Gatedalite, Zr(Mn2+2Mn3+4)SiO12, a new mineral species of the braunite group from Långban, Sweden

Abstract

AbstractGatedalite, Zr(Mn22+Mn43+)SiO12, is a new mineral of the braunite group. It is found in hausmannite-impregnated skarn together with jacobsite, Mn-bearing calcite, tephroite, Mn-bearing phlogopite, långbanite, pinakiolite and oxyplumboroméite at the Långban Mn-Fe oxide deposit, Värmland, central Sweden. The mineral occurs as very rare, small (≤60 μm), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm3. It is opaque and weakly anisotropic with reflectivity in air varying between 17.1 and 20.8% in the visible spectral range. Gatedalite is tetragonal, space groupI41/acd, with the unit-cell parametersa= 9.4668(6) Å,c= 18.8701(14) Å,V= 1691.1(2) Å3andZ= 8. The crystal structure was refined to anR1 index of 5.09% using 1339 unique reflections collected with MoKαX-ray radiation. The five strongest powder X-ray diffraction lines [din Å, (I), (hkl)] are: 2.730(100)(224), 2.367(12)(040), 1.6735(12)(440), 1.6707(29)(048) and 1.4267(16)(264). Electron microprobe analyses in combination with single-crystal structure refinement resulted in the empirical formula: (Zr0.494+Mn0.402+Mg0.07Ca0.02Zn0.01Ce0.013+)Σ1.00(Mn4.443+Fe0.593+Mn0.572+Mg0.41Al0.01)Σ6.02Si0.99O12. Gatedalite is a member of the braunite group (general formula AB6SiO12). It is related to braunite (Mn2+Mn63+SiO12) through the net cation exchange (Zr4++ Mn2+) → 2Mn3+, which results from the substitutions Zr4+→ Mn2+at the 8-fold coordinated site (Ain the general formula) coupled with a 2Mn2+→ 2Mn3+substitution at the 6-fold coordinated sites (Bin the general formula).