Three samples of bulk smectite clay from the Pilori and Velanida bentonite deposits of the Ventzia basin, western Macedonia, Greece, were characterized in detail. Chemical analyses and X-ray diffraction (XRD; Rietveld method) showed that the samples are rich in Fe and Mg (8–13 mass% and 6–10 mass% as oxides, respectively) and contain ~60–65 mass% smectite. The high Fe and Mg content as well as the high Cr (0.3–0.5 mass%) and Ni content (0.1–0.3 mass%) are consistent with the formation of these clays from ultramafic precursor sediments of the Vourinos ophiolite complex. Both XRD and infrared spectroscopy indicate the presence of other clay minerals besides smectite, such as minor amounts of kaolinite, trioctahedral-rich palygorskite, serpentine and talc (depending on the sample). The position of the d060 reflection (1.51–1.52 Å) is compatible with either high Fe content or partial trioctahedral character, or probably both. The predominance of ferruginous smectite or nontronite was confirmed by infrared spectroscopy, but the minor presence of a trioctahedral magnesian smectite in some of the samples cannot be excluded. Layer charge densities, determined by the νO-D (oxygen-deuterium IR-stretching) method calibrated against the structural formula method, are in the 0.48–0.52 eq/FU range. Various methods point to the existence of a significant amount of tetrahedral charge, which is typical of ferruginous smectites. The major exchangeable cation is Mg2+ (73–90%), which represents a special property of these materials compared with other bentonites.