Crystal chemistry of trioctahedral micas-2M 1 from Bunyaruguru kamafugite (southwest Uganda)

The crystal chemistry of 2M1 micas from Bunyaruguru kamafugite (southwest Uganda) was studied by electron probe microanalysis, single-crystal X-ray diffraction, Mössbauer and Fourier transform infrared spectroscopy. Chemical analyses showed that the studied crystals are Ti-rich, F-poor phlogopites with an annitic component, Fetot/(Fetot + Mg), ranging from 0.15 to 0.22. 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 2.77 ≤ R1 ≤ 3.52% and 2.91 ≤ wR2 ≤ 4.02%. 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 [IVFe3+ ↔ IVAl]; Ti-oxy [VIM2+ + 2 (OH)− ↔ VITi4+ + 2 (O2−) + H2↑] and Al, Fe3+, Cr-oxy [VIM2+ + (OH) − ↔ VIM3+ + O2− + ½ (H2)↑]; Al, Fe3+-Tschermak [VIM2+ + IVSi4+ ↔ VIM3+ + IVAl]; kinoshitalite [XIIK + IVSi4+ ↔ XIIBa2+ + IVAl] and [XIIK+ + IVAl3+ ↔ IVSi4+ + XII□]. The estimation of the OH− content for Ugandan mica-2M1 was obtained, for the first time, from the linear regression equation c = 0.20(2) × OH− (gpfu) + 19.93(2) derived from literature data of 2M1-samples with known OH− content. The orientation of the O-H vector with respect to c* was found in the range from 2.0 to 6.9°.