Clay decomposition in seasonally wet, acid soils: Micromorphological, chemical and mineralogical evidence from individual argillans

Individual argillans in thin sections from a seasonally wet, acid soil were studied by petrographic microscope, X-ray diffraction microcamera, and electron microprobe.

The data suggest that, under seasonally reducing and leaching conditions, free iron is reduced and partly leached; smectite and illite are decomposed while at least part of the aluminium, magnesium and potassium liberated is removed by leaching; silica liberated from the smectite and illite is reprecipitated as microcrystalline quartz; and the argillans are residually enriched in rutile and kandite.

Birefringent argillans are altered to isotropic, characteristically grainy cutans by this process. Strongly altered parts show a bright opalescent reflection in incident light.