CHARACTERS OF CLAY AND SHALE
The question of what is a true Clay has been much discussed, especially by agriculturists, in recent years[939]. The material, as a rock, is regarded as a massive kaolin, and, if pure, should have the following percentage composition:—silica 46·3, alumina 39·8, water 13·9. Some Pipe-clays, white and uncontaminated, closely approximate to this ideal. True clays are very plastic when moistened, and shrink on drying, forming a compact mass the particles of which do not fall apart. When thoroughly dried, however, and placed in water, lumps of clay break up readily; the water creeps in along their capillary passages and expels trains of air-bubbles as it goes. This fact has been utilised in the extraction of fossils from a matrix of stiff clay. If the clay thus reduced to powder is now "puddled" by the finger, it again forms a closely adherent plastic mass.
The individual spaces between adjacent particles in a clay are very minute, and this accounts for its practical impermeability to water; but the total pore-space or "porosity" may amount to more than fifty per cent. of the volume of the rock. Unless earth-pressures have brought the mass into the condition of shale or slate, the tiny flaky kaolin particles, and the associated very small grains of other minerals, have not shaken themselves down into a closely aggregated state. When moistened, however, and again dried, the surface-tension of the film of water about any group of grains, increasing as evaporation thins the film, draws the grains nearer to one another, and a considerable shrinkage of the mass results. Alternate wetting and drying tends to make a clay less obdurate and sticky, by increasing the number of separate aggregates of grains. The passages between these aggregates are no longer so minutely capillary, and a clay soil becomes by this process distinctly "lighter" from the farming point of view.
The larger cracks caused by shrinkage greatly increase the evaporation of water, by exposing new surfaces, which penetrate deeply into the clay. Often the mass shrinks so as to develop hexagonal structure, from the drying surface downwards [Fig. 8]).
The natural "flocculation" of clays, the process by which compound grains are formed in place of individual soil-particles, is assisted by the action of water bearing certain salts in solution. Calcium carbonate is an excellent flocculator, and this fact has long led farmers to place burnt lime or powdered limestone on their lands. Sodium carbonate, on the other hand, is brought up in some dry regions by capillary action, and exercises a reverse effect, keeping the minute particles apart from one another, and thus promoting thorough clayiness in the clay.
Experiment has shown that fineness of grain is responsible for most of the characters of a clay, and from this point of view the small size of kaolin flakes as compared with grains of other minerals will account for the "clayiness" of this particular mineral when it constitutes a rock. Clays, however, when shaken up in a column of distilled water, cause what seems to be a perpetual cloudiness, since it remains after the great bulk of the clay has settled down. Flocculation by salts alone removes it. Some authors have urged that a colloid substance, amounting perhaps to only one or two per cent. of the whole clay, imparts this distinctive character. Such colloids are believed to arise during the decomposition of aluminous silicates under tropical and probably alkaline influences; but they are not known to be associated with the processes by which kaolin is formed from felspars.
Fig. 8. Shrinkage-cracks in Clay, with footprints of birds in the foreground. Tundra of Mimer Bay, Spitsbergen.
A. D. Hall[40] points out that the cloudiness is probably due to the extreme minuteness of certain of the particles. True clayiness thus depends on the proportion of grains smaller than ·002 mm. in diameter. Yet Hall and Russell look to other causes to explain the continued suspension of such particles in the water, and they suggest the presence of potassium and sodium silicates of the zeolite group, which liberate by hydrolysis a little alkali in contact with a large bulk of water. Free alkalies prevent flocculation, and so encourage suspension of the particles.
To the ordinary observer, a rock possesses the properties of clay, and is a clay, if it contains more than forty per cent. of particles less than ·01 mm. in diameter. But such rocks are found, on chemical analysis, to contain a large amount of kaolin, and the old view, that clays are massive kaolins, is thus substantially correct.
None the less, clays are notably impure, and in many there is a large admixture of quartz sand. The kaolin, derived originally from the decay of other silicates, is rarely freed from a variety of minerals and rock-fragments that were associated with it in its place of origin. Grains of quartz and unaltered felspar a tenth of a millimetre in diameter distinctly "lighten" a clay soil, on account of their relative coarseness. A sandy clay is styled a Loam, and a fine-grained loam furnishes the ideal soil for the general purposes of a farmer. It does not retain water too long upon its surface, nor does it dry too quickly after rain. Much of what we call boulder-clay proves to be in reality a loam.
T. Mellard Reade and P. Holland[41] have shown that even in clays of marine origin there may be a considerable proportion of very fine quartz sand.
Calcium carbonate, usually occurring as fine rock-dust derived from limestone, or as minute shell-fragments, may be mingled with clay to form a Marl. The term is not a quantitative one, and may be applied to any clay that shows a brisk effervescence with cold acids. Though unpleasantly sticky when wet, marls flocculate themselves naturally by supplying calcium carbonate in solution to waters that pass through their crevices (see [p. 80]).
The stratification of clays may be invisible throughout considerable masses, unless sandy beds are intercalated among them. Yet, when a lump of clay is dried and then placed in water, as previously described, it will often break up along parallel planes, which show that there is a regular arrangement of its particles. The fact that so many of these particles are platy becomes emphasised under the pressure of subsequent sediments, whereby the platy surfaces of the particles are brought into planes parallel with one another. The clay then becomes a Shale, with regular planes of fissility, which are parallel to those of bedding. A certain amount of deformation of the rock accompanies this change, flow being set up parallel with the bedding, and included fossils becoming sometimes flattened. This deformation is especially noticeable in the case of plant-remains. Shales may in time attain the density and fissile structure of true slate.
The colours of clays and shales are of considerable interest. Blackness is often due to organic matter, and especially to fragments of plants, which retain their woody structure and their carbonaceous character when protected by clay from oxidation.
The bluish tint of clays is due to finely divided iron pyrites (iron disulphide), which may occasionally appear as distinct crystals or nodules of one or other of its forms, pyrite or marcasite. On oxidation, limonite arises, which colours the mass brown, as is seen in the upper part of many clay-pits. The occurrence of iron pyrites often dates back to the time at which the clay accumulated. N. Andrussow[42] points out that in the Black Sea there is an enormous supply of decaying organic matter provided by the floating organisms of the upper layers. This rains continually down towards the floor. The portion that reaches depths of over 100 fathoms escapes from the voracity of free-swimming organisms and arrives at the region where bacteria alone abound. These bacteria act on dissolved sulphates, and also largely, according to Andrussow, on the albumen of the decaying matter. In both cases, sulphuretted hydrogen is produced. Andrussow treats the reduction of the marine sulphates as a minor process, due to the need that the bacteria have for oxygen in the deep waters, which are insufficiently supplied. The sulphuretted hydrogen attacks the salts of iron, and iron disulphide results.
Here we have an excellent illustration of how, in deep basins, with imperfect vertical circulation, black pyritous muds may arise, devoid of ordinary fossils. The depths of the Black Sea are practically poisoned by the abundance of sulphuretted hydrogen. But numerous cases of shales are known to us where iron pyrites replaces the shells of ammonites or forms complete casts of bivalves, and has accumulated also in concretions and crystalline groups. Such pyrites is probably of secondary origin, or arose from the reducing action of decaying organic matter on ferrous sulphate in solution in the sea.
The oxidation of iron pyrites in shales gives rise to aluminium sulphates, such as alums. Sometimes sufficient heat is evolved during this oxidation to set on fire carbonaceous matter present in the rock.
Pink-purple and green are common colours among shales, and imply that the iron is in two different states of oxidation. When the colour varies thus in successive bands, we may believe that a climatic change promoted the formation of ferric salts on the land surface when the pink layers were being formed, while ferrous (less oxidised) salts predominated when the green particles were washed into the basin. B. Smith[43] suggests that the organic matter and humic acids which are swept down in times of flood may temporarily prevent oxidation from occurring in shallow lakes and pools. Dry seasons would thus lead to the deposition of pink clays, while wet seasons would furnish green ones. The green colour in shales is mostly due to chlorite or to glauconite.
Subsequent deoxidation has been invoked to account for the green colour of certain shales. Organic matter may have been responsible, and the green spots in purple slates have been attributed to the decay of entombed organisms, the reaction having spread outwards from a centre.
Clays, owing to their impermeability, preserve fossils excellently, and the oldest shells and corals in which the original aragonite has escaped conversion into calcite occur in clays and shales of Mesozoic age (see [p. 22]).