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-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⁺) → 2O^2- + 2Fe³⁺ + H₂↑ and/or 4Fe²⁺ + 2OH^- + O₂ → 4Fe³⁺ + 3O^2- + H₂O. On the basis of the elastic and thermal parameters measured we have modeled the deformation contribution (G_deform) 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 G_deform. A comparison based on G_deform 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).