CLAY, commonly defined as a fine-grained, almost impalpable substance, very soft, more or less coherent when dry, retentive of water and often plastic when wet ; it has an "earthy odour" when breathed upon or moistened (from 0. Eng. claeg, a word common in various forms to Teutonic languages, c f . Ger. Klei) . The chem ical composition of clay varies according to the finely divided min eral matter present ; usually the latter consists essentially of hy drous silicates of aluminium, iron, and alkalies. The constituent particles are commonly less than •oosmm. in diameter and are often so small as to be colloidal. The upper limit of size in de fining "clay" is taken by soil-analysts as -002mm. diameter, but by geologists often as much as •oimm. diameter. From the indura tion and alteration of clay are formed a number of rocks which collectively are known as "clay-rocks" or "pelitic rocks" (from Gr. inXhe, clay), e.g., mudstone, shale, slate. In nature clays are rarely free from foreign ingredients, some of which are visible to the eye, but others only by means of the microscope. The eco nomic value of many clays is dependent on their freedom from (or occasionally the presence of) certain impurities. The com monest of these are (I) organic matter, humus (exemplified by clay-soils with an admixture of peat, oil-shales, carbonaceous shales) ;
fossils (such as plants in the shales of the Lias and Coal Measures, and shells in clayey deposits of all ages) ; (3 ) carbonate of lime (rarely altogether absent but abundant in marls, which may be defined as calcareous clays, cement-stones and ar gillaceous limestones) ; (4) sulphide of iron, as pyrite or mar casite, either finely diffused and giving the clay a blackish or dark grey cclour which "weathers" brown (e.g., London clay), or in nodules and concretions (e.g., Gault, Kimmeridge clay) ; (5) ox ides of iron, which stain the clay to bluish, greenish and grey tints when in the ferrous state, and bright red, brown, or yellow when in the ferric condition, hence red and yellow ochres; (6) sand and silt, which are particles of coarser grain, frequently of detrital silica (thus forming loams, arenaceous clays, argillaceous sandstones, etc.).
The principal constituent minerals of clays are hydrated ox ides of aluminium (bauxite) and iron (laterite), hydrated silicates of aluminium (such as kaolinite), finely-divided micas, chlorites and similar micaceous minerals. Recent work on French marine clays of ages varying from Trias to Middle Tertiary has shown that the dominant mineral constituent is a hydrated silicate of aluminium magnesium, and potash, bravaisite, while kaolin is present in clays formed as residuals in fresh-water, or by pneu matolytic action.
A few of the most important clay rocks may be briefly de scribed.
Almost any argillaceous material may be used for the manufacture of bricks and tiles. The colour and texture of the resulting products depend on the impurities in the clay, and also on the conditions of burning. The best materials for brick making contain a proportion of sandy and silty im purities (up to
which prevent the clay showing too great a shrinkage on being fired. For tile making, less sandy material is desirable.
This is white, friable and earthy. It occurs as a result of decomposition of granite, porphyry and syenite, and oc cupies hollows or cavities, often of considerable depth. The crude material contains kaolinite (the pure hydrated silicate of alumin ium), scaly white mica, quartz, tourmaline and other minerals. The kaolinite is produced by the action of heated gases which have escaped from the igneous rocks, on certain of the constituent minerals. Pure clay for commercial purposes is obtained by levi gation of the crude product and repeated settlement in tanks.
These are those varieties which are refractory to heat. They are free from, or contain only small quantities of, al kalies and alkaline earths. They occur frequently in Carboniferous rocks, sometimes as under-clays of coal seams, or in association with lacustrine deposits of later age (e.g., inferior oolite of York shire). In addition to hydrated aluminium silicates, some varie ties contain free hydrated alumina, when they are known as baux itic fire-clays (e.g., Ayrshire).
This is formed by ice-action and consists usually of tough sandy clay, often unstratified, containing boulders of various sizes. The latter frequently display great variety in petrological character, are angular or rounded and sometimes marked with glacial striae.
This is a fine calcareous clay deposited under the dry steppe or desert conditions which of ten followed the glaciation of a region. The loess occupies a world-wide fringing belt, lying beyond the margin of the glacial deposits, and in the main is re garded as formed of wind-borne material derived from the des iccated glacial clays.
While, as a result of drying and induration, clays which are homogeneous throughout yield mudstones, others split readily into leaflike plates or laminae parallel to their bedding and are termed shales. This laminated structure is accentuated by the presence of films of other substances such as sand, mica or vegetable debris. Shales occur frequently throughout the geo logical column, the most notable being those of the Carboniferous and Jurassic (e.g., Kimmeridge shales). Some of them contain much organic material in thejorm of globular, yellow, apparently resinous bodies, and when distilled yield paraffin oil, wax, corn pounds of ammonia, etc. In New South Wales, Scotland, Es thonia, and parts of North America such oil-shales are worked on a commercial scale (see OIL, MINERAL).
(terra rossa).—This is a type of reddish clay found in limestone caves, which represents the insoluble and thor oughly weathered impurities left behind on solution of the cal careous matter. Similar residual clays occur on the surface or in hollows in limestone-country (see also articles CAVE and LATERITE) .
For Red Clay, Green and Blue Muds, etc., see OCEAN AND OCEANOGRAPHY. The article on SEDIMENTARY ROCKS should also be consulted. (P. G. H. B.)
