Si4Al1. 5Mg0. 5O10(OH)2Ca0. 25 An aluminum silicate (smectite) with a 2:1 layer structure composed of two silica tetrahedral sheets and a shared aluminum and magnesium octahedral sheet. Montmorillonite has a permanent negative charge that attracts interlayer cations that exist in various degrees of hydration thus causing expansion and collapse of the structure (i.e., shrink-swell). The calcium in the formula above is readily exchangeable with other cations.