Abstract

Clinopyroxenes along the solid solution hedenbergite-aegirine M2[Ca2+1-xNa+xM1{Fe2+1-xFe3+x}Si2O6 were synthesized using hydrothermal techniques at 4 kbar. Different temperatures and redox conditions were used to determine optimum synthesis conditions and the stability range of individual compositions in the T - log fO2 field. Synthesized samples were characterized using microprobe analysis, X-ray powder diffraction and Mössbauer spectroscopy at 298 K and 80 K. The structure was refined in the C2/c space group by means of the Rietveld method.

Along the solid-solution series between hedenbergite (a0 = 9.8448(6) Å, b0 = 9.0296(6) Å, c0 = 5.2452(4) Å, β = 104.8130) and aegirine endmembers (a0 = 9.6547(6) Å, b0 = 8.7941(8) Å, c0 = 5.2944(4) Å, β = 107.3980), the changes in unit cell dimensions show significant deviations from linearity. Mean and individual M1-O distances decrease linearly from hedenbergite to aegirine; mean M2-O and T-O distances do not change significantly, whereas individual length may vary. While in hedenbergite the coordination of the M2 site is 6+2, it is 4+4 in aegirine. The Mössbauer spectra of the solid-solution endmembers display narrowly split resonance absorption lines with hyperfine parameters typical for Fe2+ (δ = 1.18 mm/s, Δ = 2.25 mm/s at 298 K) and Fe3+ (δ = 0.38 mm/s, Δ = 0.30 mm/s at 298 K). Fe occupies only the M1 site. The Fe2+ resonance absorption is somewhat broadened in the 80 K spectra of the solid solution, which is due to a distribution of quadrupole splittings.

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