Vermiculite octahedral sheet, composition

They found that the soil vermiculites could be placed into two groups. The first group had at least one A1 per four tetrahedral sites and A1 in the octahedral sheet (No.1-5). The charge deficiency in the tetrahedral sheet was partially compensated for by a positive charge in the octahedral sheet (two cations per three sites). These samples are similar to the coarser-grained vermiculites in composition. The second... 

The difference in the composition of these two size fractions is similar to the difference between the two types of clay vermiculite described by Barshad and Kishk. The two sets of data confirm the idea that clay vermiculites developed by mild leaching action of pre-existing sheet structures tend to inherit much of their octahedral and tetrahedral character. Clay vermiculites formed by relatively intense weathering and by diagenetic alterations and in approximate equilibrium with their soil environment will have little if any tetrahedral Al the octahedral sheet can be quite variable in composition and depends on the availability of Al, Fe, and Mg. 

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