Meccanismi di sostituzione del titano nella biotite in funzione dell'ambiente petrogenetico.

The purpose of this thesis is to investigate on the relationships between mechanisms of Ti substitution in biotite and petrogenetic environments. Ti can be accommodated in biotite according to three main substitution mechanisms: Ti-vacancy, where titanium entrance is balanced by crystalline vacant sites; Ti-oxy, where Ti change is balanced by H loss from the hydroxyle, and Ti-Tschermak, where the entry of Ti causes redistribution of charge in both octahedral and tetrahedral sites. Although Ti substitution mechanisms in biotite have been investigated extensively, the nature of this substitutions in different petrologic environments is still uncertain. In particular, it is unclear where and why the Ti-oxy substitution prevails over the Ti-vacancy and Ti-Schermak substitutions. The aim of this study is to contribute to the understanding of factors controlling the Ti entry mechanisms and to attempt to find a relationship between the dominant operating substitution mechanism and the petrologic context of the rocks.