Twinnite
Twinnite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Pb(Sb₀.₆₃As₀.₃₇)₂S₄.
It crystallizes in the monoclinic crystal system (space group P2_1/n). It is a member of the Sartorite structural group.
Key properties: specific gravity 5.323; Black, with a slightly brownish tint streak; Metallic luster.
IMA symbol Twi, IMA status: approved, named in 1966.
Found in marble with other lead sulfantimonides (Madoc, Canada).
It is found associated with Chabournéite, pierrotite, parapierrotite, stibnite, pyrite, sphalerite, zinkenite, madocite, andorite, smithite, laffittite, routhierite, aktashite.
In honor of Robert Mitchell Thompson (1918–1967), Canadian mineralogist, University of British Columbia, Vancouver, Canada. Thompson is “son of Thomas”; the latter is Aramaic for “a twin”.