The effect of Fe-for-Si substitution on the b-dimension of nontronite

A survey of 060 spacings of dioctahedral smectites has shown that for nontronites with only iron on octahedral sites, a linear relationship exists between the b -axial dimension and the number of atoms of tetrahedrally coordinated Fe 3+ per unit cell containing O 20 (OH) 4 . The slope, 0·0497 Å/Fe 3+ , is steeper than that for the increase in b resulting from Fe 3+ on octahedral sites, 0·0377 Å/Fe 3+ , demonstrating the greater importance of tetrahedral substitution in influencing b . Factors that take account of the number of Fe 3+ atoms on tetrahedral sites have been incorporated in existing formulae for calculating b -axis cell dimensions for smectites, but these are usually less successful in predicting b than a new formula involving only tetrahedral and octahedral iron contents.