Soil development on basic and ultrabasic rocks in cold environments of Russia traced by mineralogical composition and pore space characteristics

- The porosity system is a significant factor in clay mineralogy in soils from hard rocks.
- Details of the porosity system give rise to secondary mineral products in rocks.
- The zones with phyllosilicate accumulation in the rocks locate in regions with higher porosity.
- Small pores, their elongated form, and surface roughness influence the rock sensitivity to weathering.

Recent soils from basic (amphibolite and meta-gabbro amphibolite) and ultrabasic (serpentinous dunite) rocks formed in cold and humid climates of Northern Eurasia (Russia) were studied to detail the characterization of soils and rocks with special attention to the interdependence of porosity system and rock mineralogy. The study plots were located in taiga and tundra zones of East Fennoscandia and the Polar Ural Mountains. A variety of methods was used including optical microscopy, X-ray diffraction and Rietveld analysis, and three supplemental methods for the determination of pore space characteristics in rocks: (i) mercury intrusion porosimetry, (ii) a modification of this method using the intrusion of a molten alloy (Wood's metal), and (iii) scanning atomic-force microscopy. The results illustrate that the specification of the porosity system is a significant factor in tracing the clay mineralogy in soils formed from hard rocks. Ultrabasic rock is the most sensitive to weathering, as determined by (i) the high value of small pores, especially those with a radius of < 10 nm, (ii) the elongated form of the pores and surface roughness, and (iii) zones with an accumulation of phyllosilicates in regions with higher porosity causing the formation of soil enriched by clay minerals.Despite the presence of low proportions of phyllosilicates in both types of basic rocks, only soil from meta-gabbro amphibolite is enriched by clay minerals and is most probably affected by small pores (< 10 nm). The absence of phyllosilicate accumulation along the pores and the predominantly empty space inside the pores indicates the limitation of potential sources of phyllosilicates for developing soils from meta-gabbro amphibolite. Insignificant phyllosilicate accumulation in shallow soil from amphibolite, in which the fine size fractions are mostly the result of rock disintegration, is supposedly due to a particularly narrow pore size distribution with a predominance of pores between 100 and 1000 nm.