Internal Structure of Vermiculite 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 

Substitution of Si by Al in the tetrahedral site causes excess negative charge, which is balanced by interlayer cations. The most common interlayer cation in vermiculites is Mg others are Ca and Na.  

Vermiculites can readily absorb water, and in the maximum hydrated state the H2O moleeules occur along two sheets in the interlayer space. In such a H2O sheet, eaeh H2O molecule has hydrogen bond with the neighbouring one, which weakly bound them together. There is also hydrogen bond in between the H2O and neighbouring O of the t-o-t l er. The two pairs of H2O sheets are held together by the Mg, which hes midway between them. 

The general chemical formula of vermicuhtes is given below  

At high temperature and low humidity, vermiculite loses some of the interlayer H2O, and there is ordy one H2O sheet in the interlayer space. When temperature is higher, all the H2O molecules m be expelled, giving rise to fully collapsed structure. 

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