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Bicapite, KNa 2 Mg 2 (H 2 PV 14  5+  O 42 )·25H 2 O, is a new mineral species (IMA2018-048) discovered at the Pickett Corral mine, Montrose County, Colorado, U.S.A. Bicapite occurs as square tablets up to about 0.2 mm on edge on montroseite-corvusite-bearing sandstone. Crystals are dark red-brown, often appearing black. The streak is orange, and the luster is vitreous. Bicapite is brittle, has a Mohs hardness of 1½, and displays one excellent cleavage on {100}. The measured density is 2.44(2) g/cm 3 . Bicapite is uniaxial (+), w = 1.785(5), e ≈ 1.81 (white light); pleochroism is red-brown;  E  &gt;  O , slight. The electron probe microanalysis and results of the crystal structure determination provided the empirical formula (based on 67 O apfu) (K 1.23 Na 2.23 Mg 1.48 ) ∑4.94 [H 2.51 P 1.02 (V 13.91  5+  Mo 0.07  6+  ) ∑13.98 O 42 ]·25H 2 O. Bicapite is tetragonal,  I 4/ m , with  a  = 11.5446(12) Å,  c  = 20.5460(14) Å,  V  = 2738.3(6) Å 3 , and  Z  = 2. The strongest four lines in the diffraction pattern are [ d  in Å ( I ) ( hkl )]: 10.14 (100) (002,101); 2.978 (29) (134,206); 2.809 (11) (305); and 2.583 (11) (420,008). The atomic arrangement of bicapite was solved and refined to  R  1  = 0.0465 for 1008 independent reflections with  I  &gt; 2s I . The structural unit is a [H 2 PV 12  5+  O 40 (V 5+ O) 2 ] 7–  heteropolyanion composed of 12 distorted VO 6  octahedra surrounding a central PO 4  tetrahedron and capped on opposite sides by two VO 5  square pyramids; the structural unit is a modification of the a-isomer of the Keggin anion, [ XM  12 O 40 ] n- . Charge balance in the structure is maintained by the [KNa 2 Mg 2 (H 2 O) 25 ] 7+  interstitial complex. The name bicapite is in recognition of this being the only known mineral with a structure based on a bicapped Keggin anion. The discovery of bicapite and numerous other natural polyoxometalate compounds in the Colorado Plateau uranium/ vanadium deposits make that the most productive region found to date for naturally occurring polyoxometalate compounds.

american mineralogist

Bicapite, KN a2 M g2 (H 2 PV 5+  14  O 42 )·25H 2 O, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A