Mineralogical–Geochemical Features, Genesis, and Age of Refractory Clays in the Shulepovo Deposit (Ryazan Region, Central European Russia)

The Shulepovo refractory clay deposit was discovered in the 1970s. It consists of subhorizontal kaolinite lenses located at different depths (1–20 m) in a clay–sand sequence tentatively assigned to the Neogene or, possibly, Late Pliocene (?). Application of the X-ray diffraction method coupled with the UV-VIS-NIR spectroscopy established an almost monomineral composition of lenses, with a minimal (not more than 5%) content of quartz, as well as admixture of halloysite and mixed-layer smectite–vermiculite. Calculation of the Hinckley index (HI) for kaolinite revealed a high ordering degree, suggesting its mainly authigenic origin. The bulk chemical composition of kaolinite samples is close to that of pure kaolinite, with an insignificant (not more than 2%) Fe and Ti. Zonation in the chemical composition alteration was not detected within the lens. Studies of the host sand–clay sequence revealed that it can be assigned to the alluvial (channel and oxbow) facies. The formation model of kaolinite lenses in the Shulepovo deposit includes the following processes: major transformations of the terrigenous–clastic kaolinite in stagnant waters, its crystallization, and formation of the authigenic kaolinite due to the decomposition of the clastic aluminosilicate minerals in a chemically aggressive medium.