Gatedalite, Zr(Mn (super 2+) (sub 2) Mn (super 3+) (sub 4) )SiO12, a new mineral species of the braunite group from Langban, Sweden

Gatedalite, Zr(Mn (super 2+) (sub 2) Mn (super 3+) (sub 4) )SiO (sub 12) , 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, langbanite, pinakiolite and oxyplumboromeite at the Langban Mn-Fe oxide deposit, Varmland, central Sweden. The mineral occurs as very rare, small (< or =60 mu m), grey, submetallic, irregularly rounded anhedral grains. Gatedalite has a calculated density of 4.783 g/cm (super 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 (sub 1) /acd, with the unit-cell parameters a = 9.4668(6) Aa, c = 18.8701(14) Aa, V = 1691.1(2) Aa (super 3) and Z = 8. The crystal structure was refined to an R1 index of 5.09% using 1339 unique reflections collected with MoKalpha X-ray radiation. The five strongest powder X-ray diffraction lines [d in Aa, (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 (super 4+) (sub 0.49) Mn (super 2+) (sub 0.40) Mg (sub 0.07) Ca (sub 0.02) Zn (sub 0.01) Ce (super 3+) (sub 0.01) ) (sub Sigma 1.00) (Mn (super 3+) (sub 4.44) Fe (super 3+) (sub 0.59) Mn (super 2+) (sub 0.57) Mg (sub 0.41) Al (sub 0.01) ) (sub Sigma 6.02) Si (sub 0.99) O (sub 12) . Gatedalite is a member of the braunite group (general formula AB (sub 6) SiO (sub 12) ). It is related to braunite (Mn (super 2+) Mn (super 3+) (sub 6) SiO (sub 12) ) through the net cation exchange (Zr (super 4+) + Mn (super 2+) ) --> 2Mn (super 3+) , which results from the substitutions Zr (super 4+) --> Mn (super 2+) at the 8-fold coordinated site (A in the general formula) coupled with a 2Mn (super 2+) --> 2Mn (super 3+) substitution at the 6-fold coordinated sites (B in the general formula).