Kaolinite as a Carrier Mineral of Rare Earth Elements in Coal: A Geochemical and Environmental Perspective

Abstract

Kaolinite  is an Alumino-silicate mineral that occurs in coal and its ash in fine or micro-crystalline form. Its main chemical composition is Alumino-silicate (Al2Si2O5(OH)4) or Al2O3*2SiO2*2H2O, which belongs to the Kaolinite group of minerals. The presence of Kaolinite in coal directly influences the ash composition, gas productivity, and thermal behaviour of the coal. Moreover, this mineral plays an important role in evaluating the geochemical maturity, combustion quality, and environmental impact of coal-derived dust. In recent years, Kaolinite has been recognized as a potential carrier of Rare Earth Elements (REEs). This article presents a scientific analysis of the geochemical relationship between Kaolinite and REEs, their concentration in coal ash, and their environmental implications. Kaolinite in coal generally occurs as microscopic crystals filling the cracks, cell structures, and pores within the coal matrix, rather than as large visible mineral veins. Under microscopic and Scanning Electron Microscope (SEM) studies, Kaolinite appears as a white or light-coloured, extremely fine-grained mineral against the dark background of coal. It is commonly found as matrix material, coating on other minerals, or as fracture linings. In SEM imaging, Kaolinite is identified by its fine-grained, platy or flaky structure, which fills voids and micro-fractures. This morphology indicates that Kaolinite generally forms as a secondary mineral during diagenetic alteration processes, filling pores, cleats, and cracks. In some cases, it also occurs as disseminated particles intimately mixed with the organic and inorganic constituents of coal.