Bicapite, KN>M(HPV O)·25HO, a new polyoxometalate mineral with a bicapped Keggin anion from the Pickett Corral mine, Montrose County, Colorado, U.S.A

Abstract

Bicapite, KNaMg(HPV O)·25HO, 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 1, 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) (K1.23Na2.23Mg1.48)∑4.94[H2.51P1.02(V13.91 Mo0.076+ )∑13.98O]·25HO. Bicapite is tetragonal, I4/m, with a = 11.5446(12) Å, c = 20.5460(14) Å, V = 2738.3(6) Å3, 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 [HPV12 O40(VO)]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 [KNaMg(HO)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.