Synthesis and structural properties of clinopyroxenes of the hedenbergite CaFe2+Si2O6-aegirine NaFe3+Si2O6 solid-solution series

Clinopyroxenes along the solid solution hedenbergite-aegirine (M2)[Ca1-x2+N ax+](M1) {Fe1-x2+Fex3+} Si2O6 were synthesized using hydrothermal technique s at 4 kbar. Different temperatures and redox conditions were used to deter mine optimum synthesis conditions and the stability range of individual com positions in the T - log fO(2) field. Synthesized samples were characterize d using microprobe analysis, X-ray powder diffraction and Mossbauer spectro scopy 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 (a(o) = 9.8448(6) Angs trom, b(o) = 9.0296(6) Angstrom, c(o) = 5.2452(4) Angstrom, beta = 104.813 degrees) and aegirine endmembers (a(o) = 9.6547(6) Angstrom, b(o) = 8.7941( 8) Angstrom, c(o) = 5.2944(4) Angstrom, beta = 107.398 degrees), the change s in unit cell dimensions show significant deviations from linearity. Mean and individual M1-O distances decrease linearly from hedenbergite to aegiri ne; mean M2-O and T-O distances do not change significantly, whereas indivi dual length may vary. While in hedenbergite the coordination of the M2 site is 6+2, it is 4+4 in aegirine. The Mossbauer spectra of the solid-solution endmembers display narrowly split resonance absorption lines with hyperfin e parameters typical for Fe2+ (delta = 1.18 mm/s, Delta = 2.25 mm/s at 298 K) and Fe3+ (delta = 0.38 mm/s, Delta = 0.30 mm/s at 298 K). Fe occupies on ly 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 quadr upole splittings.