BENTONITES FROM MOSSBAUERAND EPR SPECTROSCOPY

ABSTRACT: M/Sssbauer 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 F content ( ~ 1-10H) which is seen as doublets in the spectra t 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. The term smectite refers to an important group of microcrystalline clay minerals with a large variation in chemical composition (Ross & Hendricks, 1945; Brindley, 1980), consisting of 2: 1 silicate layers in which planes of cations with octahedral coordination are sandwiched between sheets of tetrahedrally-coordinated cations. A variety of cations can occur in both the tetrahedral and octahedral sheets, but primarily Al and Si occur in tetrahedra