Coexistence of 1M and 2M1 polytypes in Ti-phlogopite from the Central Fields volcanics of the Southwest Uganda

In the present work, the crystal chemistry of natural Tiphlogopites from alkali-rich igneous rocks from the Central Fields of Bunyaruguru and Katwe-kikorongo (Southwest Uganda) has been investigated. The host rocks are characterized by olivine – melilitite and olivine – kalsilite – nepheline – clinopyroxene assemblages [1]. The phlogopites from the former rock are labelled BU (specifically, BU1, BU3 and BU4) whereas those from the latter are labelled KK (in detail, KK8 and KK13). All samples underwent chemical (Electron Micro Probe Analysis, EMPA), structural (Single Crystal X-ray Diffraction, SCXRD) and spectroscopic (FTIR) investigation. EMPA yielded the following ranges: MgO (17.87-21.48 wt%), FeOtot (5.40-9.22 wt%) and TiO2 (4.59-7.05 wt%) for BU samples whereas MgO (17.98-18.51 wt%), FeOtot (8.17-8.86 wt%) and TiO2 (6.02-6.49 wt%) for KK crystals. SCXRD analyses showed the coexistence of both the 1M and the 2M1 polytypes within the same sample. The BU3 mica is an exception because, to date, only crystals belonging to the 2M1 polytype have been found. Average cell parameters are a = 5.33, b = 9.22, c = 10.22 Å and = 100.06° for the 1M whereas a = 5.33, b = 9.23, c = 20.22 Å and = 95.08° for the 2M1 phlogopites. Structure refinements using anisotropic displacement parameters were performed in space group C2/m for 1M and C2/c for 2M1 samples and converged at 1.63 R 4.64 %, 1.96 Rw R Rw 4.41 % for the two polytypes, respectively. Micro-FTIR provides insightful informations about octahedral cationic environments, the substitution mechanisms trough which cations enter the mica structure and the hydrogen orientation [2-4]. The samples so far analyzed display, to different extent, fine structure in the OH- stretching region. In terms of substitution mechanisms this implies that the samples contain different combination of M3+-Tschermak, M3+- oxy substitutions, whereas they are not affected by M3+-vacancy substitutions. Indeed the bands at about 3620 and 3535 cm-1, which correspond to Al3+Al3+-OH and Fe3