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Kegginite, Pb 3 Ca 3 [AsV 12 O 40 (VO)]·20H 2 O, 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/cm 3 . Kegginite is optically uniaxial (–) with pleochroism:  O  orange-red and  E  red-orange;  E  &lt;  O . Electron microprobe analyses yielded the empirical formula Pb 2.98 Ca 2.39 Mg 0.56 V 13.05 As 0.95 O 61 H 40.15 . Kegginite is trigonal,P    3  ¯    ,    $P\bar 3,$with  a  = 14.936(5),  c  = 15.846(5) Å,  V  = 3061(2) Å 3 , and  Z  = 2. The crystal structure of kegginite ( R 1   = 0.064 for 1356  F o   &gt; 4σ F  reflections) contains a[  As5  +V12    5  +O40(  V  O  )  ]12  −${{\rm{[A}}{{\rm{s}}^{{\rm{5 + }}}}{\rm{V}}_{12}^{5 + }{{\rm{O}}_{{\rm{40}}}}{\rm{(VO)]}}^{12 - }}$polyoxometalate cluster, which is a mono-capped Keggin ε-isomer.

Kegginite, Pb 3 Ca 3 [AsV 12 O 40 (VO)]·20H 2 O, a new mineral with a novel ε-isomer of the Keggin anion