Characterization of synthetic hedenbergite (CaFeSi₂O₆)–petedunnite (CaZnSi₂O₆) solid solution series by X-ray powder diffraction and ⁵⁷Fe Mössbauer spectroscopy

Abstract

Clinopyroxenes along the solid solution series hedenbergite (CaFeSi₂O₆)–petedunnite (CaZnSi₂O₆) were synthesized under hydrothermal conditions and different oxygen fugacities at temperatures of 700 to 1200 °C and pressures of 0.2 to 2.5 GPa. Properties were determined by means of X-ray diffraction, electron microprobe analysis and ⁵⁷Fe Mössbauer spectroscopy at 298 K. Unit-cell parameters display a linear dependency with changing composition. Parameters a ₀ and b ₀ exhibit a linear decrease with increasing Zn content while the monoclinic angle β increases linearly. Parameter c ₀ is not affected by composition and remains constant at a value of 5.248 Å. The molar volume can be described according to the equation V _mol (ccm mol⁻¹)=33.963(16)−0.544(31)*Zn pfu. The isomer shifts of ferrous iron on the octahedral M1 site in hedenbergite are not affected by composition along the hedenbergite–petedunnite solid solution series and remain constant at an average value of 1.18 mm s⁻¹. Quadrupole splittings of Fe²⁺ on the M1 are, however, strongly affected by composition, and they decrease linearly with increasing petedunnite component in hedenbergite, ranging from 2.25 mm s⁻¹ for pure hedenbergite end member to 1.99 mm s⁻¹ for a solid solution containing 84 mole% petedunnite. The half-widths of intermediate solid solutions vary between 0.26 and 0.33 mm s⁻¹, indicating, in accordance with the microprobe analyses and X-ray diffraction, that samples are homogeneous and well-crystallized. The data from this study demonstrate that the crystallinity of hedenbergitic clinopyroxenes can be improved by using oxide mixtures as starting materials. Crystal sizes for intermediate compositions range up to 70 μm, suitable for standard single-crystal X-ray analysis.