Abstract: The dehydration of sodium, potassium, lithium, hydrogen, calcium, magnesium, and manganese monoionic montmorillonites (prepared with ion-exchange columns) and a vermiculite, was examined by the x-ray powder diffraction oscillating-heating method. The position and intensity of each of the first-order basal spacings were measured at intervals of 5° to 10° C from room temperature to 900° C. Position and intensity measurements were made for the first five orders of the basal spacing of the calcium montmorillonite up to 300° C and compared with computed intensities at these spacings. These data are presented as oscillating-heating x-ray diffraction diagrams showing the intensity change with temperature, with important spacing shifts indicated, and as graphs showing the change in spacing with change in temperature.
Like vermiculite, calcium, magnesium, manganese, lithium, and hydrogen montmorillonite have an octahedral coordination of their exchange cation, and upon heating they also pass through two stable hydrates (ca. 14.5 A and 11.5 A). Montmorillonites of the two larger ions, sodium and potassium, have a regular one-water-layer configuration at 12.4 A, which is distinctly different from the 11.5 A hydrate of the smaller ions.
The foreshortened oxygen-oxygen approaches of the lower hydrate (11.5 A) may be accounted for by assuming that the exchange ions take up coordinate positions between silica-oxygen and water levels or, alternatively, that some of the silica tetrahedra are inverted.