Chemical and structural variability of illitic phases formed from kaolinite in hydrothermal conditions

The transformations of kaolinite in the system Na2O–K2O–MgO–Al2O3–SiO2–H2O–HCl at 200 °C, with different MgCl2 contents, have been investigated by X-ray diffraction and transmission/analytical electron microscopy. Two contrasting mechanisms of illite formation were identified: 1) direct precipitation of illite from dissolution of zeolites, which occurs at high pH and low Mg contents; and 2) formation of illitic phases from recrystallized kaolinite, either through dissolution–precipitation processes or by topotactic replacement. The second mechanism occurs at higher Mg contents and almost neutral conditions. Under these later conditions, kaolinite dissolution–precipitation processes occur at the earliest stages of reaction, which lead to the formation of Mg-rich kaolinite. The illitic phases formed through both mechanisms have different structural and chemical characteristics. Direct precipitation from solution produces well-ordered illites with scarce phengitic substitution and high Na contents. Illitic phases formed from kaolinite show an evolution from illite/smectite mixed-layers to illite, at increasing run times. These phases are characterized by high octahedral Mg content (in the order of 0.8 apfu, for O10(OH)2), and lack of Na. The activity diagrams constructed from the data of solutions confirm the data obtained from the study of the solid products.