Pressure-induced structural phase transition of the iron end-member of ringwoodite ( -Fe2SiO4) investigated by X-ray diffraction and Mossbauer spectroscopy

Abstract

We have carried out X-ray diffraction and Mossbauer spectroscopy measurements on the spinel phase γ-Fe 2 SiO 4 (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 R 3 mR ). Mossbauer spectroscopy measurements reveal two different Fe 2+ 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 K 0 = 197(3) GPa using the second-order Birch-Murnaghan equation of state, and K 0 = 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 Mossbauer hyperfine parameters are in good agreement with previous studies, which were limited to a lower pressure range.