As a means of distinguishing between the broad groups of flay minerals, chemical analysis may be a valuable supplement to other methods, but rarely will it remove the necessity for other methods, X-ray analysis in particular. The two- and three-layer clay minerals may be distinguished by the SiO2 · Al2O3 ratio or SiO2 · sesquioxide ratio, but these ratios do not provide criteria for dividing either class. Among the three-layer clays, which embrace montmorillonite, nontronite, and the diverse hydrous micas, non-exchangeable K is a better criterion. It seems probable that montmorillonite, as found in bentonite deposits, does not contain significant amounts of non-exchangeable K, whereas all known hydrous micas contain significant, although widely variable, amounts of non-exchangeable K.

The greatest value of chemical analysis inheres in the fact that by means of calculation the specific nature of the isomorphism may be determined. But the degree of confidence that can justifiably be placed in the calculated isomorphism depends on the purity of the sample analyzed. SiO2 as impurity may be shown by an excess of calculated Si++++ ions, or by a deficiency of octahedral cations. It also affects the calculated isomorphism of the tetrahedral layers of the lattice by reducing its Al content.

On the other hand, Al2O3 as impurity affects the calculation in the opposite way; that is, it reduces the number of Si++++ ions per unit cell, while increasing both tetrahedral and octahedral Al+++ ions. Fe as impurity has essentially the same effect as Al.

In some samples it is possible to detect SiO2 as impurity by means of calculation.