Clay minerals and the development of Quaternary soils in central Italy

Abstract

Clay mineral analysis is widely used to characterize soil parent material and to relate it to the bedrock. When it is applied to a soil profile, it can give a ‘genetic signal’ of mineralogical transformations due to soil-forming processes. In this study, ten selected soil profiles in Montagnola Senese, central Italy, were analyzed for genetic signals of soil ageing, eluviation and illuviation, fragipan formation, and other processes. In previous investigations, the parent materials of benchmark soils of the area have been dated to gain an understanding of the Quaternary geomorphological evolution, which spans a time ranging from early Pleistocene to Holocene. The number of profiles selected allowed a statistical analysis of the parameters.

Illite and kaolinite are the most quantitatively important phyllosilicates in the soils studied. The other clay minerals found are vermiculite, hydroxy-interlayered vermiculite (HIV), illite–HIV and illite– chlorite mixed layers, and chlorite. Soil ageing is characterized by a change in clay mineral composition that increases with age. Argilluviation is marked by a preferential accumulation of kaolinite in illuvial horizons, while formation of tongues in fragipans does not cause any significant difference in clay mineral composition between bleached and stained parts of the horizons. Furthermore, bulk density of fragipans is not related to clay mineral content. On the other hand, large bleached masses in more mature soils contain more kaolinite and vermiculite than the surrounding mass. The trends in the contents of clay minerals through the soil horizons confirm the two types of lithological discontinuities that were predicted during the field survey.