Argillic alteration of rocks involves conversion of certain minerals to minerals of the clay group, such as kaolinite (below right) and montmorillonite (bottom right).
Clays are hydrous aluminium phyllosilicates, typically less than 2 μm (micrometres) in diameter, and are distinguished from other small soil particles, such as silt. Clays may be residual or transported, and generally result from:
▪ the chemical weathering of aluminosilicate-bearing rocks (such as granite, containing feldspars),
▪ solution of rocks containing clayey impurities, such as limestone,
▪ disintegration and solution of shales,
▪ hydrothermal alteration.
Clays exhibit the smallest size of soil particles, flake or layered shape, affinity for water, and a tendency toward high plasticity.
In soils, argillic horizons are diagnostic clay accumulations, often designated as Bt (B horizon dominated by deposited clay, "t").
links: Micromorphology of argillic horizons / Soil Formation and Classification, What is Soil?, Soil Science Glossary, USDA gallery of soil profiles, soil facts, soil education