A Mössbauer study of ferrous silicates

Abstract

The Mössbauer spectra of three types of ferrous silicates, almandine garnet, olivine and hedenbergite, have been recorded over a wide range of temperatures, and the effects on the spectra of substitution by magnetic or non-magnetic ions have been investigated. A new assignment has been proposed for the 3d spin-allowed crystal field levels of the Fe²⁺ ion in the 8-fold coordinated site of almandine garnet. Several synthetic Ca, Mg and Mn iron-olivines were prepared and the site populations determined. The low temperature spectra of fayalite have been investigated and the two subspectra corresponding to Fe²⁺ in the M(l) and M(2) sites have been identified. Single crystal studies at high temperatures have been employed to determine the EFG principal axes at both sites and the orientation of the hyperfine field at both sites. An estimate of exchange interactions in the olivine structure has been made from the compositional variation of Neel point of the Mn-Fe olivines. Natural samples of hedenbergite have been investigated over a wide temperature range and the hyperfine parameters have been determined. The spectra of an oriented single crystal are presented, and the relative peak intensities indicate that a change of orientation of the EFG principal axes occurs on substitution of Mn²⁺ for Fe²⁺ ions in the M(l) sites.