Outstanding deep red crystals of the extremely rare thallium mineral gillulyite from the type and only known locality known worldwide: Lulu Cut, South Mercur Pit, Tooele County, Utah, United States of America. This material was discovered in 1991, and has never been found in any other locality. — Photo: David Hospital · CC BY-SA 4.0
Gillulyite
Gillulyite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Tl₂As₇.₅Sb₀.₃S₁₃.
It crystallizes in the monoclinic crystal system (space group P2/n).
Key properties: Mohs hardness 2–2.5; specific gravity 4.02; Bright brick-red streak; Vitreous to metallic if tarnished luster.
IMA symbol Gly, IMA status: approved, named in 1989.
Found in a sediment-hosted disseminated gold deposit, in organic-rich unoxidized carbonates.
It is found associated with Orpiment, realgar, lorandite, raguinite, pyrite, barite, calcite.
The mineral was named to honor James C. Guilluly (1896–1980), U.S. Geological Survey, who worked in the area of the Mercur deposit.