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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 Al 3+ , Ti 4+ , and, only in a few samples, to Fe 3+ . The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [ VI Ti 4+VI (Mg,Fe) 2+  -1 (OH) -  -2 O 2-  2 ] and Ti-vacancy [ VI Ti 4+VI  □ VI (Mg,Fe) 2+  -2 ] 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 + Fe tot ) diagram, possibly as a consequence of a variation in intensive parameters (T, P, f H₂ , f O₂ , f H₂O ) during crystallization in the magmatic chamber.

american mineralogist

Crystal chemistry of trioctahedral micas in alkaline and subalkaline volcanic rocks: A case study from Mt. Sassetto (Tolfa district, Latium, central Italy)