Trioctahedral micas-1M from Mt. Vulture (Italy): Structural disorder and crystal chemistry

The crystal chemistry of trioctahedral micas-IM from Mt. Vulture phonolitic -trachitic ignimbrites has been investigated by single crystal X-ray diffra ction, 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-ferrokin oshitalite solid solution. In addition, samples with remarkably different M g/Fe ratio can coexist in the same volcano-stratigraphic level, Fe-rich sam ples having also higher Ti content. The crystal structure of all of the ana lysed 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 o f cases. In the present work, for the first time, the disorder has been suc cessfully interpreted and managed in the structure refinement. It is demons trated 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 sa mple 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 partitio ning of Ti at M2 sites. However the mechanism of Ti incorporation in the st ructure seems to be different in Mg-rich and Fe-rich samples. For the forme r the Ti-vacancy substitution mechanism seems to hold: 2(VI)Mg(2+) <----> T i-VI(4+) + (VI) []. For the latter, the occurrence of Li, as detected by em ission spectroscopy plasma measurements, structural evidence and charge bal ance considerations suggest the following novel substitution mechanism, Ti- VI(4+) + Li-VI(+) + O2- <----> 2 M-VI(2+) + OH-, to be active in the struct ure.