3T-phlogopite from Kasenyi kamafugite (SW Uganda): EPMA, XPS, FTIR, and SCXRD study

A 3T mica polytype from Kasenyi (south west Uganda) kamafugite was studied by Electron Probe Microanalysis (EPMA), Single Crystal X-ray Diffraction (SCXRD), micro-Fourier Transform Infrared Spectoscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) in order to characterize its crystal chemistry and the relationships with samples from the same rock but showing different stacking sequence. EPMA data gave: SiO2 = 38.7(2), Al2O3 = 13.08(9), MgO = 20.4(2), TiO2 = 4.8(1), MnO = 0.03(3), FeOtot = 5.51(9), Cr2O3 = 0.90(7), NiO = 0.11(5), SrO = 0.03(3), ZnO = 0.04(3), ZrO2 = 0.01(2), K2O = 9.64(5), Na2O = 0.29(1), BaO = 0.15(5), F = 0.13(5) and Cl = 0.01(1) wt%. The analysed sample may be classified as a Ti-rich, F-poor mica with a composition in the phlogopite- annite join end members. X-ray photoelectron spectroscopy provided Fe2+/Fe3+ and O2- /OH equal to ~ 0.75 and 7.14, respectively, which are in agreement with the results of previous Mössbauer investigation on the BU1 sample and with the structural formula of the studied crystal. Infrared spectra showed, in the OH- stretching region (~ 3740-3600 cm-1 cm-1), a shoulder at ~ 3660 cm-1 which is assigned to MgMgFe3+-OH--K-O2- local configurations. No evidences of vacancy substitutions were observed. Single crystal X-ray refinement using anisotropic displacement parameters was performed in the P3112 space group and converged to R1 = 4.34 and wR2 = 3.33 %. Unit cell parameters are: a = b = 5.3235(3) and c = 30.188(2) Å. Geometrical and chemical considerations point to a disordered cation distribution over T1 and T2 tetrahedral sites, whereas partial cation ordering characterizes the octahedral sites with M1 = M2 ≠ M3. Tetrahedral bond/edge lengths distortion and angle variances parameters evidence more distorted polyhedra in 3T polytype than those found in coexisting 1M and 2M1 polytypes. Finally, the overall crystal chemical features indicates the occurrence in the studied sample of the following substitution mechanisms: 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+  VI(Al3+, Fe3+) + IVAl3+]; XIIK+ + IVAl3+  IVSi4+ + XII.