P-V AND T-V EQUATIONS OF STATE OF NATURAL BIOTITE: AN IN-SITU HIGH-PRESSURE AND HIGH-TEMPERATURE POWDER DIFFRACTION STUDY, COMBINED WITH MÖSSBAUER SPECTROSCOPY

Abstract

The P-Vand T-V equations of state of a natural biotite sample (Mg/Fe ratio ≁ 1) have been studied using in-situ high-pressure (0.0001-11 GPa) synchrotron radiation powder diffraction at the European Synchrotron Radiation Facilities (ESRF) in Grenoble, France, and in-situ high-temperature (298-610 K) laboratory X-ray powder diffraction. A third-order Birch-Murnaghan model [V0 = 498.7(1) Å 3, measured value] provides the following elastic parameters: K0 = 49(1) GPa, K1 = 8.1(5). The volume thermal expansion is satisfactorily described by a constant value resulting in 37(2) 10-6 K-1. Mössbauer spectroscopy proves that REDOX reactions have occurred upon heating, presumably 2(OH- + Fe2+) → 2O2- + 2Fe3+ + H2↑ and/or 4Fe2+ + 2OH- + O2 → 4Fe3+ + 3O2- + H2O. On the basis of the elastic and thermal parameters measured we have modeled the deformation contribution (Gdeform) to the Gibbs energy. The third-order Birch-Murnaghan model with V0 fixed at its experimental value and the model with refined V0 do not significantly differ from one another in terms of Gdeform. A comparison based on Gdeform between biotite and phlogopite shows a better compliance to P of the former, though balanced in mineral reactions by a difference of molar volume, i.e., V0(biotite) > V0(phlogopite).