Transmission electron microscopy evidence for experimental illitization of smectite in K-enriched seawater solution at 50°C and basic pH

Experimental illitization of smectite was studied by TEM and 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 (S Spain). Amounts of 0.25 g were added to four different solutions: sea-water, and three K-enriched sea-water solutions prepared by adding KOH to sea-water 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 sea-water 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 with 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 and illite. The TEM/AEM studies were performed on the residue corresponding with 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 the 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.