Kegginite, Pb₃Ca₃[AsV₁₂O₄₀(VO)]·20H₂O, a new mineral with a novel ε-isomer of the Keggin anion Available to Purchase

Kegginite, Pb₃Ca₃[AsV₁₂O₄₀(VO)]·20H₂O, is a new mineral species from the Packrat mine, near Gateway, Mesa County, Colorado, U.S.A. It is a secondary mineral found on asphaltum in a montroseite-and corvusite-bearing sandstone. Other secondary minerals found in close association with kegginite are ansermetite, gypsum, mesaite, and sherwoodite. Crystals of kegginite are orange-red simple hexagonal tablets. The streak is pinkish-orange, the luster is vitreous, the Mohs hardness is about 2, the tenacity is brittle, fracture is irregular, cleavage is good on {001}, and the calculated density is 2.69 g/cm³. Kegginite is optically uniaxial (−) with pleochroism: O orange-red and E red-orange; E < O. Electron microprobe analyses yielded the empirical formula Pb_2.98Ca_2.39Mg_0.56V_13.05As_0.95O₆₁H_40.15. Kegginite is trigonal, $P3¯$⁠, with a = 14.936(5), c = 15.846(5) Å, V = 3061(2) ų, and Z = 2. The crystal structure of kegginite (R₁ = 0.064 for 1356 F_o > 4σF reflections) contains a $[As5+V125+O40(VO)]12−$ polyoxometalate cluster, which is a mono-capped Keggin ε-isomer.