LAUSR

Abstract Nontronites are iron-rich dioctahedral smectites that serve as a natural source of nanomaghemite and nanowustite, which are industrial minerals widely used to develop biomedicine and semiconductor applications. The integration of various bulk analytical techniques and computational calculations to study the structure of nontronites show that their contents of tetrahedral Fe3+ surpass theoretical values. Consequently, the microstructure of nontronite has only been partially understood. Three nontronites (NAu-1 and NAu-2 from South Australia) and NG-1 (Hogen Hagen, Germany) were studied as received using high-resolution techniques, namely, High-Resolution Scanning and Transmission Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (HRSEM and HRTEM-EDS), and Electron Spin Resonance Spectroscopy (ESR). Nontronite surfaces contained abundant nanodomains (predominantly ≤ 2-nm in diameter), homogeneously distributed, mostly circular in shape, discrete, crystalline, and composed by a single phase. Nanodomains were found invariably associated to nontronite surfaces, mostly embedded although loosely bound at times. Noteworthy were nanodomains showing non-round contours, a signature of halted growth. However, nanodomain abundance related to the tetrahedral Fe (or unit cell formulae). If theoretical contents of tetrahedral Fe in nontronite were higher (NAu-2, NG-1) then the nanodomains were found to be less abundant and more heterogeneous in size and shape. Acquired Miller diffraction indexes for Fe nanodomains confirmed the presence of maghemite [γ-Fe2O3] and wustite [Fe1−xO]. Most remarkably, wustite, a highly-reduced mineral remained stable under oxic conditions, so the small size of wustite and its interactions with nontronite surfaces inhibited the oxidation of structural Fe by atmospheric oxygen. Finally, we specuate that mineral microregions containing nanomaghemite and nanowustite may form a Fe mixed valence system showing electronic and magnetic properties resembling those characteristic of Fe(II)-Fe(III) minerals.