Abstract
Three synthetic samples of the hedenbergite-diopside join Ca(FexMg1−x)Si2O6 (x = 1, x = 0.80, and x = 0.60) were studied between ambient conditions and 10 GPa (293 K) by use of 57Fe γ resonance (Mössbauer effect) with a clamp-technique pressure cell. The isomer shift δ and quadrupole splitting ΔEQ of Fe2+ at the M1 position decrease with increasing pressure. They exhibit a discontinuity between 3.8 and 4.3 GPa in all three samples, indicating a reversible phase transition of first order. An approximately linear relationship between δ and ΔEQ was observed. The value of δ at constant pressure is independent of Fe/Mg ratio, whereas ΔEQ decreases with decreasing Fe/Mg ratio. The ∂δ/∂ ln V and ∂ΔEQ/∂ ln V values were derived.
The axial and volume compressibilities of two samples (x = 1 and x = 0.60) were determined from the unit-cell parameters measured by powder X-ray diffraction between ambient pressure and 10 GPa using a diamond-anvil cell. The axial compressibility ratio βa:βb:βc of hedenbergite is 0.65(4):1.00(6):0.79(6). Isothermal bulk moduli for Hd and Hd60Di40 were determined by fitting a Birch-Murnaghan equation of state to the pressure-volume data, assuming K0′ = 4. They are 119(2) and 82.7(1) GPa, respectively. An inverse relationship of the unit-cell parameters between pressure and temperature was observed.
Pressure change in δ below 4 GPa results mainly from changes in Fe2+ valence orbitals. The change in ΔEQ is due to changes in the three lowest 3d orbitals of Fe2+ coupled with changes in octahedral geometry. For the changes in δ and ΔEQ above 4 GPa, changes in overlap between Fe2+ and ligand orbitals are important.
The decrease in ΔEQ with decreasing Fe/Mg ratio at constant pressure results mainly from the change in geometry of the M1 octahedra occupied by Fe2+. The relatively invariable δ in the three samples at constant pressure suggests that the bond distances of the Fe2+ octahedra are distinct from those of the Mg2+ octahedra.
