Nuclear resonant X-ray spectroscopy of (Mg,Fe)SiO_3 orthoenstatites

Abstract

We present nuclear resonant inelastic X-ray scattering (NRIXS) and synchrotron Mössbauer spectroscopy (SMS) measurements, both nuclear resonant X-ray spectroscopic methods, on synthetic samples of orthoenstatite-structured (Mg,^(57)Fe)SiO_3, a representative component in Earth's upper mantle. All measurements were performed at ambient conditions. NRIXS spectra were measured for three samples of orthoenstatite containing 20, 13, and 7 mol% FeSiO_3. The Debye sound velocities were determined from the low-energy region of the partial phonon density of states (PDOS). With known density and bulk modulus, the shear modulus, compressional and shear wave velocities have been computed. The sound velocities obtained from NRIXS are in good agreement with sound velocities obtained using Brillouin spectroscopy and ultrasonic methods for similar compositions. An important advantage of NRIXS is access to additional thermodynamic information, such as the average force constant, mean-square displacement, obtained from the PDOS. We discuss the contribution of the vibrational spectra to these quantities. In addition to the PDOS, the electronic environment of the iron sites in (Mg_(0.87)^(57)Fe_(0.13))SiO_3 orthoenstatite was determined using ^(57)Fe SMS and conventional Mössbauer spectroscopy. Evaluation of the Mössbauer spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The minority and majority sites are consistent with high-spin Fe^(2+) in the M1 and M2 sites, respectively.