Crystal chemistry of trioctahedral micas-2M1 from Bunyaruguru (SW Uganda) kamafugite

In trioctahedral micas, polytype 2 M1 occurs less frequently than 1M one. For such a reason, trioctahedral 1M-polytype has been extensively studied up to date whereas studies on 2 M1-polytype are relatively rare. In several cases 2 M1-micas have been reported as coexisting with 1M-micas [1, 2, 3, 4, 5]. Other studies were focused on the characterization of phlogopite-annite 2 M1 micas with peculiar composition [6, 7, 8, 9, 10]. The crystal chemistry of 2 M1 micas from Bunyaruguru (south west Uganda) kamafugite was studied by Electron Probe Microanalysis, Single Crystal X-ray Diffraction, Mössbauer and Fourier Transform Infrared spectroscopy. To the best of our knowledge, this is the first integrated crystal chemical study of phlogopite from Ugandan kamafugites, and was undertaken to get an insight into the crystal chemistry of the trioctahedral mica 2 M1-polytype. Chemical analyses showed that the studied crystals are Ti-rich, F-poor phlogopites with an annitic component, Fe<sub>tot</sub>/(Fe<sub>tot</sub> + Mg), ranging from 0.15 to 0.23. Unit-cell parameters from single crystal X-ray data are in the range: 5.3252(1) < a ≤ 5.3307(1), 9.2231(3) < b ≤ 9.2315(3), 20.1550(6) < c < 20.1964(8) Å and 94.994(2) < β ≤ 95.131(2)°. Anisotropic structure refinements, in the space group C2/c, converged to 2.80 < R1 ≤ 3.56 % and 2.91 < wR2 ≤ 4.08 %. Mössbauer spectroscopy showed that the studied sample has: VIFe2+ = 60(1) % , VIFe3+ = 24(1) % and IVFe3+ = 16(1) %. FTIR investigations pointed to the occurrence of Fe3+-oxy substitutions and ruled out the presence of vacancy mechanisms. The overall crystal chemical features are consistent with the following substitutions: tetraferriphlogopite; Ti-oxy and Al, Fe3+, Cr-oxy; Al, Fe3+-Tschermak; kinoshitalite and XIIK+ + IVAl3+  IVSi4+ + XII. The estimation of the OH<sup>-</sup> content for Ugandan mica-2 M1 was obtained, for the first time, from the linear regression equation c = 0.20(2) x OH- (gpfu) + 19.93(2) derived from literature data of 2 M1-samples with known OH- content. The orientation of the O-H vector with respect to c* was found in the range from 2.8 to 12.6°, consistently with literature values [11].