Technical Report NTB 89-03
Transmission electron microscope study of illite/smectite mixed layers
A transmission electron microscopy study was undertaken to investigate the microstructural relationships between mixed layer illite/smectite phyllosilicates and their alteration products. The analysis of specimens from Southwestern Sweden suggested that four types of montmorillonite alteration could be distinguished. The first three possibilities involved the dissolution of montmorillonite followed by the precipitation of various phases: silica, fine grained montmorillonite which subsequently recrystallized to illite, or quartz and illite. The fourth possibility involved dehydration and isostructural replacement of montmorillonite with illite. The mobility of amorphous silica in the montmorillonite structure was demonstrated using in-situ experiments with the electron beam, and static observations of two identical but differently prepared specimens. In the latter case distinctions were observed in the residence of the silica phase. Based on the transmission electron microscope observations, it is proposed that the precipitated amorphous silica can affect the swelling capability of montmorillonite both as a replacement of interlayer water, and as an obstruction to the free movement of water into the structure.