Phengite chemistry has been investigated in experiments on a natural SiO 2 -TiO 2 -saturated greywacke and a natural SiO 2 -TiO 2 -Al 2 SiO 5 -saturated pelite, at 1.5-8.0 GPa and 800-1,050°C. High Ti-contents (0.3-3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si ? R 2? = Al IV ? Al VI ) coupled with the substitution:The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si 3 Al]O 10 (OH) 2 , TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel ? Rt = TiP ? Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T-P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crust.
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