Tiny, prismatic, lustrous, red fangite crystals on dolomite. Field of view: 0.75 mm. Photo: Mischa Crumbach. From Lengenbach quarry. — Photo: Mischa Crumbach · CC BY 4.0
Fangite
A thallium sulfosalt discovered in Utah's Mercur gold deposit — surviving specimens sit in the Smithsonian.
Intro adapted from Wikipedia (CC BY-SA).
Fangite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Tl₃AsS₄.
It crystallizes in the orthorhombic crystal system (space group Pnma).
Key properties: Mohs hardness 2–2.5; specific gravity 6.185; Vitreous, tarnishing to metallic luster.
IMA symbol Fg, IMA status: approved, named in 1991.
Found in a sediment-hosted disseminated gold deposit, in sulfide-rich carbonaceous ore, probably produced through alteration of Tl-As-rich sulfosalts or sulfides.
It is found associated with Pyrite, realgar, orpiment, calcite.
In honor of Dr. Jen-Ho Fang (1929–), crystal chemist, University of Alabama, Tuscaloosa, Alabama, USA.