Comparative analysis of clay mineral evolution under the conditions of humid and arid lithogenesis

Humid lithogenesis is characterized by the accumulation and postsedimentary transformation of eluvial products within terrigenous and terrigenous-carbonate (with calcite) formations containing mainly dioctahedral clay minerals. Platform fresh-water sediments of early-catagenesis zone accumulate 1 : 1 and 2 : 1 varieties, with a predominance of expanding layers in the latter. In sand-silty rocks authigenic kaolinite also appears. Trioctahedral chlorite of generation I is developed from the top of this zone in both clayey and sand-silty marine deposits. Also, mixed-layer minerals are synthesized: montmorillonite-glauconite at the top of the zone and montmorillonite-hydromica at the bottom.Arid lithogenesis is controlled by the accumulation of sediments of terrigenous-carbonate (with dolomite), carbonate, and halogen formations. In contrast to the terrigenous and aclimatic volcanogenic formations, these ones are dominated by trioctahedral clay minerals: chlorite, sepiolite, and mixed-layer chlorite-based minerals. In early-catagenesis zone, sepiolite, palygorskite (di-analog of sepiolite), and halloysite quickly transform into talc- and chlorite-saponite and kaolinite, respectively. In hypergenesis zone climate is the main factor, whereas in early-catagenesis zone, especially in arid environments, the chemical composition of the medium plays a significant role. In late-catagenesis zone, degradation products of mica and montmorillonite transform first into varieties with less than 40% and somewhat more than 40% expanding layers, respectively, whereas trioctahedral minerals preserve up to 50% such layers. This process is accompanied by the recrystallization of both allothigenic and authigenic minerals with the transition of scaly montmorillonite into bladed hydromica, with the authigenic minerals of halogen-formation sediments characterized by the most perfect crystal habits and regular structures. In metagenesis zone, di-minerals of orogenic-area sediments transform into sericite, and trioctahedral minerals, into more magnesian chlorite of generation II. This indicates that in late-catagenesis and metagenesis zones, the PT-conditions of the environment are of crucial importance.