CLASSIFICATION OF CLAYS.
It has seemed best to carry on the discussion of the general problems involved in the geology of clays as we have done in the preceding para-. graphs and then discuss the characteristics of each of the groups of clays and the conditions under which they were formed separately.
§ 33. No really satisfactory classification of clays has yet been pro posed, but the following grouping adapted from Orton and Wheeler seems better suited to our purpose than any classification we have seen.
34. As has already been said, kaolin results from the decomposition of complex silicates whose principal base is aluminum and whose other bases form, with carbonic acid, compounds which are soluble in earth-water. As used commercially the word does not stand for a definite mineral substance, but rather for series of silicates, hydro-sili Cates, oxides and hydroxides of aluminum, or occasionally of magnesium.
Mineralogically kaolin or kaolinite is a well defined mineral with the composition and as this approximates the average composition of the whole group the name is used commercially to designate the widely varying mixtures of its members. This will be better understood by study of the table of analyses in § 17.
The minerals most commonly entering into commercial kaolin are so far as known—Kaolinite, Al208.2SiO2.2H20; Pholerite, Al2O3.SiO2.4H2O; Halloysite, Al203.2SiO2.3H20; Cimolite, 2Al203.9SiO2.6H20; Montmoril lonite Al203,4Si02H20; Pyrophyllite, 4Al208.15SiO2.4H20; Allophane, Al2O3.SiOs; Collyrite, 2Al2Os.SiO2.9H2O; Schrotterite, 8Al2O8.3SiOs. 30H20 ; Gibbsite, Al203.3H20 ; Diaspore, Al203.1120 ; Sepiolite, 2MgO. 3SiO2.2H20; the Zeolites, quartz and undecomposed fragments of the minerals contained in the original rock. Admixtures of these minerals when pure form a white mass, pulverulent or easily made so by 'weather ing, and but little plastic until finely ground, but its lack of plasticity is probably due to cementation of its grains. Absolutely pure kaolin can only be formed by the decomposition of rocks whose minerals contain no bases which form compounds with carbonic or other earth acids that are relatively insoluble in water, but commercially pure kaolins may contain any light colored minerals which do not act as fluxes or tend to discolor the ware.
Granitoid rocks sufficiently pure to form such kaolin are occasionally found, but are rare, but when such rocks are fractured and open fissures formed, these wounds are frequently healed by the deposition of a sort of scar-tissue called vein rock, made up almost entirely of quartz, potash or soda feldspar, and light-colored mica. These veins are often sufficient
ly pure to form kaolin of excellent quality when decomposed. Ex amples of this vein rock may often be seen in the ridges projecting from the boulders which are sometimes so abundant on our prairies.
Occasionally when white argillaceous limestones decompose they leave a clayey mass sufficiently pure to be classed as a kaolin.
Our deposits of kaolin then are formed in three different ways. First, by the decomposition of highly feldspathic granitoid rocks; second, by the decomposition of vein rocks; third, by the decomposition of white argillaceous limestones.
Principally on account of their lack of plasticity kaolins are seldom used alone, but they form the basis of most white or light wares, being mixed for this purpose with plastic clays, flint, spar, and often small amounts of ground bones and other ingredients.
Ball Clays.—§ 35. Ball clays are simply plastic kaolins. They are said always to have been transported, but there seems no good reason why any deposit of kaolin should not become plastic if exposed to conditions which favor the movement of ground-water through its mass and espe cially if this movement is accompanied by considerable changes in tem perature.
A study of the table of analyses of ball clays (§ 17) shows that they are often somewhat richer in alumina than the kaolins. This would only be true of those which have been transported, because in these only would .the assorting power of water come into play, causing the coarser and heavier grains of silica to be deposited as the current loses velocity before the finer and more buoyant grains of clay. They are comparable in this respect to washed kaolins. This table also seems to indicate that ball clays are richer in minerals whose alumina content is too high for kaolin than are kaolins, but this is more apparent than real because the presence of free silica in kaolin masks that of minerals high in alumina. Ball clays are principally used to make kaolin more plastic. When used alone they shrink badly.