Coquandite, a new antimony oxy-sulfate hydrate, occurs as spheroidal knobs of silky fibers or, rarely, as tiny transparent colorless lamellar crystals on stibnite at the Pereta mine, Tuscany, Italy; it is assocd. with klebelsbergite, peretaite, valentinite, sulfur, gypsum, stibiconite, and senarmontite. Coquandite is triclinic P‾1, with a 11.434(7), b 29.77(4), c 11.314(4) Å, α 91.07(7)°, β 119.24(3)°, γ 92.82(1)°. It has a cell vol. of 3352(5) Å3 with Z = 12 and a calcd. d. of 5.78 g cm-3. The crystals, elongated along [001] and flattened on {010}, display polysynthetic twinning with (010) as the twin plane. Optically, they are biaxial (+) with z ≈ c, 2V » 60°, n = 2.08(5). The strongest lines of the x-ray powder pattern are [d in Å, (I), (hkl)] 14.84(50)(020), 9.27(41)(111, 110) 6.81(67)(130), 3.304(93)(090), 3.092(100)(330). Coquandite has also been found at the Cetine mine, Tuscany, Italy, and at the Lucky Knock mine, Tonasket, Okanogan County, Washington, USA. Microprobe chem. analyses (elemental microanal. for H) gave Sb2O3 88.91, SO3 8.35, CaO 0.04, Na2O 0.03, H2O 1.43, total 98.76 wt.%, corresponding to the empirical formula (Sb + S = 7) Sb5.98Ca0.01Na0.01O7.96(SO4)1.02.0.78H2O, and to the idealized formula Sb6O8(SO4).H2O. The IR spectrum, which confirms the presence of water in the formula, is given. A partial structural arrangement is also given: the Sb polyhedra lie in nine layers perpendicular to [010] and form 'hexagonally' shaped groups surrounded by SO4 tetrahedra.

Coquandite, Sb6O8(SO4).H2O, a new mineral from Pereta, Tuscany, Italy, and two other localities / C., Sabelli; P., Orlandi; Vezzalini, Maria Giovanna. - In: MINERALOGICAL MAGAZINE. - ISSN 0026-461X. - STAMPA. - 56(1992), pp. 599-693.

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