The basic process in thermal transformation of smectite to hydrous mica is generally thought to be a lattice charge change caused by partial replacement of tetrahedral silica by aluminum. This is assumed to yield preferential uptake of potassium with concomitant contraction and loss of expandability of smectite aggregates as well as release of silica that migrates by diffusion from interlayer space into interaggregate voids, where it may form amorphous silica hydrogels and possibly quartz.

The latter process was investigated by use of hydrothermal tests of fully water saturated, as well as partly saturated Na-montmorillonite gels, the intention being to identify possible heat-induced changes in expandability on a molecular scale by applying electron microscopy. The gels were exposed to a temperature of 225°C for 18 days in a first test series and the microstructural patterns compared with those of non-heated material.

A clear tendency of lattice contraction was observed in the heated clay gels, particularly in the non-saturated one. The microstructure had the form of networks of virtually non-expandable, interwoven dense stacks.

A possible physical explanation of the contraction is that the heat caused instability of the interlayer water lattices, yielding dominant interatomic mass forces which caused contraction of the stacks. In connection herewith, silica was released from the smectite lattices and precipitated at the edges of the stacks, which reduced or eliminated the expandability. Minute, precipitated silicic bodies, amorphous as well as crystal-line, appeared in both clays.