Ansermetite, MnV2O6•4H2O, a new mineral species with V5+ in five-fold coordination from Val Ferrera, Eastern Swiss Alps

Ansermetite, MnV2O6.4H(2)O, is a new mineral species from the metamorphosed synsedimentary exhalative Mn deposit of Fianel, Val Ferrera, Eastern Swiss Alps. The crystals are carmine red, transparent, with an orange streak and adamantine luster. The Mobs hardness is similar to3, D-calc = 2.49 g/cm(3), D-meas = 2.57(2) g/cm(3). Ansermetite has an excellent cleavage along {110}, the prism, and an uneven fracture. It is biaxial with n(min) = 1.797 and n(max) = 1.856 (Na-D, 22.5degreesC), strongly pleochroic between yellow orange (X) and ruby red (Z), and non-fluorescent. Chemically, ansermetite is near the end-member composition MnV2O6.4H(2)O, containing only small amounts of arsenic (0.93 wt% As2O5) and strontium (0.20 wt% SrO). Ansermetite is monoclinic, C2/c, a 13.171(2), b 10.128(1), c 6.983(1) Angstrom, beta 111.572(2)degrees, V 866.3(2) Angstrom(3), Z = 4. The crystal structure was solved with direct methods on the basis of 701 reflections with I > 2sigma(I) and refined to R-1 = 7.01%. The structure is isotypic with that of synthetic CoV2O6.4H(2)O and MnV2O6.4H(2)O. It contains V5+ in distorted square pyramidal coordination with five atoms of oxygen, and Mn2+ in octahedral coordination with four atoms of oxygen and two H2O groups. The VO5 polyhedra share edges to form chains extending along [001], and linked by MnO4(H2O)(2) octahedra. This arrangement results in a three-dimensional framework defining tunnels that host the two H2O groups per formula unit belonging to MnO4(H2O)(2) octahedra as well and two additional free H2O groups. Ansermetite forms crystalline crusts up to similar to500 mum in thickness and several square centimeters in surface filling thin open fractures cross-cutting V-rich Mn-Fe silicate-oxide ores. Ansermetite is associated to fianelite, Mn2V(V,As)O-7.2H(2)O, Fe oxyhydroxides and silica. Ansermetite and fianelite represent the latest stage of V mobility at the Fianel deposit. Ansermetite is stable at ambient conditions, but is unstable in air and aqueous solutions at temperatures above similar to50 to 85degreesC. Ansermetite is likely to have crystallized from near neutral to mildly acidic groundwater during the Quaternary.