MOSSBAUER STUDIES OF IRON SILICATE SPINEL AT HIGH-PRESSURE

We have measured in situ Mossbauer transmission spectra of iron silicate spinel (gamma-Fe2SiO4) in a diamond anvil cell at room temperature and pressures up to 16 GPa. The observed spectra show a doublet characteristic of the paramagnetic state. The isomer shift and quadrupole splitting at atmospheric pressure are 1.10 and 2.63 mm/s, respectively, which are smaller than those of fayalite (alpha-Fe2SiO4). Both the isomer shift and quadrupole splitting decrease linearly with pressure with slope of -0.003(1) and -0.020(1) mm/sec.GPa, respectively. This simple linear trend suggests that no electronic or polymorphic transitions occur under 16 GPa except for those due to the small and continuous changes of volume and local symmetry under pressure. On the basis of a crystalline field calculation, the negative pressure derivative of the quadrupole splitting is associated with a trend towards an ideal cubic symmetry of the oxygen sublattice.