Chapter III: The Origins of Clays

The terms 'primary' and 'residual' are applied to those clays which are found overlying or in close association with the rocks from which they have been derived, and distinguish them from the 'secondary' or 'transported' clays which have been carried some distance away from their place of origin.

=Residual clays= may be formed by the simple removal of other materials, the clay remaining behind, as in the decomposition of some argillaceous limestones, in which the calcareous matter has been removed by solution whilst the clay is unaffected. Such a clay is not a primary one as it has probably been derived from some distant source and, having been deposited along with the limestone ooze, has formed an intimate mixture from which the limestone has, at a later geological epoch, been removed in the manner indicated. Residual clays are seldom pure, being often rich in iron compounds, though the white clays of Staffordshire and Derbyshire are highly refractory.

It is seldom necessary to distinguish residual clays from other secondary or transported ones (Chapters II and IV).

=Primary clays=, on the contrary, have been derived from rocks which have undergone chemical decomposition, one of the products being clay. The most important primary clays are the kaolins, which are derived from the decomposition of felspar, but other primary clays derived from other minerals are known, though less frequently mentioned.

The _kaolins_ are primary clays[10] formed by the decomposition of felspar and occur in many parts of the world. In Great Britain the most important are the china clays found in Devon and Cornwall, which occur in association with the granite from which they have been formed. The kaolins in Germany are, apparently, of similar origin, though some are derived from porphyry and not from granite; they are the chief material used in the manufacture of Dresden, Meissen, Berlin and other porcelains. The French kaolins from St Yrieux and Limousin are said by Granger (17) to be derived from gneiss amphibole. The American kaolins have, according to Ries (6), been chiefly formed from the weathering of pegmatite veins, but the origin of some important deposits in Texas and Indiana has not yet been fully explained.

[Footnote 10: Some kaolins in central Europe appear to have been transported and of secondary origin.]

The corresponding material used by the Chinese for the manufacture of porcelain bears a name which is really that of the place from whence it was originally obtained; the term _Kao-ling_ indicates merely a high ridge. According to Richthofen (18) the rock from which Chinese porcelain is made is not a true kaolin, but is allied to the _jades_. The term 'kaolin' is therefore a misnomer when applied to white-burning, primary clays generally, but its use has become so firmly established as to render it permanent.

Kaolins are seldom found in a sufficiently pure state to be used direct, but must be freed from large amounts of undecomposed rock, quartz, mica, etc., by a process of washing and sedimentation. When purified in this manner, the best qualities of china clay yield, on analysis, alumina, silica and water in the proportions indicated by the formula H4Al2Si2O9 together with about 5 per cent. of mica and other impurities. Some high class commercial kaolins contain over 30 per cent. of mica and 10 per cent. of quartz.

The chief constituents of rocks which take part in the production of kaolins appear to be the felspars, but the natural processes by which these felspars are decomposed are by no means perfectly understood. Some kaolins appear to have been formed by weathering and others by subaerial action. Thus Collins (19) has stated very emphatically that the kaolinization of Cornish felspar has been chiefly effected by fluorine and other substances rising from below and not by carbonic acid and water acting from above. Ries (6) and other American observers are equally convinced that certain kaolins they have examined are the result of 'weathering.' German and French investigators are divided in their opinions, and Fuchs has found that the Passau (Saxony) kaolin is derived from a special mineral, not unlike a soda-lime felspar deficient in silica, to which he has given the name 'porcelain spar.'

The _felspars_ form a class of minerals whose chief characteristic is the combination of an alkaline or alkaline-earth base with silica and alumina. Orthoclase (K2OAl2O3·6SiO2)--the chief potassium felspar--is typical of the whole class. When treated with water under suitable conditions, the felspar appears to become hydrolysed and some of the water enters into combination, the potash being removed by solution. Attempts to effect this decomposition artificially have proved abortive though several investigators appear to have effected it to a limited extent by electrolysis or by heating under great pressure (3).

The effect on felspars of waters containing carbon dioxide in solution has been studied by Forschammer, Vogt, and others, and they have concluded that kaolinization may occur with this agent though it does not appear to be the chief cause in the formation of Cornish china clays.

The probable effect of fluoric vapours has been studied by Collins (19) who confirmed von Buch's observation that fluorides (particularly lepidolite and tourmaline) are constantly associated with china clay; he found by direct experiment that felspar is decomposed by hydrofluoric acid at the ordinary temperature without the other constituents of the granite in which it occurs being affected. This theory is confirmed by the great depths of the kaolin deposits in Cornwall and in Zettlitz (Bohemia) which appear to be too great to render satisfactory any theory of simple weathering though kaolins in other localities, especially in America, appear to be largely the result of weathering. According to Hickling (36) the product of the action of hydrofluoric acid 'has not the remotest resemblance to china clay.'

Kaolin, when carefully freed from its impurities, as far as this is possible, is peculiarly resistant to the action of water. This resistance may be due to its highly complex constitution, as the simpler hydro-alumino-silicates, such as collyrite, show an acid reaction when ground with water. Rohland (5), therefore, suggests that kaolinization is effected by water first hydrolysing the felspar and forming colloidal silica and sodium or potassium hydroxides which are removed whilst the complex alumino-silicate remains in the form of kaolin. Hickling (36), on the contrary, believes that the action of the weather on felspar produces secondary muscovite--a form of mica--and that this is, later, converted into kaolinite or china clay (fig. 17, p. 105).

The various theories which have been propounded may be summarized into three main classes, and whilst it is probable that any one of them, or any one combination, may be true for a particular kaolin, yet the whole process of kaolinization is so complex and the conditions under which it has occurred appear to be so diverse that it is doubtful if any simple theory can be devised which will satisfactorily meet all cases.

(_a_) The decomposition of the granite, and particularly of the felspar within it, may be ascribed to purely chemical reactions in which the chief agents are water and carbon dioxide.

(_b_) Other substances--possibly of an organic nature and derived from the soil--may have played an important part.

(_c_) Wet steam and hot solutions of fluorine, boron or sulphur compounds may have effected the decomposition.

The recent progress made in the application of the laws of physical chemistry to geological problems is continually throwing fresh light on this interesting subject. Thus, studies of the dissociation pressures and transition points between the anhydrous and the hydrous states of various substances and the effect of water as a powerful agent of decomposition (hydrolysis) have shown that hydration is a characteristic result of decompositions occurring in the upper portions of the earth's crust and not in the lower ones, and that it is usually checked, or even reversed, when the substance is under great pressure. At great depths kaolins and other complex hydrous silicates give place to anhydrous ones such as muscovite, andalusite and staurolite. There is, therefore, good reason to believe that the kaolinization of Cornish felspar has occurred at only moderate depths from the surface and that it has been chiefly produced by the action of water containing acid gases in solution. The acid in the water may have been absorbed from the atmosphere, or it may be due to vapours rising from below through the felspathic material.

In Great Britain, china clay occurs in the form of powdery particles apparently amorphous, but containing some crystals, scattered through a mass of harder rock, the whole being known as china clay rock or 'carclazite.' The softer portions of this china clay rock are known as 'growan' and the china clay in it represents only a small proportion of the whole material.

The finer particles of clay and other materials are removed by treatment with water, whereby one-third to one-eighth of the material is separated. This small proportion is then subjected to further washing and sedimentation in order to obtain the china clay in a state of commercial purity. It will thus be understood that the Cornish china clays are not 'deposits' in the usual acceptation of that term, the soft growan from which they are obtained being almost invariably the result of decomposition _in situ_ of some species of felspar in disintegrated granite.

The commercial kaolins of France, Germany, America and China very closely resemble the Cornish china clays in composition, but when used in the manufacture of porcelain they create differences in the finished material which are clearly noticeable, though microscopical examination and chemical analysis, at present, fail to distinguish between them in the raw state on account of their great resistance to ordinary chemical and physical forces.

In addition to the breaking up of felspathic rocks with the formation of china clay or kaolin (kaolinization), other decompositions which occur may result in the formation of clays, and an examination of a considerable number of clays by J. M. van Bemmelen (26) has led him to suppose that several different clay-forming forces have been at work in the production of clays. He classifies these under four heads:

(1) _Kaolinization_, or the decomposition of felspathic and similar rocks by the action of telluric water containing active gases in solution.

(2) _Ordinary weathering_ in which the action is largely mechanical, but is accompanied by some hydrolysis owing to the impurities contained in the water which is an essential factor.

(3) _Lateritic action_--or simple decomposition by heat--which occurs chiefly under tropical conditions, but may also occur in temperate climates, and has for its main product a mixture of free silica and alumina, the latter being in the form of (amorphous) 'laterite.' It may not improbably be a result of the decomposition of the clay molecule similar to that which occurs when china clay is heated, as there is no temperature below which it can be said that china clay does not decompose into free silica and alumina (29).

(4) _Secondary reactions_ in which the products of one of the reactions previously described may undergo further changes, as the conversion of amorphous clayite into crystalline kaolinite, or amorphous laterite into crystalline hydrargillite.

Weathering.

The action of the forces conveniently classed under the term _weathering_ are of two main kinds:

(_a_) The _mechanical grinding_ of sandstone, quartzite, limestone, and other rocks, causes an addition of adventitious material to clay, the proportions being sometimes so large as to render it necessary to term the material an argillaceous sand, rather than a sandy clay. Some of these grains of mineral matter are so minute and so resistant to the ordinary chemical reagents as to make it extremely difficult to distinguish them from clay.

(_b_) The _chemical decomposition_ due to the action of very dilute solutions. By this means simple silicates are decomposed with the formation of colloidal silica which may either remain in solution or may be deposited in a coagulated form. At the same time, some alumino-silicates will be similarly decomposed into colloidal alumino-silicic acids or clays.

The ultimate results of the action of ordinary weathering on silicate rocks are, therefore, sands and clays, the latter being in some ways quite distinct in their origin and physical properties from the china clays. According to J. M. van Bemmelen (26) such clays also contain an alumino-silicate soluble in boiling hydrochloric acid followed by caustic soda, whereas pure china clays are unaffected by this treatment.

The variety of silicates and other minerals which--in a partially decomposed condition--go to form 'clays' is so great that the complete separation of the smallest particles of them from those of the true clay present has never been accomplished and our knowledge of the mineralogical constitution of many of the best known clays is far from complete.

It is highly probable that the action of water does not cease with the formation of clay, but that it slowly effects an increase in the plasticity of the clay. There thus appear to be at least three kinds of primary clay, viz.:

_Kaolinic_ or _china clays_ which are chiefly derived from felspar and can be isolated in a relatively pure state. They are highly refractory, but only slightly plastic.

_Epigenic_ or _colloidal clays_ derived from kaolinic clays, as a secondary product, or directly from felspar, mica, augite and other alumino-silicates by 'weathering.' They are usually less refractory and much more plastic than the china clays and contain a large percentage of impurities--sometimes in the form of free silica (sand) or of metallic oxides, carbonates, sulphides, sulphates, silicates, or other compounds. Many so-called secondary clays such as pipe clays, ball clays and fireclays may be of this type, though their origin is difficult to trace owing to their subsequent transportation and deposition.

_Lateritic_ or _highly aluminous clays_, of a highly refractory character, but low plasticity. They are usually somewhat rich in iron oxide which materially affects their plasticity. Unlike the china clays, pure lateritic clays are completely decomposed by hydrochloric acid. Bauxite and some of the highly aluminous clays of the Coal Measures appear to be of this type.

Unfortunately these different types of clay are extremely difficult to distinguish and in many instances they have become mixed with each other and with other materials during the actions to be described in the next chapter, that it is often almost impossible to decide whether the true clay in a given specimen possessed its characteristics _ab initio_ or whether it has gained them since the time when it ceased to be a primary clay.

=Secondary clays= are those which have been produced by the action of the weather and other natural forces on primary clays, the changes effected being of a physical rather than a chemical nature (see Chapter IV).

The essential constituent of secondary clays has not been positively identified. In so far as it has been isolated it differs from the true clay in the primary clays in several important respects, and until its nature has been more fully investigated great caution must be exercised in assigning a definite name to it. For many purposes the term _pelinite_ (p. 149) is convenient, being analogous to the corresponding one used for material in china clays (_clayite_, p. 107). These terms are purely provisional and must be discarded when the true mineralogical identities of the substances they represent have been established.