Pomite and pseudopomite, two new carbonate-encapsulating mixed-valence polyoxovanadate minerals

Abstract

Pomite (IMA2021-063), ideally Ca₃[V⁴⁺₅V⁵⁺₁₀O₃₇(CO₃)]·37H₂O, and pseudopomite (IMA2021-064), ideally Ca₃.₅[V⁴⁺₆V⁵⁺₉O₃₇(CO₃)]·32H₂O, are two new polyoxometalate minerals from the Blue Streak mine, Bull Canyon, Montrose County, Colorado, U.S.A. Pomite properties: striated blades up to ~1 mm long; very dark green-blue color; green-blue streak; vitreous luster; brittle; Mohs hardness ≈2; irregular, splintery fracture; good cleavages on {010} and {001}; 2.19(2) g/cm⁻³ density; refractive indices in the vicinity of 1.70; weakly birefringent with little or no pleochroism. Pseudopomite properties: striated prisms and blades up to ~1 mm; very dark blue-green color; blue-green streak; vitreous luster; brittle; Mohs hardness ≈2; curved, irregular fracture; probably two fair cleavages, {100} and {001}; 2.40(2) g/cm⁻³ density; refractive indices in the vicinity of 1.72; no discernable birefringence or pleochroism. Electron microprobe analyses provided the empirical formulas Ca₃.₁₁[V⁴⁺₅.₂₃V⁵⁺₉.₇₇O₃₇(CO₃)]·37H₂O and Ca₃.₄₉[V⁴⁺₅.₉₈V⁵⁺₉.₀₂O₃₇(CO₃)]·29H₂O for pomite and pseudopomite, respectively. Pomite is triclinic, P1̅, with a = 12.3668(10), b = 12.9692(12), c = 22.068(2) Å, α = 99.038(7), β = 95.689(7), γ = 103.249(7)°, V = 3368.7(5) ų, and Z = 2. Pseudopomite is triclinic, P1̅, with a = 12.2910(18), b = 12.6205(15), c = 20.917(3) Å, α = 77.381(6), β = 85.965(5), γ = 64.367(7)°, V = 2853.6(7) ų, and Z = 2. The crystal structures of both minerals (pomite, R₁ = 0.103; pseudopomite, R₁ = 0.116) contain a novel [V⁴⁺ₓV⁵⁺₁₅₋ₓO₃₇(CO₃)]⁽¹⁺ˣ⁾⁻ heteropolyanion, which is unique in natural and synthetic materials but has similarities to the [V⁴⁺₈V⁵⁺₇O₃₆(CO₃)]⁷⁻ and [H₈V⁴⁺₁₅O₃₆(CO₃)]⁶⁻ heteropolyanions reported in synthetic phases.