Dickthomssenite, Mg(V2O6)•7H2O), a new mineral species from the Firefly-Pigmay mine, Utah: Descriptive mineralogy and arrangement of atoms

Abstract

Dickthomssenite, Mg(V2O6)•7H2O, is a new mineral species from the Firefly-Pigmay uranium-vanadium mine, San Juan County, Utah. The phase crystallizes as platy light golden brown crystals up to 1.5 mm in length, with white streak and vitreous luster. Its Mohs hardness is 2 1/2, its calculated density, 2.037(1), and its observed density, between 1.96 and 2.09 g/cm3. Dickthomssenite displays a perfect {100} cleavage. In 589.3 nm light, the mineral is translucent, β 1.6124(3), β 1.6740, and γ 1.7104(4), 2Vmeas 74(1)°, 2Vcalc 73°, b = Z, c Λ Y = 17°. Electron-microprobe analyses yielded (in wt.%) MgO 15.38%, FeO 0.46%, V2O5 73.92%, H2O (difference) 10.24%, yielding (Mg0.94Fe0.94Fe0.02) V2O6 1.4H2O. Dickthomssenite dehydrates rapidly under the electron beam; the crystal-structure determination reveals that there are seven H2O groups, giving the ideal formula Mg(V2O6) 7H2O. Dickthomssenite is monoclinic, C2/c, a 38.954(2), b 7.2010(4), c 16.3465(9) A, β 97.602(1)°, V 4544.0(4) A3, Z = 16. The strongest three measured X-ray lines [d in Å(I)(hkl)] are: 9.704(100)(400), 5.843(100)(402), and 3.139(90)(1202). The phase is formed of (100) planes of chains of V2O6 divanadate edge-sharing polyhedra containing [2+3]-coordinated vanadium atoms, linked in (100) by corner-sharing with Mg octahedra. These (100) planes are linked along [100] by layers of Mg(H2O)6 octahedra and non-bonded H2O molecules. Dickthomssenite is named after Richard W. Thomssen (b. 1933), consulting geologist from Dayton, Nevada, U.S.A.