Realgar, also known as ″arsenic blende″, ″ruby sulphur″ or ″ruby of arsenic″, is an arsenic sulfide mineral with the chemical formula α-As4S4. 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. 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 pararealgar and bonazziite.
Realgar crystals, Royal Reward Mine, King County, Washington, US
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
Arsenic is a chemical element; it has symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is a notoriously toxic heavy metal. It occurs naturally in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.
Arsenic
A large sample of native arsenic from Sainte-Marie-aux-Mines, France
Realgar
The arsenic labyrinth, part of Botallack Mine, Cornwall