Formation of kaolinite nano- and microcrystals by weathering of phyllosilicates

Abstract

The relationship between morphology, size, and real structure of kaolinite microcrystals and the structure and composition of the primary minerals, by the weathering of which kaolinite is formed, was studied with analytical transmitted electron microscopy (ATEM), including vacuum decoration with gold, imaging in transmitted light, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS). Muscovite, biotite, and chlorite that have been weathered (replaced with kaolinite) to varying degrees were examined. It has been shown that kaolinite is formed from the solution, which arises owing to decomposition of primary minerals, and crystallizes immediately at the surface of these minerals without participation of any intermediate amorphous or crystalline phases. Kaolinite grows over primary minerals by mechanism of periodical 2D nucleation at the early stage of its formation and by helical mechanism at the late stage. The difference in morphology, dimensions, and real structure of the kaolinite replacing muscovite, biotite, and chlorite is determined by various degrees of similarity with primary minerals in parameters and structural motifs, distinct strains of kaolinite structure at the stages of epitactic and autoepitactic growth, different resistance of primary minerals to weathering and variable intensity of this process.