Mossbauer studies of selected synthetic silicates

Mossbauer spectra of fayalite-, alpha-Fe2SiO4, powder and single crystals were taken in the antiferromagnetic temperature region below T-N congruent to 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.