The crystal chemistry of Ti-rich phlogopite from deposits of the oldest activity (~740 ka) of Mt. Vulture, Potenza, Italy, was investigated to identify the substitution mechanisms in Ti-rich phlogopite and to determine its potential as a geohygrometer. Substitution mechanisms were determined by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), Mössbauer spectroscopy, and micro-Fourier transform infrared (FTIR) spectroscopy. Magnetite and sanidine from the mica-rich host rocks were also analyzed. Use of the biotite-magnetite-sanidine geohygrometer in these volcanics (essentially trachytic-phonolitic ignimbrites) is exploited.

All investigated phlogopite samples consist of the 1M polytype, with 5.3275(2) ≤ a ≤ 5.3635(4) Å, 9.2211(4) ≤ b ≤ 9.2958(8) Å, 10.1211(5) ≤ c ≤ 10.281(1) Å, and 99.980(3) ≤ β ≤ 100.097(2)°. Structure refinements in space group C2/m converged to 2.42 ≤ R1 ≤ 4.00% and 2.04 ≤ wR2 ≤ 4.50%. VIFe3+/Fetot from Mössbauer analyses ranged from 34(1)–89(1)%. The main bands in the OH-stretching region are 3709, 3682, and 3658 cm−1, and were assigned to 3Mg-OH-K-OH, 3Mg-OH-K-O2−, and 2MgFe3+-OH-K-O2− local configurations, respectively. The overall crystal chemical features are compatible with the M3+-Tschermak substitutions (VIM2+ + IVSi4+VIM3+ + IVAl, with M3+ = Al,Fe3+), M3+-oxy [VIM2+ + (OH) ↔ M3+ + (O)2− + ½H2↑] and Ti-oxy substitutions [VIM2+ + 2(OH)VITi4+ + 2(O)2− + H2↑]. The magnetite composition varies between Mt82Uv18 and Mt93Uv07, whereas the proportion of orthoclase with respect to albite plus anorthite is 0.77 ≤ XOr ≤ 0.82. A partly ionic model was used to estimate annite activity, starting from experimentally determined substitutions in phlogopite. The resulting water fugacity was an order of magnitude lower than that calculated using stoichiometry to determine the phlogopite formula. The results are consistent with the occurrence of a well-developed hydrothermal system that altered the magmatic oxygen fugacity conditions at Mt. Vulture.

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