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

Ansermetite, MnV₂O₆·4H₂O, 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 Mohs hardness is ∼3, D_calc = 2.49 g/cm³. D_meas = 2.57(2) g/cm³. 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, 22.5°C), strongly pleochroic between yellow orange (X) and ruby red (Z), and non-fluorescent. Chemically, ansermetite is near the end-member composition MnV₂O₆·4H₂O, 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) Å. 111.572(2)°, V 866.3(2) ų, Z = 4. The crystal structure was solved with direct methods on the basis of 701 reflections with 1 > 2σ(1) and refined to R₁ = 7.01%. The structure is isotypic with that of synthetic CoV₂O₆·4H₂O and MnV₂O₆·4H₂O. 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 ∼500 μm 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, Mn₂ V(V,As)O₇·2H₂O, 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 ∼50 to 85°C. Ansermetite is likely to have crystallized from near neutral to mildly acidic groundwater during the Quaternary.