COMPLEXITY OF _MINERALOGICAL COMPOSITION OF CLAY.
In the section, the "geology of clays," Professor Rolfe has set forth in detail the most accepted theory of the origin of clays, the effective agencies of rock decomposition, and the manner in which these agencies operate. It has been shown clearly that the residual mass resulting from rock decomposition may be comprised of a variety of silicates, the kind and number of silicates formed being dependent upon the con ditions attending the rock decomposition. Professor Cook,' after giving the analyses of several of the New Jersey kaolins that differ widely in chemical composition, remarks: "The examples above stated prove conclusively that clays are not altogether uniform in composition, even after throwing out all the ac cidental or foreign constitutents. Either the essential kaolinite is not constant, or our clays consist of this mineral mixed in varying propor tions with other hydrous silicates of alumina. Inasmuch Os the greater number of the rich fire and ware clays of the State, and also others which have been here examined, do correspond very closely to the com position and formula assigned to this mineral, the latter explanation is more plausible." After nearly thirty years of constant research Dr. Cook's problem is no nearer solution, for Dr. Clark in Bull. 125 of the II. S. Geological Survey, suggests that there are seven possible combinations of alumina, silica and water of combination, which might form crystalized kaolin.
Professor Rolfe has shown that a pure kaolin can he formed only by the decomposition of rocks that consist almost altogether of feldspathic or other highly aluminous minerals, together with comparatively unde composed minerals like quartz and mica, that are in large part separ able from the kaolinite grains by elutriation. If the parent rock con tains iron or other metallic oxide hearing minerals the residual kaolin will be contaminated with these coloring oxides in such a manner as to render its purification by elutriation impossible. If it is impossible to determine the mineralogical constitution or makeup in the former case, where the residual deposit is largely' pure kaolin, contaminated only with substances that are separable in running water, it is obvious that in the latter case, that of the impure deposit, the problem is far more com plicated.
The difficulty of determining the mineralogical composition of a clay is increased many fold in the case of those that have been transported from the place of origin and contaminated with a heterogeneous as sortment of inorganic materials encountered en route. Shales may vary so widely in their mineralogical constitution that in one case the mass may be highly ferruginous, in another highly calcareous, and so on, depending upon the amount and kind of contaminating substance. Because the shale is highly calcareous it does not follow that it is a simple mixture of calcium carbonate and kaolin, but rather that the predomin ant adulterant is calcium carbonate. Silica, iron compounds, etc., may be and usually are present in considerable quantities in the calcareous shales. The nearest that geologists or ceramists have come to deter mining what inorganic substances are present in a given shale, is sim ply to say that it is calcareous or silicious, etc. It has not been found possible to determine the mineralogical composition of any of the com plex secondary clays either by the microscope or by chemical analysis Approximation to the mineralogical composition of the purer secondary clays like the ball clays, is made possible by "rational analysis," in which the differentiation of the minerals depends upon their relative solubility in sulphuric acid, yet by this method it is incorrectly assumed that only three mineral components are present, i. e., clay substance, feldspar, and quartz, and the results are forced to tally with this as sumption.
It is obvious, therefore, that an attempt to distinguish the minerals that occur commonly in clays would be useless in discussing the min eralogical composition of clays in general, and much more so in the ease of any particular clay.