Clay Minerals

a group of hydrous silicates comprising the majority of clays and determining their physicochemical, mechanical, and other properties. Clay minerals are predominantly the product of weathering of aluminosilicates and silicates of magmatic and metamorphic rocks at the surface. In the process of weathering the clay minerals undergo sequential transformations of their structure and chemical composition, depending on the changes in the physicochemical conditions in the weathering and sedimentation medium. The particle size of clay minerals in clays rarely exceeds 0.01 mm. According to their crystalline structure, clay minerals belong to the schistous or pseudoschistous silicates. The crystal lattices of typical clay minerals consist of alternating networks of silicon-oxygen tetrahedrons (silicon ions in tetracoordinate configuration) and of hydroxyl octahedrons surrounding atoms of aluminum, iron, or magnesium. In this case, divalent magnesium is present in every octahedron (trioctahedral silicates), whereas trivalent aluminum is present in two out of three octahedrons (dioctahedral silicates).

Clay minerals with two-level structures consist of tetrahedral and octahedral networks, such as the kaolinite group—for example, kaolinite, dickite, nacrite, and halloysite. Clay minerals with three-level structures consist of two outer tetrahedral networks and an inner octahedral network. Members of this category are the hydromica group, for example, hydromuscovite and glauconite (potassium atoms are located in the spaces between layers); the montmorillonite group, for example, Al-montmorillonite and Fe-montmorillonite, or nontronite (water and exchange cations are located in the spaces between layers); and the chlorite group, with alternating three-level layers and spaces between the layers (octahedral networks) in the structure. Clay minerals of more complex structures are also known.

Definite differences in the chemical composition of the clay minerals correspond to the crystallochemical differences in their structure. As a result of this property, the clay minerals differ sharply from one another. Thus, for example, montmorillonitic minerals have very high exchange capacities and adsorption properties, but these properties are weakly developed in kaolinitic materials. The clay minerals belonging to the hydromica group increase greatly in volume upon heating. Structural determinations on clay minerals are performed by infrared spectroscopy and by chemical, X-ray, electron-diffraction, electron-microscope, and thermal methods.