Å, c = 6.587(3) Å, β = 106.43°; Z=16 |
| Identification | |
| Color | Red to yellow-orange; in polished section, pale gray, with abundant yellow to red internal reflections |
| Crystal habit | Prismatic striated crystals; more commonly massive, coarse to fine granular, or as incrustations |
| Crystal system | Monoclinic prismatic |
| Twinning | Contact twins on {100} |
| Cleavage | Good on {010}; less so on {101}, {100}, {120}, and {110} |
| Tenacity | Sectile, slightly brittle |
| Mohs scale hardness | 1.5–2 |
| Luster | Resinous to greasy |
| Streak | Red-orange to red |
| Diaphaneity | Transparent |
| Specific gravity | 3.56 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 2.538 nβ = 2.684 nγ = 2.704 |
| Birefringence | δ = 0.166 |
| Pleochroism | Nearly colorless to pale golden yellow |
| 2V angle | 40° |
| Dispersion | r > v, very strong |
| Other characteristics | Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. |
| References | [1][2][3][4] |
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 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 and Medieval Latin, and its earliest record in English is in the 1390s.[5]
Contents |
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.
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, as well in the cores of some types of crackling stars.
Realgar is poisonous. The ancient Greeks, who called it "sandaracha", knew that it was poisonous. It was used to poison rats in medieval Spain and in 16th century England.[6] It is still sometimes used to kill weeds, insects, and rodents,[7] even though more effective arsenic-based agents are available.
Realgar performs corrosive work when put in contact with various substances. It was commonly applied in leather manufacturing to remove the hair from animal pelts, for example. Because realgar is a known carcinogen, and an arsenic poison, and because competitive substitutes are available, it is rarely used today as a corroding agent.
It was, along with orpiment, a significant item of trade in the ancient Roman Empire and was used as a red paint pigment[8] 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.
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