The basic structure of 2:1 clay minerals is two silicon tetrahedral layers and one aluminum octahedral layer. This layer is weakly held to another 2:1 layer to make the 2:1 family of clay minerals. An interlayer or the space between the sheets becomes an important difference between 2:1 and 1:1 clay minerals. Smectite and Vermiculite are two kinds of 2:1 clay minerals.