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

• Published in: The American mineralogist Vol. 92; no. 4; pp. 468 - 480
• Main Authors: Laurora, Angela, Brigatti, Maria Franca, Mottana, Annibale, Malferrari, Daniele, Caprilli, Enrico
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.04.2007; De Gruyter; Walter de Gruyter GmbH
• Subjects: alkalic composition; Case studies; central Italy; Chemistry; crystal chemistry; Crystallization; Crystals; electron probe data; Europe; hydrothermal conditions; hydrothermalism; Igneous rocks; Italy; Latium Italy; lattice parameters; mica group; mica oxidation; Mineralogy; Minerals; Monte Sassetto; Petrology; sheet silicates; silicates; Southern Europe; substitution; Ti-substitutions; Tolfa Hills; Topology; unit cell; Volcanic micas; Volcanic rocks; Volcanoes; X-ray diffraction data; Italy
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2007.2339

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+VI[]VI(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).