Crystal structure of Realgar (AsS, also As4S4) as "ball-and-stick-model" with view parallel to the a-axis. — Photo: Ra'ike (Wikipedia) · CC0
Realgar
Also known as: Ruby sulphur.
The red-orange arsenic sulfide — 'ruby sulphur' — a soft mineral that light slowly transforms into pararealgar.
Intro adapted from Wikipedia (CC BY-SA).
Realgar is an IMA-recognized mineral species in the sulfide class, with the chemical formula AsS.
It crystallizes in the monoclinic crystal system (space group P2_1/n).
Key properties: Mohs hardness 1.5–2; specific gravity 3.56; typically red or orange; Red-orange to red streak; Resinous to greasy luster; optically biaxial.
IMA symbol Rlg, grandfathered status — known before the IMA’s founding, named in 1747.
Most commonly as a low-temperature hydrothermal vein mineral in association with As–Sb minerals. It is also found as volcanic sublimations and in hot spring deposits; in carbonate and clay sedimentary rocks.
It is found associated with Orpiment, arsenolite, other arsenic minerals, calcite, barite.
The name comes from the Arabic rahj al ghar for powder of the mine.
Collections: Collector Favorites, Rare & Remarkable.