Donowensite, Ca(H2O)3Fe3+2(V2O7)2, and Mikehowardite, Fe3+4(VO4)4(H2O)2·H2O, Two New Vanadium Minerals from the Wilson Springs Vanadium Mine, Wilson Springs, Arkansas, USA

Abstract

ABSTRACT Donowensite (IMA2020-067), Ca(H2O)3Fe3+2(V2O7)2, and mikehowardite (IMA2020-068), Fe3+4(VO4)4(H2O)2·H2O, are intimately associated new secondary minerals from the Wilson Springs vanadium mine, Wilson Springs, Arkansas, USA. Donowensite has the following properties: needles up to 1 mm in length; yellow color; orange streak; subadamantine luster; brittle; Mohs hardness 3; splintery fracture; three cleavages ({001} perfect, {100} and {010} very good); density 2.97(2) g/cm3; biaxial (+), α > 1.95, β > 1.95, γ > 1.95; 2V = 72(2)°; moderate r > v dispersion; orientation X ^ b = 7°, Z ≈ c; pleochroism X brown orange, Y orange yellow, Z yellow. Mikehowardite has the following properties: equant prisms up to 0.15 mm in length; very dark brown color; yellow-orange streak; subadamantine luster; Mohs hardness 3½; irregular, stepped fracture; three cleavages ({100} very good, two undetermined good cleavages); density 3.19(2) g/cm3; biaxial with slight pleochroism in shades of brown-orange; Gladstone-Dale nav = 2.034. Electron probe microanalyses provided the empirical formulae Ca0.93Fe3+1.92Mn3+0.01V4.06P0.01O17H6.00 for donowensite and K0.11Ca0.02Fe3+3.78Mn3+0.03V3.67P0.33O18.87H6.18 for mikehowardite. Donowensite is triclinic, P with a = 7.3452(4), b = 9.9291(4), c = 10.0151(7) Å, α = 94.455(7), β = 98.476(7), γ = 100.779(7)°, V = 705.52(7) Å3, and Z = 2. Mikehowardite is triclinic, P with a = 6.6546(17), b = 6.6689(14), c = 9.003(2) Å, α = 76.515(5), β = 84.400(6), γ = 75.058(5)°, V = 375.11(15) Å3, and Z = 1. The structure of donowensite (R1 = 0.0561 for 2615 I > 2σI reflections) contains zig-zag chains of edge-sharing FeO6 octahedra that are linked to one another by V2O7 pyrovanadate groups to form sheets between which are Ca2+ cations and H2O groups. The structure of mikehowardite (R1 = 0.0678 for 1098 I > 2σI reflections) has similarities to the structure of schubnelite. In both mikehowardite and schubnelite, edge-sharing dimers of Fe3+O6 octahedra are linked by distorted VO4 tetrahedra.