Realgar

Realgar is a member of sulfide mineral with chemical composition is As4S4 or AsS. It is bright red or orange in color. Crystals are not often found, but when they occur they are short, prismatic, and striated. Realgar mostly occurs as coarse to fine granular masses and as encrustations. Realgar disintegrates on prolonged exposure to light, forming an opaque yellow powder, which is principally pararealgar. Therefore, specimens are kept in darkened containers. Realgar is typically found in hydrothermal deposits at low temperature (up to 400°F/200°C) often with orpiment and other arsenic minerals. It also forms as a sublimate around volcanoes, hot springs, and geyser deposits and as a weathering product of other arsenic-bearing minerals. Realgar is often found with stibnite and calcite.

Polymorphism & Series: Trimorphous with alacr´anite and pararealgar

Name: From the Arabic rahj al ghar for powder of the mine

Association: Orpiment, arsenolite, other arsenic minerals, calcite, barite

Crystallography: Monoclinic; prismatic. Found in short, vertically striated, prismatic crystals. Frequently coarse to fine granular and often earthy and as an incrustation.

Composition: Arsenic monosulfide, AsS. As = 70.1 percent, S = 29.9 percent.

Diagnostic Features: Realgar can be distinguished by its red color, resinous luster, and almost invariable association with orpiment. Its orange-red streak serves to distinguish it from other red minerals

Chemical Properties of Realgar

Chemical Classification	Sulfide mineral
Chemical Composition	As4S4 or AsS

Physical Properties of Realgar

Color	Red to yellow-orange; in polished section, pale gray, with abundant yellow to red internal reflections
Streak	Red-orange to red
Luster	Resinous to greasy
Cleavage	Good on {010}; less so on {101}, {100}, {120}, and {110}
Diaphaneity	Transparent
Mohs Hardness	1.5–2
Specific Gravity	3.56
Diagnostic Properties	Toxic and carcinogenic. Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment.
Crystal System	Monoclinic
Tenacity	Sectile
Density	3.56 g/cm3 (Measured) 3.59 g/cm3 (Calculated)

Optical Properties of Realgar

Type	Anisotropic
Color / Pleochroism	Nearly colorless to pale golden yellow
Twinning	Contact twins on {100}
Optic Sign	Biaxial (-)
Birefringence	δ = 0.166
Relief	Very High

Occurrence

Realgar is found in veins of lead, silver, and gold ores associated with orpiment, other arsenic minerals, and stibnite. It also occurs as a volcanic sublimation product and as a deposit from hot springs

Uses Area

An ore of arsenic.
Historically used as a pigment
Depilatory
Poison
Ingredient in explosives and fireworks
Ritualistic “medicine,” cosmetic.

Distribution

Only localities for finely crystallized material are given.

In Germany, from Schneeberg, Saxony, and at St. Andreasberg, Harz Mountains.
In the Lengenbach quarry, Binntal, Valais, Switzerland.
In Romania, from Baia Sprie , Cavnic (Kapnikbanya), and at Sacarımb (Nagyag).
In the Czech Republic, from Jachymov (Joachimsthal).
At Alsar (Allchar), near Rosden, Macedonia.
From Men-Kyule, Sakha, Russia.
At Saimoku, Prefecture, Japan.
In the Shimen mine, 33 km southeast of Shimen, Hunan Province, China. In the USA, fine large crystals at the Getchell mine, Potosi district, Humboldt Co., Nevada; in Tooele Co., Utah, at Mercur; in Washington, in the Monte Cristo district, Snohomish Co., and in the Royal Reward mine, in the Green River Gorge, King Co.

References

Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
Mindat.org. (2019). Realgar: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/min-727.html [Accessed 4 Mar. 2019].