Theoretical modelling of cis-vacant and trans-vacant configurations in theoctahedral sheet of illites and smectites

The tis-vacant configurations of smectites and illites have been studied th eoretically by using transferable empirical interatomic potentials. A wide range of compositions of octahedral and tetrahedral cation and interlayer c harge has been considered. All results have been compared with the trans-va cant configurations in each sample. The calculated values reproduce the dif ferences in the lattice parameters between the cis- and trans-vacant config urations of experimental studies. Taking into account the cis-/trans-vacant proportion. the calculated structures agree with experiment for the main s tructural features of the crystal lattice. The effect of the cation substit ution in the octahedral and tetrahedral sheets on the cell parameters has b een also studied. finding good linear relationships. The calculated cation substitution effects are consistent with experimental results. Although the energy difference between the cis- and trans-vacant configurations is smal l, the cis-vacant is more stable when the composition of clays is more smec titic, like the experimental behaviour. Similar trends of the cation substi tution effect on the cis-/trans-vacant proportion to the experimental resul ts are found. The structure of the hydroxy groups has also been analysed. T he OH bond length, the orientation of the O- H bond with respect to the (00 1) plane and the nonbonding H...O distances have been studied.