A chemical classification system for argillaceous sediments and factors affecting their composition

Abstract

Chemical analyses of clays, shales and slates from different parts of the world and from different sedimentological environments have been collected, together with analyses of the most common minerals found in these sediments. It is shown how these chemical compositions are distributed within a triangular diagram with the following molecular numbers in the corners: FeO (total iron)+MgO, Al₂O₃ and K₂O + Na₂O + CaO (does not include calcium present as carbonate). The most important factor influencing the composition of the sediments is weathering, while grain size has a relatively small effect. The influence of source rock is only noticeable as regards the most unweathered materials.

On the basis of this triangular diagram a new classification system is introduced.