Mineralogy and pore space characteristics of traprocks from Central Siberia, Russia: Prerequisite of weathering trends and soil formation

Pore space issues and mineral paragenesis of traprocks from the central part of the basaltic province (Central Siberia, Russia) were studied, as was the fine size fraction (< 1 μm) of well-drained soils from two groups — “shallow with hard rock” and “deeper and mature with saprolite.” The explanation of coexisting of these two groups was given via rocks` characteristics. The methods used included mercury intrusion porosimetry (MIP), impregnation of connective pores with a molten alloy (Wood`s metal) combined with subsequent electron microscopy, optical microscopy, X-ray diffraction, and IR spectroscopy. Rock from the lithic contact is represented by slightly weathered dolerite and shows a tendency to bimodal pore size distribution with a second maximum in pore size at 10 nm, which is due to the occurrence of phyllosilicates. Coarse pore systems are mainly due to cracks, which allow an easy exchange of water. Higher total porosity in the rock in one of two pits (~ 12 vol.% compared with 4 vol.%) appears to be a prerequisite for the formation of a deeper and more mature profile. Secondary products of dolerite weathering are smectites determining the association of clay minerals in both soil profiles and Fe-oxides. Most likely, desegregation and weathering of the rock fragments from soil horizons are fast enough to keep sufficient amounts of smectite only in the fine size fraction of the shallow profile. In the mature profile, pedogenesis leads to acidification of upper soil horizons and to a pronounced decrease in the smectite proportion, not only in the soil but also in the rock fragments from soil horizons. Thus, smectite(s) that was stable in early stages of rock weathering and soil formation and became unstable due to soil acidification illustrates the metastable nature of clay mineralogy in the well-drained soils from basic rock.