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Reversed phase equilibrium constraints on the stability of Mg-Fe-Al biotite

¹ Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario, Canada K1A 0E9
² Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemisty, 119017 Moscow, Russia
³ Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
⁴ Institute for Chemical Process and Environmental Technology, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario, Canada K1A 0R6

Correspondence: ^* E-mail: RBerman{at}NRCan.gc.ca

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:

(1)

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:

(2)

(3)

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:

(4)

The combined experimental results place tight constraints on the thermodynamic properties of phlogopite, annite, eastonite, and siderophyllite. The resulting nonzero (H₂₉₈ = –9.4 kJ/mol, with S = V = 0) energetics for the internal equilibrium:

(5)

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

Key Words: Biotite • phase equilibria • experimental petrology • mixing properties • annite • phlogopite • siderophyllite • eastonite

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