Moessbauer spectra of several magnetic sheet silicates in external magnetic field

Moessbauer spectra of nontronite, glauconite and montmorillonite recorded at temperatures down to 1.3 K and in applied fields of up to 4.5 T confirm that nontronite and glauconite are antiferromagnetic at 1.3 K. Glauconite becomes ferromagnetic in applied field. Fesup(III) ions in glauconite are deduced to be coupled ferromagnetically in the plane with alternate ferromagnetic planes coupled by a weak antiferromagnetic interaction. Magnetic-hyperfine field distributions are computed for the magnetic spectra giving some indication of the relative disorder at particular iron sites. (Auth.)