guettardite

{{infobox mineral | name = Guettardite | image = | alt = | caption = | category = Sulfosalt mineral | formula = | IMAsymbol = Gue | molweight = | strunz = 2.HC.05a | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/a | unit cell = a = 20.17, b = 7.94 c = 8.72 [Å]; β = 101.12°; Z = 8 | color = Grayish black; white with reddish internal reflections in polished section | colour = | habit = Acicular crystals and anhedral grains | twinning = Polysynthetic twinning on {100} | cleavage = Perfect on {001} | fracture = Conchoidal | tenacity = Very brittle | mohs = 4 | luster = Metallic | streak = Brown | diaphaneity = Opaque | gravity = 5.2 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = Relatively strong | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }} Guettardite is a rare arsenic-antimony lead sulfosalt mineral with the chemical formula . It forms gray black metallic prismatic to acicular crystals with monoclinic symmetry. It is a dimorph of the triclinic twinnite.

==Discovery and occurrence== It was first described in 1967 for an occurrence in the Taylor Pit, Madoc, Hastings County, Ontario, Canada. It was named for French naturalist Jean-Étienne Guettard (1715–1786).