Abstract: A sodium-saturated Texas bentonite was fractionated into six particle-size ranges. The expansion of each fraction was observed by X-ray diffraction under controlled humidity conditions ranging from 0–100 per cent r.h. At all humidities, the fine fractions expanded more than the coarse. A single size fraction expands through a series 10 Å, 12.4 Å and 15.5 Å. The expansion from 10–12.4 Å occurs at progressively lower relative humidities for smaller size fractions in the same way as the expansion from 12.4–15.5 Å. The gradation from high resistance to expansion in the coarse fractions to low resistance to expansion in the fine fractions appears to be continuous.
Interlayer cation density, which controls the resistance to expansion, is dependent upon the structural charge density on the silicate layer. The continuous variation of resistance to expansion that is a function of particle size is interpreted to indicate a corresponding continuous variation in structural charge with particle size.