Four natural bentonites, from Texas, Wyoming, Mexico, and Puerto Rico, have been fractionated by use of a super-centrifuge. The fractions have been investigated for base exchange capacity, apparent density, X-ray diffraction character and gel strengths of their suspensions.

Based on (001) spacings from X-ray diffraction and cation composition from ammonium acetate base exchange data, it was found that upon fractionation, clays of heterogeneous cation composition divide into two groups. Fractions larger than 0.150 micron contain the divalent cations Ca2+-Mg2+, while the finer fractions contain the monovalent cation Na+. Based on indirect evidence provided by X-ray diffraction, particle size fractionation, and gel strength, and correlation of the data with the literature, it is concluded that Na+ clays exist as unit layers in suspensions while Ca2+-Mg2+ particles are aggregates of a few layers.

Base exchange capacity and density were found to be independent of particle size.

A new structure is postulated and proposed for aqueous montmorillonite gels. This structure, descriptively called “hinge,” comprises clay particles oriented randomly to form a network in which each particle is hinged to other particles through sharing of bound particle-water.