White tufts of the rare mineral coquandite in a matrix of stibnite from the type locality (Pereta Mine, Scansano, Tuscany, Italy). — Photo: David Hospital · CC BY-SA 4.0
Coquandite
Coquandite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Sb³⁺₆₊ₓO₈₊ₓ(SO₄)(OH)ₓ(H₂O)₁₋ₓ (x = 0.3).
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: specific gravity 5.78; White streak; Adamantine to pearly luster; optically biaxial.
IMA symbol Cqd, IMA status: approved, named in 1991.
Probably produced by action of H2 SO4 on stibnite, in a stibnite vein in silicified limestone (Pereta mine, Italy).
It is found associated with Klebelsbergite, peretaite, valentinite, sénarmontite, stibiconite, stibnite, sulfur, gypsum, quartz.
The name refers to Henri-Jean-Baptiste Coquand (1813–1881), Professor of Geology and Mineralogy, University of Marseilles, Marseilles, France, for his early work on the antimony deposits of Tuscany.