Mossbauer investigations and molecular orbital calculations on epidote

The electronic structure of iron-rich epidote has been investigated by clus ter molecular orbital calculations in local spin density approximation. Cal culated quadrupole splittings for Fe(III) at both the MI and M3 sites are i n quantitative agreement with the experimental values obtained by Mossbauer spectroscopy. A detailed analysis of the theoretical results shows that a strong tetragonal compression of the M3 octahedron is responsible for the u nusually large value of the quadrupole splitting of Fe-M3(III). The corresp onding electric field gradient (efg) is dominated by the anisotropy of the valence shell of iron, whereas the ligands contribute only about 15% to the efg. The calculations emphasize that rather large clusters, extending beyo nd the second coordination sphere of iron, are necessary for a reliable des cription. Small clusters including only the first coordination sphere of ir on generally yield misleading results due to unsaturated oxygen bonds and r elatively large cluster charges.