Optical properties of illuvial clay coatings
The characteristics by which the illuvial origin of clay is recognised in thin sections are: optical continuity, preferred strong orientation, intense interference colour, existence of laminations, textural contrast with the adjacent matrix, abrupt limit with the surrounding material (in both parallel and crossed nicols), own natural colour and localised, covering the walls of macrovoids or the surfaces of aggregates. But it will not always show all these characteristics and therefore a limited number of these features can be present in every situation.
Localisation. As the illuvial clay accumulations are produced by the immobilisation of suspensions, these are locally present in soils, related to the large voids (although in dynamic horizons they may change to be incorporated in the soil mass). They are found forming more or less thick pellicles,
coating the walls of macrovoids
on mineral grains
or completely filling voids.
In general, it is not always easy to judge if a certain oriented clay domain is illuvial or not, but perhaps the microscope is the main tool in the identification of the clay transformation. This figure shows a series of microscopic fields in which the recognition of the illuvial origin of the clay is becoming more and more problematic.
Difficulties in the recognition of illuvial
clay coatings by means of the petrographic microscope are due
to two different situations.
On the one hand, there is the pellicles' own stability, which will regulate their permanence (clay coatings can be destroyed after a while, mixing with the soil mass and totally disappearing; humidity changes that lead to swellings and contractions, the precipitation of carbonates and biotic activity are the most frequent causes).
The following images reproduce an illuvial clay infilling that has been deformed by a later carbonate accumulation.
And on the other hand there may be confusion problems with other clay coatings and clayey domains with very different origins. The most common confusion problem of illuvial clay is the clay coatings oriented by pressure, originating from contraction and swelling processes due to humidity changes in the soil. In general, these coatings have less orientation, they do not have microlaminations and do not have clear boundaries (figure).
Below there are a series of examples of illuvial clay accumulations.
To summarise, the correct recognition of the
illuvial process is a very important fact and its wrong diagnosis
is the most frequent cause of mistaken interpretations in the
study of soils. Coatings with illuvial origin are, in general,
very difficult to differentiate in field studies, but easy to
differentiate, most of the time, in microscopic studies.