A number of different commercial vermiculites have been examined and it has been found that the exfoliable minerals can be divided into three main groups; those which give a 26 Å spacing on an X-ray diffraction pattern, those which give a 14 Å spacing, and those which give various spacings and are partially dehydrated. All are mixed-layer minerals of one type or another, but all have some vermiculite layers in their constitution. The expansibility of these minerals is not linked with their structural types, but there is an indication that the largest expansion is associated with the occurrence of hydrobiotite and the lowest expansion with the occurrence of true vermiculite.