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 Pb2.98Ca2.39Mg0.56V13.05As0.95O61H40.15. Kegginite is trigonal, P3¯, with a = 14.936(5), c = 15.846(5) Å, V = 3061(2) Å3, and Z = 2. The crystal structure of kegginite (R1 = 0.064 for 1356 Fo > 4σF reflections) contains a [As5+V125+O40(VO)]12 polyoxometalate cluster, which is a mono-capped Keggin ε-isomer.

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