Experimental illitization of smectite was studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Experiments were performed on the <2 μm fraction consisting entirely of smectite separated from a soil formed on subvolcanic rock located in the External Zone of the Betic Cordilleras (southern Spain). Amounts of 0.25 g were added to different solutions: seawater, and three K-enriched seawater solutions prepared by adding KOH to seawater whose final [K] values were 0.1, 0.5 and 1 M, respectively. The experiments were performed at 50°C over a period of 30 days. The XRD patterns showed no mineralogical changes in residues from seawater or from the 0.1 M [K] solution. With increasing pH and K molarity, the smectite peak, initially at 1.4 nm, became broader. This change in the smectite peak was more significant in the residue from the 1 M [K] solution. The appearance of a small shoulder at 1.0 nm in the residue from a 0.5 M [K] solution showed the beginning of illite formation. However, its appearance was clearer in XRD patterns of the residue corresponding to the 1 M [K] solution. The XRD data from air-dried, glycolated, and heated samples from the 1 M [K] solution indicated the presence of smectite, disordered interstratified illite-smectite (I-S) and illite.
The TEM/AEM studies were performed on the residue corresponding to the 1 M [K] experiment. The HRTEM images revealed that smectite and illite occurred as separated packets with a ferroan lizardite, as a by-product of the smectite-to-illite reaction, interstratified and intergrown with illite. Smectite occurs both as ‘rims’ on the illite packet and in its core. The presence of smectite in the core of illite packet indicates that the lateral transition from smectite to illite was incomplete, taking place by direct replacement of smectite layers as a whole through a dissolution-precipitation mechanism. The experimental study shows that smectite may transform in a wide range of geological and artificial environments involving high-pH K-rich solutions.