realgar

{{infobox mineral | name = Realgar | category = Sulfide mineral | boxwidth = 330px | boxbgcolor = | image = Realgar-229713.jpg | imagesize = | alt = | caption = Realgar crystals, Royal Reward Mine, King County, Washington, US | formula = or AsS | IMAsymbol = Rlg | molweight = | strunz = 2.FA.15a | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/n (no. 14) | unit cell = a = 9.325(3) Å b = 13.571(5) Å c = 6.587(3) Å β = 106.43°; Z = 16 | color = Red to yellow-orange; in polished section, pale gray, with abundant yellow to red internal reflections | colour = | habit = Prismatic striated crystals; more commonly massive, coarse to fine granular, or as incrustations | twinning = Contact twins on {100} | cleavage = Good on {010}; less so on {101}, {100}, {120}, and {110} | fracture = | tenacity = Sectile, slightly brittle | mohs = 1.5–2 | luster = Resinous to greasy | streak = Red-orange to red | diaphaneity = Transparent | gravity = 3.56 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 2.538 nβ = 2.684 nγ = 2.704 | birefringence = δ = 0.166 | pleochroism = Nearly colorless to pale golden yellow | 2V = 40° | dispersion = r > v, very strong | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = Toxic and carcinogenic. Disintegrates on long exposure to light to a powder composed of pararealgar or arsenolite and orpiment. | alteration = | references = }}

Realgar ( ), also known as arsenic blende, ruby sulphur or ruby of arsenic, is an arsenic sulfide mineral with the chemical formula . It is a soft mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment (). It is orange-red in color, and burns with a bluish flame releasing fumes of arsenic and sulfur. It is trimorphous with pararealgar and bonazziite.