Caseyite, [(V5+O2)Al10–x(OH)20–2x(H2O)18–2x]2[H2V4+V95+O28][V105+O28]2[(Na,K,Ca)2–y(SO4)2–z⋅(60+8x+y+4z) H2O], where x = 0–2.5, y = 0–2, z = 0–2, is a new mineral (IMA 2019-002) occurring in low-temperature, post-mining, secondary mineral assemblages at the Burro, Packrat, and West Sunday mines in the Uravan Mineral Belt of Colorado, U.S.A. Crystals of caseyite are yellow tapering needles or blades, with a pale yellow streak, vitreous luster, brittle tenacity, curved fracture, no cleavage, Mohs hardness between 2 and 3, and 2.151 g/cm3 calculated density. Caseyite is optically biaxial (+) with α = 1.659(3), β = 1.670(3), γ = 1.720(3) (white light), 2V = 52.6(5)°, has strong r < ν dispersion, optical orientation Za (elongation of needles), and no pleochroism. Electron-probe microanalysis provided the empirical formula [(V5+O2)Al8.94(OH)17.88(H2O)15.88]2[H2V4+V95+O28][V105+O28]2[(Na0.82Ca0.35K0.27)Σ1.44 (SO4)1.33⋅70.24H2O] (+0.94 H). Caseyite is monoclinic, P21/n, a = 14.123(8), b = 30.998(15), c = 21.949(11) Å, β = 97.961(8)°, V = 9516(9) Å3, and Z = 2. The crystal structure (R1 = 0.0654 for 9162 Io>2σI reflections) contains both normal [V10O28]6– and doubly protonated mixed-valence [H2V14+V95+O28]5– decavanadate isopolyanions, and a novel vanadoaluminate heteropolycation (“flat-Al10V☐2”), ideally [(V5+O2)Al10(OH)20(H2O)18]11+, closely related to the technologically important flat-Al13 polyoxocation.

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