(4) Secondary reactions in which the products of one of the reactions previously described may undergo further changes, as the conversion of amorphous clayite into crystalline kaolinite, or amorphous laterite into crystalline hydrargillite.
Weathering.
The action of the forces conveniently classed under the term weathering are of two main kinds:
(a) The mechanical grinding of sandstone, quartzite, limestone, and other rocks, causes an addition of adventitious material to clay, the proportions being sometimes so large as to render it necessary to term the material an argillaceous sand, rather than a sandy clay. Some of these grains of mineral matter are so minute and so resistant to the ordinary chemical reagents as to make it extremely difficult to distinguish them from clay.
(b) The chemical decomposition due to the action of very dilute solutions. By this means simple silicates are decomposed with the formation of colloidal silica which may either remain in solution or may be deposited in a coagulated form. At the same time, some alumino-silicates will be similarly decomposed into colloidal alumino-silicic acids or clays.
The ultimate results of the action of ordinary weathering on silicate rocks are, therefore, sands and clays, the latter being in some ways quite distinct in their origin and physical properties from the china clays. According to J. M. van Bemmelen ([26]) such clays also contain an alumino-silicate soluble in boiling hydrochloric acid followed by caustic soda, whereas pure china clays are unaffected by this treatment.
The variety of silicates and other minerals which—in a partially decomposed condition—go to form 'clays' is so great that the complete separation of the smallest particles of them from those of the true clay present has never been accomplished and our knowledge of the mineralogical constitution of many of the best known clays is far from complete.
It is highly probable that the action of water does not cease with the formation of clay, but that it slowly effects an increase in the plasticity of the clay. There thus appear to be at least three kinds of primary clay, viz.:
Kaolinic or china clays which are chiefly derived from felspar and can be isolated in a relatively pure state. They are highly refractory, but only slightly plastic.
Epigenic or colloidal clays derived from kaolinic clays, as a secondary product, or directly from felspar, mica, augite and other alumino-silicates by 'weathering.' They are usually less refractory and much more plastic than the china clays and contain a large percentage of impurities—sometimes in the form of free silica (sand) or of metallic oxides, carbonates, sulphides, sulphates, silicates, or other compounds. Many so-called secondary clays such as pipe clays, ball clays and fireclays may be of this type, though their origin is difficult to trace owing to their subsequent transportation and deposition.