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

Donowensite (IMA2020-067), Ca(H2O)(3)Fe-2(3+)(V2O7)(2), and mikehowardite (IMA2020-068), Fe-4(3+)(VO4)(4)(H2O)(2)center dot 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/cm(3); biaxial (+), alpha > 1.95, beta > 1.95, gamma > 1.95; 2V = 72(2)degrees; moderate r > v dispersion; orientation X (boolean AND) b = 7 degrees, Z approximate to 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 3 1/2; irregular, stepped fracture; three cleavages ({100} very good, two undetermined good cleavages); density 3.19(2) g/cm(3); biaxial with slight pleochroism in shades of brown-orange; Gladstone-Dale n(av) = 2.034. Electron probe microanalyses provided the empirical formulae Ca0.93Fe1.923+Mn0.013+V4.06P0.01O17H6.00 for donowensite and K0.11Ca0.02Fe3.783+Mn0.033+V3.67P0.33O18.87H6.18 for mikehowardite. Donowensite is triclinic, P (1) over bar with a = 7.3452(4), b = 9.9291(4), c = 10.0151(7) angstrom, alpha = 94.455(7), beta = 98.476(7), gamma = 100.779(7)degrees, V = 705.52(7) angstrom(3), and Z = 2. Mikehowardite is triclinic, P (1) over bar with a = 6.6546(17), b = 6.6689(14), c = 9.003(2) angstrom, alpha = 76.515(5), beta = 84.400(6), gamma = 75.058(5)degrees, V = 375.11(15) angstrom(3), and Z = 1. The structure of donowensite (R-1 = 0.0561 for 2615 I > 2 sigma I 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 (R-1 = 0.0678 for 1098 I > 2 sigma I 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.