ANSERMETITE, MnV2O6{middle dot}4H2O, A NEW MINERAL SPECIES WITH V5+ IN FIVE-FOLD COORDINATION FROM VAL FERRERA, EASTERN SWISS ALPS

Abstract

Ansermetite, MnV 2 O 6 •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 Mohs hardness is ~3, 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 , 22.5°C), strongly pleochroic between yellow orange (X) and ruby red (Z), and non-fluorescent. Chemically, ansermetite is near the end-member composition MnV 2 O 6 •4H 2 O, containing only small amounts of arsenic (0.93 wt% As 2 O 5) 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) Å 3 , Z = 4. The crystal structure was solved with direct methods on the basis of 701 reflections with I > 2(I) and refined to R 1 = 7.01%. The structure is isotypic with that of synthetic CoV 2 O 6 •4H 2 O and MnV 2 O 6 •4H 2 O. It contains V 5+ in distorted square pyramidal coordination with five atoms of oxygen, and Mn 2+ in octahedral coordination with four atoms of oxygen and two H 2 O groups. The VO 5 polyhedra share edges to form chains extending along [001], and linked by MnO 4 (H 2 O) 2 octahedra. This arrangement results in a three-dimensional framework defining tunnels that host the two H 2 O groups per formula unit belonging to MnO 4 (H 2 O) 2 octahedra as well and two additional free H 2 O groups. Ansermetite forms crystalline crusts up to ~500 m 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, Mn 2 V(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 ~50 to 85°C. Ansermetite is likely to have crystallized from near neutral to mildly acidic groundwater during the Quaternary.

Argandite, Mn7(VO4)2(OH)8, the V analogue of allactite from the metamorphosed Mn ores at Pipji, Turtmann Valley, Switzerland

“…This difference in bulk ore chemistry appears to control the difference in As-V mineralogy between the two occurrences. Vanadates dominate at Pipji (Brugger et al 2001;, but in Val Ferrera different ore lenses, or different locations within the major lenses, are either As rich or V rich Gieré 1999, 2000); accordingly they contain either As-rich (Brugger and Berlepsch 1997;Brugger et al , 1998 or V-rich minerals (Brugger and Berlepsch 1996;Brugger et al 2003.…”

“…Chemically, argandite is the fourth known Mn vanadate, after reppiaite, Mn 5 (VO 4 ) 2 (OH) 4 (Basso et al 1992), fianelite, Mn 2 V(V,As)O 7 ·2H 2 O (Brugger and Berlepsch 1996), and ansermetite, MnV 2 O 6 ·4H 2 O (Brugger et al 2003). Remarkably, the occurrence of the four minerals has so far been restricted to metamorphosed sediment-hosted exhalative (or near-seafloor) Mn-deposits of the Central Alps (e.g., Brugger and Gieré 2000;Cortesogno et al 1979).…”

Analysis and visualization of vanadium mineral diversity and distribution

“…Minerals in each of the other five clusters are found in specific environments or associated with specific ore deposits, and no connection exists among them. Cluster III minerals are mostly Mn vanadates formed in metamorphic Fe-Mn ores (Brugger et al, 2003), and they could co-occur with vanadates from clusters I and II in Mn-deficient zones. Cluster IV is composed of V 3+ minerals formed during high-grade (>500°C) metamorphism, and its connection to cluster I indicates retrograde alteration or prograde metamorphism (Ito, 1965).…”

PIZGRISCHITE, (Cu,Fe)Cu14PbBi17S35, A NEW SULFOSALT FROM THE SWISS ALPS: DESCRIPTION, CRYSTAL STRUCTURE AND OCCURRENCE

“…The sulfosalt pizgrischite, (Cu,Fe)Cu 14 PbBi 17 S 35 , is a new mineral species from an Alpine metamorphosed pre-Tertiary Cu-Bi quartz vein situated in the steep northern face of Piz Grisch, Val Ferrera, Graubünden, eastern Swiss Alps. Small stratabound Mn-deposits (e.g., Fianel and Starlera) located in Triassic carbonates on the northwestern and southwestern face of Piz Grisch, are well known because of the occurrence of new or rare mineral species, in some cases in striking samples, described during the last decade (i.e., Þ anelite, manganlotharmeyerite, ansermetite, sailauÞ te, wallkilldellite, romeite, tilasite, medaite, palenzonaite, and långbanite; Brugger & Berlepsch 1996, Brugger & Gieré 1999, 2000, Brugger et al 2003a, b, Meisser et al 2004.…”