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 See also Appendix I, Table A3