Origin of Impurities Occurring in 16. These tables (§ 17, 18, 19,) show also that small percentages of alkali, iron and the alkaline earths are present in nearly all clays, even those of the highest grades. This is brought about in three ways. It may be due to a lack of drain age which permits a recomposition of recently-liberated bases in the way described above. A second explanation involves the fact that slight differences in the molecular structure of crystals, even those of the same mineral, enables the acids to break up some of them much more readily than others; consequently in any mass resulting from the decomposition of crystalline material, a considerable percentage of unaltered or but slightly changed fragments of crystals is mixed with the products of de composition, and these fragments usually represent most of the minerals originally present. (Users of high-grade clays usually remove the coarsest of these fragments, and like minerals produced in other ways, by washing.) Or, third, they may have been introduced by circulating earth-water bringing these ingredients from other sources and leaving them in this deposit. All these are real and active causes for the con tamination of clays.
From the foregoing we learn that rain-water falling on granitoid rocks sinks into them and permeates their whole structure; that through the action of the acids which it carries, and to some extent of the water itself, the silicates of which the granitoid rock is made are broken into their elements, and that these enter into new combinations, the bases uniting in part with acids carried in earth-water, and in part recom bining with silicic acid, or with silicic acid and water to form new silicates; that in those situations where the movement of earth-water is free the soluble compounds formed by these unions are carried away to be deposited elsewhere; that, in proportion as the movement of earth water is obstructed, these soluble salts enter into new combinations and are redeposited as silicates, oxides and salts of other acids ; that the more insoluble salts formed, such as the silicates, hydrosilicates and oxides of aluminum and magnesium, and to some extent of iron and other elements, are left behind as earthy masses containing fragments of undecomposed minerals, and the products of recrystallization men tioned above, mixed with the earthy matters ; that as shown by Table I (§ 8) alumina is much mote abundant in most crystalline rocks than any other base, and hence its salts enter more largely into these earthy de posits than do those of any other; that nearly pure deposits of such aluminous material cannot form unless the rocks from which they are derived are made up almost exclusively of minerals carrying as bases only alumina and such other substances as form soluble compounds during decomposition, and these last only when conditions are such as to permit free circulation of water during the decomposition; (earthy de posits of other materials would be formed under similar conditions) ; that these combinations of conditions rarely occur, and consequently high-grade aluminous deposits are not common; that most granitoid rocks contain considerable percentages of magnesia, lime, iron and many other substances not shown in the tables, whose salts, formed during rock decomposition, are more or less insoluble and so enter into the earthy residual mass, and also that conditions which interfere with free drain age are more common than those which favor it, and so the earthy masses generally contain the products of the recomposition and redepositiou of soluble salts; that these residual masses of earthy aluminous material are called clays, and that clays produced by the decomposition of rocks in situ are called residual clays.
It is also evident that the composition of residual clays will vary with that of the rocks from which they are derived, and will include nearly pure deposits of salts of alumina as found in so-called kaolin, ball, flint and fire clays, on the one hand, and the very impure brick, tile, and adobe clays, on the other. The variation in composition of such grades of clay as are used commercially may be seen by reference to the following table of analyses taken from the official publications of the United States, and those of the various states. The analyses are selected with the purpose of showing variations in composition. All are of clays that are highly esteemed for their several uses.
Percentage of Clay Substances.—§ 18. In a preceding paragraph (§ 16) the statement was made that clay contains a great variety of silicates, hydrosilicates, oxides and hydroxides of alumina, and these minerals differ widely in chemical composition and physical properties. It was further stated that the chemical composition of kaolin comes nearer a fair average of all these minerals than any other. Assuming for our present purpose that it is a true average, we may use its formula to estimate the amount of true clay matter in any clay. This formula tells us that 39.5% of pure clay is alumina, or in other words, that if we multiply the alumina as given in any complete analysis by 2.53, we will have the proportion of true clay' substance which the commercial clay contains.