Clay oriented by pressureConfusion of illuvial clay with clay of other origins
Occasionally, illuvial clay accumulations can be confused with other clay masses of very different origins. In general, when the illuvial clay shows well-developed micro-features, it is easily recognisable.
Clay oriented by pressure
The most common confusion with illuvial clay is the clay coatings oriented by pressure, originating from contraction and expansion processes resulting from changes of humidity in the soil.
In general, these coatings have a weaker orientation, do not have microlaminations, they can not be observed in PPL and do not have clear boundaries.
Weathered clay
This does not often cause confusion, although it may sometimes be confused with fragmented accumulations (papules).
This problem seems to be limited to a few soils in which there are enough large mica particles (mainly biotite, which by transformation gives pseudomorphous units of oriented clay).
Inherited clay
Oriented clay may be inherited directly from rock fragments. Generally, these are clayey sedimentary materials which are strongly oriented or originate from other previously existing soils.
The main types of original materials in which this phenomenon can occur are clays, claystones, lutites, marls, slates and schists, and they may also be present in soils developed over alluvial and colluvial deposits.
Clay from neoformation
Initially, it was thought that clay coatings were due to neoformation. Nowadays, this may only be the cause of problems in some soils in which recrystalisations are formed, especially of goethite, surrounding pores and aggregates, showing an apparent orientation and birefringence similar to those of illuvial clay (they may even have laminations). However, they can be distinguished, despite their similarity, because illuvial clay establishes itself parallel to the pore walls, whereas in neoformations, the newly formed clay does so perpendicularly.