Mössbauer and molecular orbital study of chlorites

The different Fe2+ lattice sites in iron-rich chlorites have been characterized by Mossbauer spectroscopy and molecular orbital calculations in local density approximation. The Mossbauer measurements were recorded at 77 K within a small velocity range (±3.5 mm s−1) to provide high energy resolution. 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 γ-beam. The zero-field spectra were analyzed with discrete Lorentzian-shaped quadrupole doublets to account for the Fe2+ sites 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+ quadrupole splitting than cationic disorder. The spectra recorded in applied field were analyzed in the spin-Hamiltonian approximation, yielding a negative sign for the electric field gradient (efg) of Fe2+ in the M1, M2, and M3 positions. The results of the MO calculations are in quantitative agreement with experiment and reveal that differences in the quadrupole splittings (ΔE Q ), their temperature dependence and in the isomer shifts (δ) of Fe2+ in M1, 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 ΔE Q -δ parameter pairs. With the calculated average iron-oxygen bond strength, the MO study provides an explanation for the observed trend that the population of the three lattice sites by Fe2+ increases according to the relation M1 < M2 < M3.