The Natural History of Clay

by Alfred B. Searle

which on removal of the hydroxyl groups might be expected to give the anhydride

though in practice this substance—if formed at all—appears to be instantly split up into Al₂O₃ and SiO₂.

By regarding the aluminium as a nucleus, as above, and some aluminium silicates as hypothetical alumino-silicic acids, as suggested by Ulffers, Scharizer, Morozewicz ([29]) and others, clay substance may be conveniently considered, along with analogous substances, as forming a special group quite distinct from the ordinary silicates. In this way Mellor and Holdcroft ([29]) consider that clay substance is not a hydrated aluminium silicate—as is usually stated in the text-books—but an alumino-silicic acid, the salts of which are the zeolites and related compounds. From this hypothesis it naturally follows that clay substance is analogous to colloidal silica which has been formed by the decomposition of a silicate by means of water and an acid.

If this view be correct, pure clay substance or true clay is a tetra-basic alumino-silicic acid H₄Al₂SiO₉ or Al₂Si₂O₅(0H₄). That its acid properties are not readily recognizable at ordinary temperatures is due to its inertness; at higher temperatures its power of combination with lime, soda potash and other bases is well recognized, though the reactions which occur are often complicated by decompositions and molecular re-arrangements which occur in consequence of the elevated temperature.

There are a number of minerals which closely resemble clayite or pure clay substance in composition, the chief difference being in the proportion of water they evolve on being heated. Thus Rectorite H₂Al₂Si₂O₈, Kaolinite H₄Al₂Si₂O₉, Halloysite H₆Al₂Si₂O₁₀ and Newtonite H₁₀Al₂Si₂O₁₂. In the crystalline form these minerals may be distinguished from each other by means of the microscope, but as the chief materials of which clays are composed appears to be amorphous it is impossible to ascertain with certainty whether a given specimen of clay substance is composed of a mixture of these analogous minerals in an amorphous form or whether it consists entirely of clayite, i.e. the clay substance obtained from china clay. As already stated, the thermal reactions which occur on heating clayite appear to be characteristic of kaolinite whilst halloysite is completely decomposed at a temperature somewhat below 200° C.; but the not improbable presence of two or more of these alumino-silicic acids in clays of secondary or multary origin makes it almost impossible to determine whether clayite is an essential constituent of all clays or whether the purest clay substance (pelinite) obtained from some of the more plastic clays does not possess a different chemical composition as well as different physical properties.

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.