Temperature dependence of the hyperfine parameters of syntheticP21/cMg-Fe clinopyroxenes along the MgSiO3-FeSiO3join

Abstract

Transmission 57 Fe Mossbauer measurements were acquired in the temperature range 11–745 K from a suite of nine synthetic Ca-free P 2 1 / c Mg-Fe clinopyroxenes (cpx) along the MgSiO 3 -FeSiO 3 join. The paramagnetic Mossbauer spectra (MS) consist of one doublet produced by Fe 2+ ions at an almost regular octahedral M1 site and a second doublet at a more distorted octahedral M2 site. The temperature dependencies of the Fe 2+ center shifts were fit to equations derived from the Debye model for the lattice vibrations, allowing the determination of the characteristic Mossbauer temperatures for the two Fe sites. The temperature variations of the M1 and M2 quadrupole splitting Δ E Q ( T ) are consistent with the higher distortions of the M2 octahedra. Applied-field MS revealed that the principal component of the electric field gradient, V zz , is positive, implying a tetragonal compression of both octahedral sites. The crystal-field model was used to analyze Δ E Q ( T ) and to calculate the energy gaps Δ 1 and Δ 2 of the first excited electronic states within the 5 D orbital term, both at M1 and M2. The various physical quantities derived from the MS are discussed in terms of the Fe/(Fe + Mg) ratio.