Clay illuviation


Clay illuviation is the process that has been most exhaustively considered in micromorphological studies, and that is why it is going to be dealt with thoroughly here.

The clay illuviation process represents the mechanical migration of clay from the surface horizons to the profile's deep horizons. The water from atmospheric precipitations mobilises the clay from the surface horizons and the suspensions infiltrate the soil through the macrovoids. When these suspensions reach the deep horizons where the soil is dry, the water of the suspensions is suctioned by the microvoids of the surrounding areas and fine clay pellicles are formed with their particles arranged in parallel to each other and at the same time parallel to the voids' walls. These pellicles that coat the walls of the macrovoids are called clay skins, or cutans, or clay coatings, or argillans.

Similarly, when the suspensions reach the aggregates, as the water goes towards the interior of the edaphic aggregates, it deposits clay particles on their surface, also forming illuviation argillans that cover the aggregates.

The study of soil in the microscope (micromorphology) consists, in our opinion, of the most efficient technique to recognise illuvial clay, since clay coatings show the whole of their characteristics in thin sections, and after a thorough study of their nature, morphology, localisation and fabric, it is generally possible to differentiate them from the clay cutans with another type of origin, such as those from pressure.


Recognition of illuvial clay

Illuvial clay accumulations are characterised by having their own characteristics, both with reference to their optical properties and the way in which they are distributed in the soil.

Optical properties of illuvial clay coatings

With only the polarizer. Although in themselves they are colourless, clayey masses generally have yellowish colours due to the Fe+++ they incorporate.

When the Fe content is very high their colours are reddish.

They only have greyish colours in hydromorphic soils (without incorporating Fe+++, or saturates with Fe++).

Polarizer and analyser. With crossed polarizer and analyser, the clayey masses should not show anisotropy, appearing with black or dark grey colours, since, although they are anisotropic crystals, due to their small size, the birefringence they produce is minimal. On the other hand, since the thickness of a microscopic preparation is about 30 microns, the light will pass through many particles when crossing a clayey mass and as they are randomly oriented the phase lags introduced in every crystallite will be compensated, finally cancelling the optical effects.

Due to their platy habit, the clay particles tend to be oriented in parallel. The result is that when the polarised light crosses them, the optical phenomena of every individual particle is added to those of the adjacent particles, and the oriented clayey domain behaves as if it were a single crystal, showing different optical properties: interference colour,

extinction phenomena

optical continuity, etc., that enable interpretations to be made on the environment in which the particles were distributed, as well as the way they accumulated.

The extinction pattern is a measurement of the degree of orientation:

good orientation

bad orientation


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