Abstract
An experimental determination has been made of the solubility of Ti in phlogopite, in the system K 2OMgOTiO 2Al 2O 3SiO 2H 2O. The fixation of Ti 4+ in the phlogopite structure is considered to follow the substitutional model: 2Si IV, Mg VI ⇌ 2Al IV, Ti VI. Titanium substitutes in octahedral sites and does not replace Si 4+ in tetrahedral sites. The solubility of Ti in phlogopite is very small at 600°C and 1,000 bar (0.07 Ti atom per formula unit); it increases up to 0.2 Ti atom per formula unit at 800°C and 1,000 bar, and 0.7 Ti atom per formula unit at 1,000°C and 1,000 bar. Pressure reverses this trend and reduces the solubility (0.2 Ti atom per formula unit at 1,000°C and 7,000 bar. The experimental results explain why Ti-rich phlogopites are usually encountered in lavas (lamproites), and also, why the Ti content of biotite increases with metamorphic grade. However, any use of the solubility of Ti in biotites as a geothermometer or a geobarometer would necessitate a determination of the influence of Fe on that solubility.
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