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

The P-V and 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) Å3, measured value] provides the following elastic parameters: K0 = 49(1) GPa, K’ = 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 Þ xed at its experimental value and the model with reÞ ned V0 do not signiÞ cantly 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).