Transformation process from phlogopite to vermiculite under hydrothermal conditions

The phlogopite from the Qeganbulak vermiculite deposit in Yuli County, Xinjiang is selected as raw material and then transformes into phlogopite-vermiculite mixed-layer mineral and vermiculite through hydrothermal synthesis in the laboratory conditions. Furthermore, the influence of experimental conditions, such as temperature, K+ concentration, pH value, and duration of hydrothermal reaction on the transformation product is investigated. Modern analysis and determination methods, such as X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), inductively coupled plasma emission spectroscopy (ICP), and transmission electron microscopy(TEM), are used to analyze the phase, chemical component, crystal structure, and microstructure of the product before, during, and after transformation. Results indicate that phlogopite can be transformed into vermiculite in MgCl2 or NaCl solutions under acidic hydrothermal conditions; in addition, the transformation rate increases with temperature. To promote transformation, the solution is changed during transformation to remove K+, accoding to change the chemical equilibrium in the conversion reaction so that more K+ can be exchanged. The transformation effect under weak acid conditions is higher than that under neutral or alkaline conditions. During transformation, K+ is leached from the interlayer, whereas phlogopite transformes into a phlogopite-vermiculite mixed layer (i.e. the phlogopite layers is the most), and then into a vermiculitephlogopite mixed layer (i.e. the vermiculite layers is the most), and finally completely into vermiculite. These results are valuable for studying the crystal structure and genesis of the layer silicate mineral vermiculite.