Particles of montmorillonite representing a considerable range of sizes wow separated from crude hentonites by means of sedimentation and supercentrifugation. Except for the effect of impurities, it was found that base-exchange capacity was independent of particle size.

The ratios of the several exchangeable cations on the niontmorillonites vary widely as they occur in the crude bentonites. Sodium is the dominant cation in the Clay Spur, Wyominj, and Belle Fourche, South Dakota, samples; calcium in those from Polkville, Mississippi, Santa Rita, New Mexico, Chambers, Arizona, and Plymouth, Utah; in the Little Rock, Arkansas, sample, hydrogen is the most abundant exchangeable cation. The hydrogen ion is reflected in the low pH of the sample. The Otay, California, and Merritt, British Columbia, samples contain more nearly equal amounts of exchangeable calcium, magnesium, and sodium.

Formulae calculations indicate that these samples present a rather wide range in isomorphism and, in consequence, the source of the negative charge on the lattice is widely variable. However, the calculated charges depend, to a large extent, on the purity of the sample. Evidence is presented that silicon dioxide in some form is the most abundant impurity and that significantly large amounts of uncombined silicon dioxide may be present in the finest fractions. The Merritt, British Columbia, sample was found to contain about 35 percent cristobalite. The corresponding particle size fraction from Amory, Mississippi, bentonite contains considerable hydrous mica. Unless allowance is made for impurities, the calculated lattice positions of the electro-positive atoms, and therefore the formula derived therefrom, are essentially meaningless.