Designating or relating to a group of clay minerals that comprises montmorillonite together with chemically related minerals which undergo a reversible expansion on absorbing water and have a characteristic structure in which water molecules are situated between sheets composed of two layers of silicon atoms sandwiching one nominally of aluminium, the differences between the minerals consisting in the extent and nature of the replacement of silicon and aluminium by other elements.

A member of the montmorillonoid group; the group itself.