Various Ti substitutions in biotite were tested by hydrothermal syntheses of Fe and Mg biotites over the range of 600-800 °C at 1000-bars pressure and at the oxidation conditions of the methane-graphite buffer. Yields of greater than 95% biotite were obtained in a few experiments, but complete syntheses were generally not possible. Microprobe analyses of the run products indicate that (1) the Ti-Tschermak's substitution (Ti + 2A1 = R2+ + 2Si) can be the dominant Ti substitution in both Fe and Mg biotites; (2) the Ti-vacancy substitution (Ti + [6]□ = 2R2+) can occur as the dominant Ti substitution in Mg biotite and in conjunction with other substitutions in Fe biotite; and (3) the experimental data do not support, but cannot rule out, the Ti-oxy substitution (Ti + 2O2+ = R2+ + 2OH). Specific substitution types appear to be governed by bulk composition and by the crystal-chemical constraints of the biotite structure.

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