Internal Structure of Illite Group

This group has trioctahedral or dioctahedral t-o-t-c structure, in which each o-sheet is sandwiched between two inward-pointing t-sheets. In the o-sheet, four out of six apical anions of each octahedron are 0 , the remaining two are OH- 

The illite group minerals have a structure similar to that of muscovite, but there is less substitution of Sr by in the tetrahedral sites. As a consequence, 

Because of possible charge imbalance, Ca and Mg can also sometimes 

The basal spacing of illite is 10 A. The K, Ca or Mg interlayer cations prevent the entrance of H2O into the structure. For this reason the basal spacing in illite is constant. Thus, the illite group minerals cannot expand by absorption of water like halloysite, vermiculite and smectite. 

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