The stability of Mg-Fe-Al biotite has been investigated with reversed phase-equilibrium experiments on four equilibria. Experimental brackets in pure H2O and H2O-CO2 mixtures for the equilibrium:

\[phlogopite\ +\ 3\ quartz\ =\ enstatite\ +\ sanidine\ +\ H_{2}O\]

are in good agreement with previous experiments in mixed-volatile fluids (Bohlen et al. 1983) and H2O-KCl solutions (Aranovich and Newton 1998), while indicating a reduced stability field for phlogopite compared to previous data in pure H2O (Wood 1976; Peterson and Newton 1989). Aluminum solubility in biotite has been determined in the Fe-, Mg-, and Fe-Mg systems from reversed phase-equilibrium data for the equilibria:

\[3\ eastonite\ +\ 6\ quartz\ =\ 2\ phlogopite\ +\ 3\ sillimanite\ +\ sanidine\ +\ H_{2}O\]
\[3\ siderophyllite\ +\ 6\ quartz\ =\ 2\ annite\ +\ 3\ sillimanite\ +\ sanidine\ +\ H_{2}O\]

over the P-T range ~600–750 °C and 1.1–3.4 kbar. Over the investigated temperatures, the brackets define nominal Al saturation levels of 1.60 ± 0.04 in Mg-biotite, 2.08 ± 0.05 in Fe-biotite, and 1.81 ± 0.03 in biotite with Fe/(Fe + Mg) = 0.43–0.44. The slight decrease in Al with increasing T and decreasing P suggested by the data is less than experimental uncertainties.

Compared to biotite on the Phl–Ann join, Al-saturated biotites have a markedly larger stability field, particularly in the Fe-system. This effect has been quantified in the Fe-system with one reversal between 691–709 °C at 2.4 kbar for the equilibrium:

\[biotite\ +\ sillimanite\ +\ quartz\ =\ almandine\ +\ sanidine\ +\ H_{2}O\]

The combined experimental results place tight constraints on the thermodynamic properties of phlogopite, annite, eastonite, and siderophyllite. The resulting nonzero (ΔH298 = −9.4 kJ/mol, with ΔS = ΔV = 0) energetics for the internal equilibrium:

\[Eastonite\ +\ 2/3\ Annite\ =\ 2/3\ Phlogopite\ +\ Siderophyllite\]

reflect strong Fe-Al affinity in biotite, which has a marked effect on thermobarometers involving biotite.

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