Guettardite
A rare gray-black lead arsenic-antimony sulfosalt forming monoclinic prismatic needles.
Intro adapted from Wikipedia (CC BY-SA).
Guettardite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Pb₈(Sb₀.₅₆As₀.₄₄)₁₆S₃₂.
It crystallizes in the monoclinic crystal system (space group P2_1/a). It is a member of the Sartorite structural group.
Key properties: specific gravity 5.26; Metallic luster.
IMA symbol Gue, IMA status: approved, named in 1966.
Of low-temperature hydrothermal origin, in marbles (Madoc, Canada; Seravezza, Italy).
It is found associated with Pyrite, sphalerite, wurtzite, galena, stibnite, orpiment, realgar, enargite, tetrahedrite, zinkenite, jordanite, bournonite, sterryite, boulangerite.
The mineral was named to honor Jean Etienne Guettard (1715–1786), French mineralogist and geologist.