High-gradient magnetic separation (HGMS) performed on the clay fraction (< 1 µm) of acid soils facilitates the separation and concentration of mineral fractions according to their Fe and Mg contents. As a result, smectitic layers formed by weathering of a chlorite were identified although they were not detected in the bulk sample. The concentration in the non-magnetic fraction of minerals with an intergrade vermiculite-chlorite behaviour enabled more accurate information to be obtained on these minerals.