Evidence for the multiphase nature of bentonites from Mössbauer and EPR spectroscopy

Mössbauer spectra of several smectites demonstrate the existence of at least three phases with distinct Fe populations: (i) a component with very low Fe content (< 1%), which shows slowly-relaxing paramagnetic hyperfine structure at both 4·2 K and 77 K; (ii) a component with intermediate Fe content (∼ 1–10%) which is seen as doublets in the spectra at 4·2 K, 77 K and ambient temperature; (iii) an Fe-rich phase (> 30% Fe), which shows magnetic ordering at 4·2 K and 77 K. These data are consistent with components (i) and (ii) corresponding to Fe incorporated in aluminosilicate structures from distinct phases, whereas (iii) is characteristic of an iron oxide phase, probably goethite in most cases. These conclusions are supported by EPR measurements which show magnetically-dilute Fe in more than one type of structural environment plus an additional component with magnetically-interacting ions.