Moessbauer studies of selected synthetic silicates

Moessbauer spectra of fayalite-, 7x03B1;-Fe2SiO4, powder and single crystals were taken in the antiferromagnetic temperature region below TN≅65 K. The orientation of the efg and H(0) with respect to the crystallographic axes was determined as function of the temperature and compared to neutron diffraction and magnetic susceptibility data. A similar study was performed with Li-acmite LiFeSi2O6, whose magnetic structure can be regarded as a model compound for quasi one-dimensional systems. Synthetic annite KFe3[AlSi3O10(OH)2] has to contain at least about 10% Fe3+ in order to be stable. A comparison of the evaluation of the spectra assuming either Lorentzian line shape of the doublets or quadrupole splitting distributions QSDs shows that Fe3+/Fe2+-ratios can be determined precisely by both methods. However, M2/M1-site distributions cannot be evaluated with great accuracy. In ilvaite CaFe3[Si2O7/O/(OH)], there is a thermally activated intersite electron hopping between Fe2+ and Fe3+ at temperatures around 300 K in a double octahedron chain. At temperatures above 395 K there is extended electron delocalization in a narrow d- or polaron-band. The substitution of Fe by Mn favours both effects and lowers the temperature of the crystallographic phase transition monoclinic to orthorhombic distinctly. In the solid solution series member acmite(50%)-hedenbergite(50%) Na0.5Ca0.5Fe2[Si2O6] just intersite electron hopping between Fe2+ and Fe3+ in the M1 octahedron chain and no formation of polaron bands is observed. This is explained by the larger Fe-Fe-intrachain distances compared to those in ilvaite or magnetite