In the purer clays the problem is much simpler and in their case an answer of at least approximate accuracy can be given to the question 'What is clay?'
Even with these purer clays it is not sufficient to study an analysis showing the total amount of the silica, alumina and other oxides present; it is still necessary to effect some kind of separation into the various minerals of which they are composed. When, however, the accessory minerals do not exceed 5 per cent. of the total ingredients their influence is less important and the nature and characteristics of the 'clay substance' itself can be more accurately studied. By careful treatment of well selected china clays, for example, it is possible to obtain a material corresponding to the formula Al2O32SiO22H2O within a total error of 1 per cent., the small amount of impurity being, as far as can be ascertained, composed of mica. So pure a specimen of clay is found on microscopical examination to consist of minute irregular grains of no definite form, together with a few crystals of the same composition and identifiable as the mineral 'kaolinite' ([p. 107]). This 'amorphous' material, which appears to be the chief constituent of all china clays and kaolins, has been termed clayite by Mellor ([22]).
Johnson and Blake, Aron and other observers have stated that the majority of the particles in china clays and kaolins are crystalline in form. Owing to their extreme smallness it is exceedingly difficult to prove that they are not so, though for all ordinary purposes they may be regarded as amorphous, the proportion of obviously crystalline matter present in British china clay of the highest qualities being so small as to be negligible.
Hickling ([36]), using an exceptionally powerful microscope, claims to have identified this 'amorphous' substance in china clay as 'worn and fragmental crystals of kaolinite,' and recently Mellor and Holdcroft and Rieke have shown that the apparently amorphous material shows the same endo- and exothermal reactions as crystalline kaolinite.
So far as china clays or kaolins are concerned, kaolinite or an amorphous substance of the same composition appears to be identical with the 'ideal clay' or 'true clay' whose characters have so long been sought.
This term—clayite—is very convenient when confined to china clays and kaolins, but it is scarcely legitimate to apply it, as has been suggested, to material in other clays until it has been isolated in a sufficiently pure form to enable its properties to be accurately studied. This restriction is the more necessary as in one very important respect clayite obtained from china clay and some kaolins differs noticeably from the nearest approach to it obtainable from the more plastic clays: namely, in its very low plasticity. This may be explained by the fact that it is only obtainable in a reasonably pure form in clays of a primary character, whilst the plastic clays have usually been transported over considerable areas and have been subjected to a variety of treatments which have had a marked effect on their physical character. Moreover, the fact that the purest 'clay' which can be isolated from plastic clays appears to be amorphous and to some extent colloidal greatly increases the difficulty of obtaining it in a pure state, especially as no liquid is known which will dissolve it without decomposing it. The fact that it is not an elementary substance, but a complex compound of silica, alumina and the elements of water, also increases the intricacy of the problem, for these substances occur in other combinations in a variety of other minerals which are clearly distinct from clay.
Ever since the publication of Seger's memorable papers ([7]), and to a small extent before that time, it has been generally understood that china clay or kaolin represented the true essential constituent of clays, but several investigators have been so imbued with the idea that all true clay substance must have a crystalline form that they have frequently used the term 'kaolinite' to include the 'amorphous' substance in plastic clays. This is unfortunate as it is by no means proved that the latter is identical with kaolinite, and a distinctive term would be of value in preventing confusion. Other investigators have used the word 'kaolin' with equal freeness, so that whilst it originally referred to material from a particular hill or ridge in China[13] it has now entered into general use for all clays whose composition approximates to that of china clay ([p. 16]) in which the plasticity is not well developed. Thus, in spite of the difference in origin between many German and French kaolins and the china clays of Cornwall, it is the custom in Europe generally to term all these materials 'kaolin.' Yet they are very different in many respects from the material originally imported from China.
[13] Kao-ling is Chinese for a high ridge or hill.
As the essential clay substance has not yet been isolated in a pure form from the most widely spread plastic clays, but is largely hypothetical as far as they are concerned, the author prefers the term pelinite [14] when referring to that portion of any plastic clays or mixtures of clays with other minerals which may be regarded as being the constituent to which the argillaceous portion of the material owes its chief properties. In china clay and kaolin the 'true clay' is identical with clayite—or even with kaolinite ([p. 108])—and there is great probability that this identity also holds in the case of the more plastic clays of other geological formations, but until it is established it appears wisest to distinguish the hypothetical or ideal clay common to all clays (if there is such a substance) by different terms according to the extent to which its composition and characters of the materials most closely resembling it are experimentally known.