SUPPOSED. HISTORY OF THE DEVELOPMENT OF PLASTICITY OF CLAYS IN NATURE.
Daubreel, Cushman' and Mellor' have disintegrated feldspar in water either by grinding or by boiling. In all cases, the liquid in which the feldspar was ground contained alkali in solution. Mellor found that not only did the solution give alkaline reactions, but the "outlines (of the solid particles) could be more readily stained with saffranine or with malachite green than before the action." Since the larger. part of the clay substance is derived from the dis integration of feldspar, it can be considered that there was formed at the time of "kaolinization" insoluble hydrous silicate of alumina, soluble potash salt and soluble silicic acid. If feldspar has been disintegrated by atmospheric agencies, water and carbonic acid, and the residual mass is so situated as not to allow the soluble salts to be washed away, they will be retained in part by adsorption and in part by recombination, forming zeolitic masses. Data are not available to warrant the state ment that plastic kaolins formed in situ owe their plasticity to these ad sorbed salts, or that they even contain adsorbed salts. We do know, however, that nearly all kaolins contain alkalies that can not be shown to be present as constituents of such minerals as feldspar or mica. Fur ther we know that "the less' free alkali a clay contains the more will it adsorb." We know also that clays which have been formed from feldspar under the disintegrating influence of fluorine are not plastic, and con tain and other fluorine compounds.
Cushman' reports that the residue left after disintegrating the feldspar and washing out the portions which have been rendered soluble, is com posed of very minute particles. If this insoluble portion, kaolin, had been formed by nature, in whose laboratory reactions, precipitations, etc., extend over an almost infinitely longer time than is given to similar physical-chemical phenomena in the laboratory, there is no doubt but that these minute particles of kaolin would arrange themselves in the thin plate-like clusters that are characteristic of this substance, just as did the Leasbuig clay cited by Wheeler. Conditions will control the
size of the plate-like crystals so developed. In many kaolins these plate forms are discernable, ranging from those of sub-microscopic dimensions up to those that can be readily measured in the microscope. In a clay examined by the writer not only were there crystals of measureable size, but they appeared to be compounded, i. e., made up of several crystals, which could not be separated to an appreciable extent by vigorous shak ing in distilled water. Under natural conditions, therefore, where the disintegrating water readily runs or seeps away, carrying the soluble por tions and leaving the insoluble "residual clay" in situ, we can expect to find a deposit that is more or less crystalline, depending upon the attend ing conditions.
These deposits, we know, are practically non-plastic. We know fur. they from Ackison's experiments and the testimony of many agricultural chemists, that these grains can be deflocculated by organic solutions. Since surface waters generally contain organic substances in solution, and since proximity of vegetable growth can give rise to a deposit of decaying vegetable matter on kaolin beds, it is easy to see how such a de flocculation can take place in situ, and especially so if the clay be moved by running water and deposited in the lower lands. By virtue of this deflocculation the clay has a smoother feel, i. e., texture, and thereby assumes a pseudo-plasticity. This fact has given rise to the fineness of grain theory of plasticity.
These deflocculated particles of kaolin have, as has been shown, a high adsorptive power. Whatever salts may be in solution in the passing waters, or may be carried upward from lower strata by rising waters, will be adsorbed by the kaolin particles. Now, depending upon the de gree of deflocculation, amount of adsorption, and the kind of salt so ad sorbed, plasticity will be developed.