An Fe-rich smectite from Ölberg (Germany) was characterized using X-ray diffraction, infrared spectroscopy and analytical electron microscopy. Progressive reduction of the octahedral charge was performed through the Hofmann & Klemen effect at increasing temperatures. The sample was heterogeneous, consisting of two smectite populations. One population, which comprises a minor portion of the sample, is an ‘Fe3+-montmorillonite’ with little or no tetrahedral charge and Fe3+ as the major octahedral cation. The other population, a major constituent of the sample, contains less Mg and more Al than the first one and exhibits some tetrahedral charge. This fraction may be considered as an inter-grade between nontronite (dominant tetrahedral charge) and Fe3+-montmorillonite (dominant octahedral charge). These two populations may occur as separate particles but also as interstratified layers.

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Under closed geochemical conditions, the weathering of a serpentinite rock composed of serpentine (70–85%) and magnesian chlorite (10–15%) associated with magnetite and chromite leads to the complete replacement of serpentine and chlorite by 2:1 layer silicates and produces new Fe oxides. The serpentine minerals crystallize under different habits issued from the serpentinization processes: mesh and hourglass pseudomorphic textures were formed from olivine, and thin-bladed pseudomorphic textures from pyroxene and amphibole crystals. Serpentine veins crosscut the whole rock with locally non-pseudomorphic interpenetrating and interlocking serpentines.Specific weathering microsystem habits with specific clay mineral crystallizations originate from these different habits: a poorly aluminous saponite in thin-bladed textures, two Fe-rich montmorillonites in mesh and hourglass (MH) textures, and in veins (V) which differentiate on Al, Mg and Fe contents. Magnesian chlorites, isolated from serpentine by hand-picking under a stereomicroscope, are found to weather to trioctahedral vermiculite. Magnetite and chromite extracted from the bulk samples are replaced by newly formed Fe oxides, maghemite, goethite and hematite, which give way to specific Fe accumulation habits in the regolith zone of the weathering profile.