Mössbauer spectra of several magnetic sheet silicates in external magnetic field

Mössbauer 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. Fe^III ions in glauconite are deduced to be coupled ferromagnetically in the plane with alternate ferromagnetic planes coupled by a weak anti ferromagnetic interaction. Magnetic-hyperfine field distributions are computed for the magnetic spectra giving some indication of the relative disorder at particular iron sites.