Bicapite, KNa2Mg2(H2PV5+14 O42)·25H2O, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A.

Bicapite, KNa₂Mg₂(H₂PV₁₄⁵⁺ O₄₂)·25H₂O, 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/cm³. Bicapite is uniaxial (+), w = 1.785(5), e ≈ 1.81 (white light); pleochroism is red-brown; E > O, slight. The electron probe microanalysis and results of the crystal structure determination provided the empirical formula (based on 67 O apfu) (K_1.23Na_2.23Mg_1.48)_∑4.94[H_2.51P_1.02(V_13.91⁵⁺ Mo_0.07⁶⁺)_∑13.98O₄₂]·25H₂O. Bicapite is tetragonal, I4/m, with a = 11.5446(12) Å, c = 20.5460(14) Å, V = 2738.3(6) ų, 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 R₁ = 0.0465 for 1008 independent reflections with I > 2sI. The structural unit is a [H₂PV₁₂⁵⁺ O₄₀(V⁵⁺O)₂]^7– heteropolyanion composed of 12 distorted VO₆ octahedra surrounding a central PO₄ tetrahedron and capped on opposite sides by two VO₅ square pyramids; the structural unit is a modification of the a-isomer of the Keggin anion, [XM₁₂O₄₀]^n-. Charge balance in the structure is maintained by the [KNa₂Mg₂(H₂O)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. [ABSTRACT FROM AUTHOR]