SCHEUCHZERITE, NA(MN,MG)9[VSI9O28 (OH)](OH)3, A NEW SINGLE-CHAIN SILICATE

Abstract

Scheuchzerite, Na(Mn,Mg)9[VSi9O28 (OH)](OH)3, is a new mineral from the metamorphosed syn-sedimentary exhalative Mn deposit of Fianel, Val Ferrera, Central Alps, Switzerland. It is dedicated to the Swiss naturalist Johann Jakob Scheuchzer (1672-1733). Scheuchzerite is associated with saneroite and tiragalloite in veins resulting from the remobilization of ore components during retrograde Tertiary Alpine metamorphism. Scheuchzerite forms yellow-orange, transparent acicular crystals up to 0.5 mm in length with yellow-orange streak and vitreous luster, Mohs' hardness ~2.5, dcalc 3.47 (electron microprobe) to 3.52 g/cm3 (structure refinement); 4meas 3.50(2) g/cm3, good cleavage parallel to fiber elongation. Scheuchzerite is biaxial positive, nmin = 1.74 and nmax = 1.75; nmean (Gladstone-Dale) 1.74; weakly pleochroic, X = brown yellow, Y = pale yellow. The empirical chemical formula is Na0.97(Mn7.79Mg0.95 Zn0.16Ni0.04Ca0.03 Al0.01)∑>=8.98(V0.95 As0.02Si9.08)∑=10.05 O32.05H4. Scheuchzerite is triclinic, P1̄, a = 9.831(5) Å, b = 10. 107(5) Å, c = 13.855(7) Å, α = 86.222(10)2*, β = 73.383(9)°, γ = 71.987(9)°; V = 1254.2(10) Å3; Z = 2. The crystal structure was solved with direct methods on the basis of 1616 unique reflections with I > 4σF and refined to R1 = 9.4%. The crystal structure consists of tetrahedral layers separated by layers containing chains of edge-sharing [Mn(O,OH)6] octahedra as well as [Na,O8] polydedra. The tetrahedral layers consist of [Si9O25(OH)] loop-branched chains of corner-sharing silicate tetrahedra extending along [011]. The loops contain 6 tetrahedra and are separated by 3 tetrahedra in a broken 4-loop arrangement. A hydrogen atom is probably shared by two O atoms (symmetrical hydrogen bond), replacing the missing silicon atom. A vanadate (VO4)3- tetrahedron branches off the 6-tetrahedra loop, and hence the overall formula of the tetrahedral chains is [VSi9O28(OH)]. In the notation of Liebau (1985), scheuchzerite is a single chain silicate (monopolysilicate) {o1B,11∞}[VSi9O28(OH)]. The topology of the scheuchzerite structure is reminiscent of that of the double-chain silicates of the amphibole group, but scheuchzerite contains a new type of silica chain.