3T-phlogopite from Kasenyi (SW Uganda) kamafugite: crystal chemistry and structure

3T trioctahedral micas are rarer than it is thought. This is likely due to the occurrence of apparent polytypism, so that 1M polytype twins result in a diffraction pattern simulating a 3T periodicity [1, 2]. Most of the 3T trioctahedral micas found in nature to date belong to muscovite-polylithionite-annite system [3, 4, 5, 6]. X-ray diffraction studies on these micas have often reported partial tetrahedral ordering and/or different patterns of octahedral ordering [3, 6]. In the present work, a 3 T trioctahedral mica from Kasenyi (south west Uganda) kamafugite was studied via Electron Probe Microanalysis (EPMA) and Single Crystal X-ray Diffraction (SCXRD). Main EPMA data gave: SiO2 = 38.7(2), Al2O3 = 13.08(9), MgO = 20.4(2), TiO2 = 4.8(1), FeOtot = 5.51(9), Cr2O3 = 0.90(7), K2O = 9.64(5), Na2O = 0.29(1), BaO = 0.15(5) and F = 0.13(5) wt%. The analysed crystal may be classified as a Ti-rich, F-poor mica with a composition intermediate between the annite and phlogopite end members. Anisotropic single crystal X-ray refinement was performed in the <i>P</i>3<sub>1</sub>12 space group and converged to R1 = 4.34 and wR2 = 3.33 %. Unit cell parameters were: a = b = 5.3235(3) and c = 30.188(2) Å. Mean bond length distances of M1, M2 and M3 follow the pattern M1 = M2 < M3, suggesting partial octahedral cation ordering. Conversely, mean bond lengths of T1 and T2 point to a disordered cation distribution over tetrahedral sites. Finally, the overall crystal chemical features indicates the occurrence in the studied sample of the following substitution mechanisms: tetraferriphlogopite; Ti-oxy and Al, Fe3+, Cr-oxy; Al, Fe3+-Tschermak; kinoshitalite and XIIK+ + IVAl3+ « IVSi4+ + XII. Such substitutions are the same as those found in 1M-2M1 coexisting micas from the same rock sample [7].