Kaolinite‒Smectite from Deposits on Holocene Terraces of the Black Sea Coast of the Caucasus (Northwest of the Colchis Lowland)

Holocene terrace systems and isolated terraces have been identified on the Black Sea coast of the Caucasus, including the northwestern part of the Colchis Lowland. Their composition was studied using a combination of lithological and physicochemical methods. It has been shown that the fine fraction (<0.001 mm) of sediments consists predominantly of newly formed mixed-layer kaolinite‒smectite with a terrigenous admixtures of smectite, kaolinite, micaceous mineral, and occasionally chlorite. Using diffraction pattern modeling for air-dry and ethylene glycol-saturated natural samples, the structural parameters of clay minerals and their quantitative content were determined. No correlation was found between these parameters and the facies settings in which the terrace deposits formed. No correlation was also found between the structural parameters of kaolinite‒smectite and the depth of sediment occurrence or their location within the study area. All samples contain kaolinite‒smectite with a fairly narrow range of the ratio of kaolinite and smectite layers, Wk : Wsm, on average from 0.55 : 0.45 to 0.65 : 0.35. It is shown that the average crystallochemical formula calculated for kaolinite‒smectite with Wk : Wsm = 0.65 : 0.35 exactly corresponds to the dioctahedral structure of the mineral. The decomposition of this formula into smectite and kaolinite components indicates that the 1 : 1 layers in the studied kaolinite‒smectites differ in their crystallochemical structure from true kaolinite ones. A solid-phase mechanism for the formation of kaolinite‒smectites in a smectite matrix is proposed via the removal of fragments of tetrahedral sheets from 2 : 1 layers and adjacent smectite interlayers. The structural evolution of mixed-layer kaolinite-smectites is discussed. The authigenic origin of mixed-layer kaolinite‒smectite minerals in the studied sediments is substantiated.