Mössbauer spectroscopy and Rietveld refinement on synthetic ferri-Tschermak’s molecule CaFe3+(Fe3+Si)O6 substituted diopside
Redhammer, Günther J.
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 439 - 452
published: Jun 22, 1998
manuscript accepted: Jan 21, 1998
manuscript received: Apr 23, 1997
ArtNo. ESP147051003004, Price: 29.00 €
Abstract Clinopyroxenes on the join diopside (di) CaMgSi2O6 - ferri-Tschermak's molecule (fts) CaFe3+Fe3+SiO6 were synthesized at different temperatures between 1050°C and 1325°C under atmospheric pressure. The solubility of fts in diopside increases with temperature up to 1150°C where it is less 30 % by weight and decreases as temperature further increases. Mossbauer spectra show at least three symmetric doublets corresponding to ferric and ferrous iron on the octahedral M1 site and ferric iron on the tetrahedral site. A markedly high amount of Fe2+ is found in diopside rich samples synthesized at high temperatures and decreases with both decreasing synthesis temperature and increasing fts component. The isomer shift of Fe3+tet increases distinctly with fts component as a consequence of increasing mean T-O bond length. The isomer shifts of Fe3+ and Fe2+ on the octahedral sites are rather insensitive to changes in composition, in contrast to quadrupole splittings of all three components. The negative correlation of quadrupole splitting versus fts content of Fe3+ on the tetrahedral and on the octahedral site can be interpreted as decreasing Fe-polyhedral distortion as fts content is increased, whereas the increase of Fe2+ quadrupole splitting is a sign of increasing distortion of the Fe2+O6 octahedra in Fe3+ rich diopsides. Lattice parameters, fractional atomic coordinates and interatomic distances were determined by Rietveld refinements of X-ray powder diffraction data. The Fe3+Fe3+ for Mg2+Si4+ substitution increases ao and co lattice parameters as a consequence of increasing size of the tetrahedra (significant increase of T-O bond lengths), whereas a moderate shrinking of the size of the M1 octahedra, as seen by a decrease of M1-O distances, causes the shrinking of bo.The monoclinic angle ß remains unaffected by fts substitution.