Metacinnabar

Metacinnabar is the high-temperature polymorph of mercuric sulfide (HgS). It is the cubic polymorph commonly referred to as beta-HgS, in contrast to cinnabar, which is the trigonal (alpha-HgS) form. Metacinnabar forms when cinnabar is subjected to elevated temperatures or pressures and can persist as a metastable phase at ambient conditions.

Crystal structure and appearance: Metacinnabar adopts a cubic zinc blende-type structure (space group F-43m). Crystals are typically small to microcrystalline, often appearing black to dark gray with a metallic luster, and are relatively soft like cinnabar.

Occurrence and formation: In nature it is rare; metacinnabar is mainly reported from heated cinnabar deposits, hydrothermal environments, or laboratory syntheses. It can replace cinnabar in situ during thermal metamorphism or rapid heating and is metastable at ordinary conditions, gradually transforming back to cinnabar over time.

Identification and safety: Metacinnabar is distinguished from cinnabar by its cubic crystal structure, best identified by X-ray diffraction, with Raman spectra and infrared data also showing characteristic differences. As a mercury sulfide, it is toxic, and handling should follow appropriate safety protocols to avoid mercury exposure.