• journal_title：American Mineralogist
• Contributor：R.G. Berman ; L. Ya. Aranovich ; D.G. Rancourt ; P.H.J. Mercier
• Publisher：Mineralogical Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2007.2051
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：92
• issue：1
• firstpage：139
• section：Articles

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:

lass="disp-formula" id="disp-formula-1">lass="math tex" alt="Formula" src="139/embed/tex-math-1.gif" />lass="disp-formula-label">(1)

are in good agreement with previous experiments in mixed-volatile fluids (lass="xref-bibr">Bohlen et al. 1983) and H₂O-KCl solutions (lass="xref-bibr">Aranovich and Newton 1998), while indicating a reduced stability field for phlogopite compared to previous data in pure H₂O (lass="xref-bibr">Wood 1976; lass="xref-bibr">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:

lass="disp-formula" id="disp-formula-2">lass="math tex" alt="Formula" src="139/embed/tex-math-2.gif" />lass="disp-formula-label">(2)

lass="disp-formula" id="disp-formula-3">lass="math tex" alt="Formula" src="139/embed/tex-math-3.gif" />lass="disp-formula-label">(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:

lass="disp-formula" id="disp-formula-4">lass="math tex" alt="Formula" src="139/embed/tex-math-4.gif" />lass="disp-formula-label">(4)

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:

lass="disp-formula" id="disp-formula-5">lass="math tex" alt="Formula" src="139/embed/tex-math-5.gif" />lass="disp-formula-label">(5)

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