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

Joël Brugger, Peter Berlepsch, Nicolas Meisser, Thomas Armbruster

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10 Citations (Scopus)

Abstract

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, Dcalc = 2.49 g/cm3. Dmeas = 2.57(2) g/cm3. Ansermetite has an excellent cleavage along [110], the prism, and an uneven fracture. It is biaxial with nmin = 1.797 and nmax = 1.856 (NaD, 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) Å3, 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.

Original languageEnglish
Pages (from-to)1423-1431
Number of pages9
JournalCanadian Mineralogist
Volume41
Issue number6
DOIs
Publication statusPublished - 1 Jan 2003
Externally publishedYes

Keywords

  • Ansermetite
  • Crystal structure
  • Eastern Swiss Alps
  • Metavanadate
  • New mineral species
  • Starlera mine
  • Val Ferrera

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