The crystal chemistry of trioctahedral micas-1M from Mt. Vulture phonolitic-trachitic ignimbrites has been investigated by single crystal X-ray diffraction, microprobe analysis and plasma emission spectroscopy.
Chemical analyses have shown that Mt. Vulture trioctahedral micas belong to the phlogopite-annite join, with minor component of kinoshitalite-ferrokinoshitalite solid solution. In addition, samples with remarkably different Mg/Fe ratio can coexist in the same volcano-stratigraphic level, Fe-rich samples having also higher Ti content. The crystal structure of all of the analysed samples is affected, to varying degrees, by structural disorder, due to ± b/3 slips of octahedral sheet along the [0 1 0], [3 1 0] and [3 −1 0] directions. The latter has been reported in the literature for a handful of cases. In the present work, for the first time, the disorder has been successfully interpreted and managed in the structure refinement. It is demonstrated that it leads to desymmetrization of the TOT layer and therefore of the whole structure, with reduction of symmetry from C2/m to C2. For the sample mostly affected by disorder (17 %) the R-factor drops from 9.97% (C2/m) to 2.74 % (C2).
The chemical and geometrical features show that Mt. Vulture micas are homo-octahedral, even if some structural details suggest a preferential partitioning of Ti at M2 sites. However the mechanism of Ti incorporation in the structure seems to be different in Mg-rich and Fe-rich samples. For the former the Ti-vacancy substitution mechanism seems to hold: