The view that clays may be regarded as impure varieties of clayite is considered erroneous by several investigators for various reasons. For instance, felspar is rarely found in china clays, but is a common constituent of secondary (plastic) clays. J. M. van Bemmelen (26), who has found that the alumina-silica ratio of clays produced by weathering is always higher than that in clays of the china clay type produced by hypogenic action. In a number of clays examined he found that a portion was soluble in boiling hydrochloric acid whereas clayite is scarcely affected by this treatment. He also found a varying proportion of alumino-silicate insoluble in hydrochloric acid but dissolved on treatment with boiling sulphuric acid and subsequently with caustic soda solution; this latter he considers to be true clayite. Unfortunately, his results were obtained by treating the crude clay with acid, instead of first removing such non-plastic materials as can be separated by washing, so that all that they show is that some clays contain alumino-silicates of a nature distinct from clayite in addition to any clayite which may be found in them.
The fact that all clays when heated to 700 or 800° C. readily react with lime-water to form the same calcium silicates and aluminates indicates so close a resemblance between the clay substance obtainable from different sources as to constitute strong evidence of the identity of this substance with clayite or with materials so analogous to it as to be indistinguishable from it under present conditions.
In all probability, the plastic clays have been derived from a somewhat greater variety of minerals than the primary clays (p. 71) and under conditions of decomposition which differ in details, though broadly of the same nature as those producing china clays. The presence of colloidal matter suggests a more vigorous action--or even a precipitation from solution--instead of the slower reactions which result in the formation of the kaolinite crystals.
The much smaller particles present in plastic clays also indicate a more complete grinding during the transportation of the material or some form of precipitation. If, as Hickling suggests, all clays are direct products of the decomposition of _mica_, the fact that several varieties of mica are known and that the conditions under which these decompose must vary considerably, afford a good, if incomplete, explanation of some of the widely diverse characteristics observed in different clays.