Mixing properties of the enstatite-ferrosilite solid solution : II. A microscopic perspective

Infrared spectra of natural, synthetic and heat treated orthopyroxene samples belonging to the enstatite-ferrosilite join were investigated in the range 70–1400 cm⁻¹. Correlations of the wavenumber shifts and line broadening systematics with the thermodynamic mixing properties were found. Wavenumber shifts of the highest energy modes correlate closely with the Si-O bond lengths and give an indirect view of the distortions of tetrahedra across the solid solution. One phonon line dominated by the bending of the tetrahedral chains is sensitive to changes in the degree of cation order and correlates with the average of the kinking angles of the same samples determined by X-ray diffraction. All the other wavenumber shifts (at lower energies) are correlated linearly with compositional changes only. A phonon line shift at low energy correlates with unit cell volume. Local strains arising from cation substitution and ordering have been quantified by means of an autocorrelation function. Local heterogeneities are associated with Fe/ Mg substitution and disordering in samples at intermediate composition. Line broadening is more pronounced in the energy region of the spectra, corresponding to modes involving primarily the M cations. The shape of line broadening in this region is similar to the enthalpy of mixing, ΔH_mix. Ordering reduces broadening and hence local strains and excess mixing properties. A fully ordered orthopyroxene at En₅₀Fs₅₀ would be collinear with pure En and pure Fs, implying that ordering eliminates the local heterogeneities which arise due to Fe/Mg mixing.