Résumé
We have carried out X-ray diffraction and Mössbauer spectroscopy measurements on the spinel phase γ-Fe2SiO4 (ringwoodite) at ambient temperature and pressures up to 66 GPa using diamond anvil cells. At pressures above 30 GPa, a previously unknown structural phase transition to a rhombohedrally distorted spinel phase has been observed (space group R3̅mR). Mössbauer spectroscopy measurements reveal two different Fe2+ sites at high pressure with an abundance ratio of 3:1, in agreement with the two crystallographic sites occupied by the iron in this distorted spinel structure. The unit-cell volume of the low-pressure spinel phase as a function of pressure results in a bulk modulus of K0 = 197(3) GPa using the second-order Birch-Murnaghan equation of state, and K0 = 201(8) GPa and K′ = 3.7(7) when using a third-order equation of state. The pressure evolution of the unit-cell volume and the Mössbauer hyperfine parameters are in good agreement with previous studies, which were limited to a lower pressure range.