cinnabar

Cinnabar is the common name for mercuric sulfide, HgS, the principal ore of mercury. It forms a bright red to reddish-orange mineral and occurs in hydrothermal veins and volcanic environments, often with quartz, calcite, pyrite, and other sulfides. Historically, cinnabar has been used to make the pigment vermilion, and it can be roasted to yield elemental mercury.

Cinnabar crystallizes in the trigonal (rhombohedral) system as red crystals, though it also occurs as massive encrustations. There are two polymorphs: alpha-cinnabar, the common red form, and beta-metacinnabar, a high-temperature cubic form. It has a resinous luster, a Mohs hardness of 2–2.5, a high specific gravity around 8.0, and a red streak. The mineral is usually opaque, but may be translucent in thin splinters.

Geographically, cinnabar occurs in notable historic sources such as Idrija in Slovenia, Almaden in Spain, and New Almaden in California, among other hydrothermal deposits worldwide. It forms by deposition of mercury sulfide from mercury-rich fluids and is commonly found with realgar, orpiment, and other sulfide minerals.

Uses and hazards: The pigment vermilion, derived from cinnabar, has been highly valued in art and decoration. However, cinnabar is toxic due to its mercury content, and mercury vapor can be released during roasting to extract mercury, posing health and environmental risks. Today, cinnabar is primarily of geological and historical interest, with mercury production governed by safety regulations.