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, 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 I> 2(I) 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 defin-ing tunnels that host the two H2O groups per formula unit belonging to MnO4(H2O)2 octahedra as well and two additional fre