This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al3+, Ti4+, and, only in a few samples, to Fe3+. The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [VITi4+VI(Mg,Fe)–12+ (OH)–2 O22–] and Ti-vacancy [VITi4+VIVI(Mg,Fe)–22+] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly affects octahedral and interlayer topology as well. Micas with the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fetot) diagram, possibly as a consequence of a variation in intensive parameters (T, P, fH2, fO2, fH2O) during crystallization in the magmatic chamber.

In micas with significant Fe3+ contents, the layer charge balance is accomplished by the following mechanisms: VIFe−32+VIFe23+VI□, VIFe−12+VIFe3+(OH)–1 O2–, and VIFe–12+VIFe3+IVSi–14+IVAl3+. These micas show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters significantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The Fe3+-vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effects related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration).

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