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

Abstract

Gatedalite, Zr(Mn 2+2 Mn 3+ 4 )SiO 12 , is a new mineral of the braunite group. It is found in hausmanniteimpregnated 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 (460 mm), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm 3 . 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 group I4 1 /acd, with the unit-cell parameters a = 9.4668 (6) Å , c = 18.8701(14) Å , V = 1691.1(2) Å 3 and Z = 8. The crystal structure was refined to an R1 index of 5.09% using 1339 unique reflections collected with MoKa X-ray radiation. The five strongest powder X-ray diffraction lines [d in 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: (Zr 4 + 0 . 4 9 M n 2 + 0 . 4 0 Mg 0 . 0 7 Ca 0 . 0 2 Z n 0 . 0 1 Ce 3 + 0 . 0 1 ) S 1 . 0 0 (Mn 3+ 4.44 Fe 3+ 0.59 Mn 2+ 0.57 Mg 0.41 Al 0.01 ) S6.02 Si 0.99 O 12 .Gatedalite is a member of the braunite group (general formula AB 6 SiO 12 ). It is related to braunite (Mn 2+ Mn 3+ 6 SiO 12 ) through the net cation exchange (Zr 4+ + Mn 2+ ) ? 2Mn 3+ , which results from the substitutions Zr 4+ ? Mn 2+ at the 8-fold coordinated site (A in the general formula) coupled with a 2Mn 2+ ? 2Mn 3+ substitution at the 6-fold coordinated sites (B in the general formula).