Abstract: Coaquandite, a new antimony oxy-sulphate hydrate, occurs as spheroidal knobs of silky fibres or, rarely, as tiny transparent colourless lamellar crystals on stibnite at the Pereta mine, Tuscany, Italy; it is associated with klebelsbergite, peretaite, valentinite, sulfur, gypsum, stibiconite, and senarmontite. Coquandite is triclinic P1¯, 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 volume of 3352(5)ų with Z = 12 and a calculated density 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. 22 microprobe chemical analyses (elemental microanalysis 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)Sb_5.98Ca_0.01Na_0.01O_7.96(SO₄)_1.02. 0·78H₂O, and to the idealised formula Sb₆O₈(SO₄).H₂O. The I.R. 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.