Mossbauer and molecular orbital study of chlorites

The different Fe2+ lattice sites in iron-rich chlorites have been character ized by Mossbauer spectroscopy and molecular orbital calculations in local density approximation. The Mossbauer measurements were recorded at 77 K wit hin a small velocity range (+/-3.5 mm s(-1)) to provide high energy resolut ion. Additionally, measurements were recorded in a wider velocity range (+/ - 10.5 mm s(-1)) at temperatures of 140, 200, and 250 K in an applied field (7 T) parallel to the gamma-beam. The zero-held spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe2+ sit es M1, M2, and M3 and with a quadrupole distribution for Fe3+ sites. Such a procedure is justified by the results obtained from MO calculations, which reveal that different anion (OH-) distributions in the first coordination sphere of M1 M2, and M3 positions have more influence on the Fe2+ quadrupol e splitting than cationic disorder. The spectra recorded in applied field w ere analyzed in the spin-Hamiltonian approximation, yielding a negative sig n for the electric held gradient (efg) of Fe2+ in the M1, M2, and M3 positi ons. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (Delta E-Q), their temperature dependence and in the isomer shifts (delta) of Fe2 in MI, M2, and M3 positions can theoretically by justified. Therefore, the combined Mossbauer and MO investigation shows that the three Fe2+ lattice sites in the chlorites investigated here can be discriminated according to their Delta E-Q-delta parameter pairs. With the calculated average iron-oxy gen bond strength, the MO study provides an explanation for the observed tr end that the population of the three lattice sites by Fe2+ increases accord ing to the relation M1 < M2 < M3.