Abstract

Pandoraite-Ba, BaV4+5V5+2O16·3H2O, and pandoraite-Ca, CaV4+5V5+2O16·3H2O, are two new vanadium-oxide-bronze minerals from the Pandora mine, La Sal district, San Juan County, Colorado, USA. Pandoraite-Ba and pandoraite-Ca are rare secondary minerals and occur on a matrix consisting of recrystallized quartz grains from the original sandstone. Crystals of carnotite are associated with pandoraite-Ba and crystals of finchite are associated with pandoraite-Ca. The minerals occur as thin, dark blue, square plates up to approximately 100 μm across and approximately 2 μm thick. Plates occur in subparallel to random intergrowths. The streak of both minerals is light greenish blue, and they display a vitreous, transparent luster and brittle tenacity; neither mineral displays fluorescence. The Mohs hardness of pandoraite-Ba and pandoraite-Ca is ca. 2½. Cleavage for both minerals is perfect on {001}. For pandoraite-Ba, densitymeas = 3.24(1) g/cm3. For pandoraite-Ca, densitymeas = 2.91(1) g/cm3. Both minerals are biaxial (pseudo-uniaxial) (–). For pandoraite-Ba, α (ε) =1.81(2), β and γ (ω) = 1.84(1). For pandoraite-Ca, α (ε) = 1.80(2), β and γ (ω) = 1.83(1). Similar greenish-blue pleochroism is found in both minerals, Y and Z (O) > X (E). For pandoraite-Ba, the empirical formula from electron probe microanalysis (EPMA) (calculated on the basis of V + Fe + Al = 7 and O = 19 apfu) is (Ba0.83Sr0.09Ca0.05Na0.03K0.02)Σ1.02(V4+4.25V5+2.38Fe3+0.35Al0.02)Σ7.00O16·3H2O, and for pandoraite-Ca it is (Ca0.62Ba0.07Sr0.02Na0.01K0.01)Σ0.73(V4+3.70V5+2.93Fe3+0.37Al0.01)Σ7.01O16·3H2O. EPMA demonstrates that solid solution exists between the phases. Pandoraite-Ba is monoclinic (pseudo-tetragonal), P2, with a 6.1537(16), b 6.1534(18), c 21.356(7) Å, β 90.058(9)°, and V 808.7(4) Å3, determined by single-crystal X-ray diffractometry. Pandoraite-Ca, inferred to be isostructural with the Ba-dominant phase, has a 6.119(8), b 6.105(8), c 21.460(9)Å, β 90.06(14)°, and V 801.7(15) Å3, determined by refinement of powder diffraction data. The atomic arrangement of pandoraite-Ba was solved and refined to R1 = 0.0573 for 3652 independent reflections with I > 2σI. Pandoraite-Ba and pandoraite-Ca have vanadium oxide bronze layer structures formed of sheets of V7O16 polyhedra that form the structural unit and (Ba,Ca)(H2O)3 interlayers; the vanadium is of mixed valence (4+, 5+), with the reduction of pentavalent vanadium occurring to balance the charge of the Ba “insertion” ions in partially occupied sites in the interlayer. A tetragonal synthetic analog is known.

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