Lateritic or highly aluminous clays, of a highly refractory character, but low plasticity. They are usually somewhat rich in iron oxide which materially affects their plasticity. Unlike the china clays, pure lateritic clays are completely decomposed by hydrochloric acid. Bauxite and some of the highly aluminous clays of the Coal Measures appear to be of this type.
Unfortunately these different types of clay are extremely difficult to distinguish and in many instances they have become mixed with each other and with other materials during the actions to be described in the next chapter, that it is often almost impossible to decide whether the true clay in a given specimen possessed its characteristics ab initio or whether it has gained them since the time when it ceased to be a primary clay.
Secondary clays are those which have been produced by the action of the weather and other natural forces on primary clays, the changes effected being of a physical rather than a chemical nature (see [Chapter IV]).
The essential constituent of secondary clays has not been positively identified. In so far as it has been isolated it differs from the true clay in the primary clays in several important respects, and until its nature has been more fully investigated great caution must be exercised in assigning a definite name to it. For many purposes the term pelinite ([p. 149]) is convenient, being analogous to the corresponding one used for material in china clays (clayite, [p. 107]). These terms are purely provisional and must be discarded when the true mineralogical identities of the substances they represent have been established.
THE MODES OF ACCUMULATION OF CLAYS
From whatever sources clays may have been originally derived, the manner in which they have been accumulated in their present positions is a factor of great importance both in regard to their chemical and physical characters and their suitability for various purposes.
As explained in [Chapter III], the china clays or kaolins may usually be regarded as primary clays derived from granitic or other felspathic or felsitic rocks by chemical decomposition. Such clays are found near to their place of origin, are usually obtainable in a comparatively pure state and are generally deficient in plasticity. They may occur in beds of small or great depth, but these are not 'accumulations' in the ordinary meaning of that term.