It has been recognized for many years that many aspects of clay technology including soil treatment and drilling mud treatment must remain in an essentially empirical state until a basis for the understanding of ion exchange reactions is established.

Much of the work on ion exchange reactions of clays in the past has been directed toward establishing total exchange capacities or determining the ionic distribution empirically. This information in general is not suitable for the evaluation of hypotheses designed to provide a basis for understanding the exchange reaction. When the techniques for characterizing the various clay minerals offered the possibility of quantitative study, the solution and exchanger phase contributions to the ionic distribution could be experimentally evaluated in principle. The particular experimental techniques which have been used to measure ionic distribution, however, frequently neglected observations which are essential if the data are to he used for testing and developing theories of ion exchange. It is now well recognized that molecular adsorption, complex ion formation in solution, and ion-pair formation between a mobile solution ion and a fixed exchanger group may occur in addition to the ion exchange reaction. Therefore, if the data are to be useful to develop theories of ion exchange, the whole system must be selected to minimize such extraneous contributions. On the basis of recent theoretical work, various experimental techniques art; evaluated from the point of view of their suitability for equilibrium ion distribution studies.

The mass action, adsorption isotherm, and Gibbs-Donnan equilibrium formulations of the ion exchange theory are discussed as they may apply to clay systems. Recent progress is summarized in (1) solution thermodynamics of mixed electrolytes as it is relevant to ion exchange processes of clays, (2) the contributions of non-ideality of the clay exchanger phase, and (S) the work of swelling of clays which affects the ionic distributions in ion exchange reactions.

It is concluded that the parameters which relate to the solid phase of the exchanger and those which relate to the solution are now sufficiently well recognized that future experiments can be planned which may more realistically provide an experimental basis for understanding the process of equilibrium ion exchange distributions in aqueous clay-electrolyte systems.

As a means of distinguishing between the broad groups of flay minerals, chemical analysis may be a valuable supplement to other methods, but rarely will it remove the necessity for other methods, X-ray analysis in particular. The two- and three-layer clay minerals may be distinguished by the SiO2 · Al2O3 ratio or SiO2 · sesquioxide ratio, but these ratios do not provide criteria for dividing either class. Among the three-layer clays, which embrace montmorillonite, nontronite, and the diverse hydrous micas, non-exchangeable K is a better criterion. It seems probable that montmorillonite, as found in bentonite deposits, does not contain significant amounts of non-exchangeable K, whereas all known hydrous micas contain significant, although widely variable, amounts of non-exchangeable K.

The greatest value of chemical analysis inheres in the fact that by means of calculation the specific nature of the isomorphism may be determined. But the degree of confidence that can justifiably be placed in the calculated isomorphism depends on the purity of the sample analyzed. SiO2 as impurity may be shown by an excess of calculated Si++++ ions, or by a deficiency of octahedral cations. It also affects the calculated isomorphism of the tetrahedral layers of the lattice by reducing its Al content.

On the other hand, Al2O3 as impurity affects the calculation in the opposite way; that is, it reduces the number of Si++++ ions per unit cell, while increasing both tetrahedral and octahedral Al+++ ions. Fe as impurity has essentially the same effect as Al.

In some samples it is possible to detect SiO2 as impurity by means of calculation.

Particles of montmorillonite representing a considerable range of sizes wow separated from crude hentonites by means of sedimentation and supercentrifugation. Except for the effect of impurities, it was found that base-exchange capacity was independent of particle size.

The ratios of the several exchangeable cations on the niontmorillonites vary widely as they occur in the crude bentonites. Sodium is the dominant cation in the Clay Spur, Wyominj, and Belle Fourche, South Dakota, samples; calcium in those from Polkville, Mississippi, Santa Rita, New Mexico, Chambers, Arizona, and Plymouth, Utah; in the Little Rock, Arkansas, sample, hydrogen is the most abundant exchangeable cation. The hydrogen ion is reflected in the low pH of the sample. The Otay, California, and Merritt, British Columbia, samples contain more nearly equal amounts of exchangeable calcium, magnesium, and sodium.

Formulae calculations indicate that these samples present a rather wide range in isomorphism and, in consequence, the source of the negative charge on the lattice is widely variable. However, the calculated charges depend, to a large extent, on the purity of the sample. Evidence is presented that silicon dioxide in some form is the most abundant impurity and that significantly large amounts of uncombined silicon dioxide may be present in the finest fractions. The Merritt, British Columbia, sample was found to contain about 35 percent cristobalite. The corresponding particle size fraction from Amory, Mississippi, bentonite contains considerable hydrous mica. Unless allowance is made for impurities, the calculated lattice positions of the electro-positive atoms, and therefore the formula derived therefrom, are essentially meaningless.

Some investigntors have used petrographic methods as the sole means of making clay mineral analyses, whereas others have considered petrographie metliods to be useless for materials as fine grained as clays. The value of the method lies between these extremes. It has consideralde value, but it has limitations and must be used with cantion.

By the use of varions auxiliary techniques it is usually possible to obtain fairly precise values for the indices of refraction and birefringence of the clay mineral components of a clay material. Frequently an interference figure can be obtained which yields farther data on the optical characteristics. The presence of very fine non-clay minerals, such as quartz and carbonate, and organic material and iron oxide or hydroxide may prevent the making of satisfactory optical measurements.

It is necessary to decide if the optical data obtained arc influenced by contamination with nonclay minerals, and if they indicate a single clay mineral or mixture of clay minerals. Fortunately the optical values themselves and the character of the particles on which they were determined give good clues regarding purity and monomineral characteristics.

Optical data in general will not permit the detection of relatively small amounts of a clay mineral in a clay mineral mixture. Also such data may l)e very misleading in studies of mixed-layer clay minerals.

In addition to optical data, petrographic studies of clay materials yield other important information. They are of value in determining the texture of the rock and the relative abundance and size of the nonclay minerals. In general microscopic studies are easy to perform and the probable value from them is so great, that they should be made in all clay material researches.

Dyestuffs and other reagents which exhibit characteristic colors when adsorbed on clay granules frequently have been employed as aids in clay identification. The colors generally are believed to result from pleochroic effects and acid-base or oxidation-reduction reaction mechanisms. Certain aniline dyes, solutions of which vary in color according to their hydrogen ion content, may be used to indicate the relative acidity of clay.surfaces as well as to heighten the natural pleochroism of kaolin minerals. When n clay sample has been pretreated with acid, the colors assumed by the adsorbed dye depend on the characteristic base exchange capacities of the various clay mineral groups present.

Aromatic diamines, amino phenols, and other compounds which can be oxidized to colored semiquinone cations permit particularly sensitive and positive identification of members of the montmorillonoid family and certain other three-layer clay minerals. Theories concerning the nature of this apparently catalytic oxidation on clay surfaces are inadequate to explain the reactions. New concepts and additional experimental information are needed.

Since X-ray diffraction patterns are directly related to crystal structures. X-ray identification is, in principal, better suited to the recognition of structural groups and structural varieties than of chemical species.

Well-formed kaolin, mica, and chlorite structures give rise to characteristic 7, 10 and 14Å spacings which are relatively easily identified. Hydrated forms, such as hydrated halloysite (d = 14Å) and montmorillonoids under normal conditions (d= 14Å), are recognized either by low-temperature dehydration giving characteristically diminished spacings, or by the formation of organic complexes giving characteristically increased spacings. This introduces at once the principle that X-ray identification may, and generally does, entail the study of mineral modification by suitable chemical and/or thermal treatment.

The main requirements in the X-ray technique are: (1) Ability to record long spacings up to 2.5Å or even higher values; (2) well focused lines with good resolution; (o) absence of background scattering and white radiation anomalies. Of these, (1) and C) lie within the control of the investigator, but (2) depends partly on the material.

The main difficulties of X-ray identification are: (1) The multiplicity of lines when many components are present, and (2) poorly defined diagrams arising from poor crystallinity and/or small size of crystals. The first can be treated experimentally, but the second is inherent in the problem. Simidification of X-ray diagrams is possible by sedimentation, acid treatment and thermal treatment and by the use of orientated.specimens. Poor quality powder diagrams (if not due to poor technique) are of interest in themselves and may still give valuable information concerning the clay minerals, more particularly when monomineralic or mainly monomineralic fractions are used. Line profiles give valuable indications of randomly displaced layers and of interstratified sequences.

The recognition of chemical species by X-rays is difficult but some progress can be made if pure or nearly pure specimens are obtainable. The recognition of di- and tri-octahedral minerals by the (060) spacing is well known but the recognition of micas and chlorites in any greater detail is difficult. A consideration of basal spacings and basal intensities may make more specific identification possible.

Electron microscope studies show that most of the clay minerals have morphological characteristics which can be effectively used to aid in their identification. In addition, detailed studies at high magnification have provided structural information not obtainable by other means.

In the kaolinite group each of the common minerals has a clear-cut and diagnostic morphology. However, much more work is needed on flint clays and the kaolins in soils before the same can be said of these varieties. Continued study of halloysite reveals significant details pertinent to the form and structure of the tubular crystals.

Illites from different sources show interesting differences in form which reflect variation in the degree of crystallinity. An illite from the Ordovician Oswego formation shows an unusual development of narrow to broad lath-shaped crystals. Except for lower content of iron and magnesium the material has all the characteristics of other illites. The thinnest laths are of the same order of thickness as the flakes of more common illites. Certain mixed-layer minerals are identical in morphology with some varieties of illite.

The minerals of the moutmorillonite group have been the most difficult of the clay minerals to characterize on the basis of morphology. The lath-shaped crystals of certain beidellites, sauconite, nontronite, and hectorite are diagnostic. The metal shadowing technique shows that many of the laths approach one unit cell in thickness.

The morphology of many montmorillonites is dependent upon the mode of sample preparation. The replica method offers a means of studying clay particles in their natural state but this technique is still in the early stages of development as it relates to the clay minerals.

Differential thermal analysis (DTA) began soon after the development of the thermocouple. It has progressed through the systematic development of better equipment and the cataloguing of typical DTA curves for a variety of materials until good technique now requires control of the composition and pressure of the furnace atmosphere as well as consideration of the thermodynamics and kinetics of the reactions involved. Although differential thermal analyses have been made for many materials, the major applications have been concerned with clay and carbonate minerals.

In DTA curves for clay minerals the low-temperature endothermic loop associated with the loss of water, and the high-temperature exothermic loop accompanying the formation of new compounds, are changed in shape, temperature, and intensity by the kind of exchange cations. The midtemperature-range endothermic loop has a temperature dependence on the partial pressure of water in the furnace atmosphere.

For the anhydrous normal carbonates the dissociation temperature and its dependence on the partial pressvire of CO2 are in the decreasing order Ca, Mg, Mn, Fe, and Zn. The lower temperature loop of dolomite, the reaction for which must be preceded by an internal rearrangement, is independent of the pressure of CO2 but may be shifted to a lower temperature by prolonged fine grinding which accomplishes a similar rearrangement.

The physical properties of clay are of extreme importance in soil science. Plant growth, and hence crop production, within any environmental condition is largely controlled by soil structure which results from reactions involving clay. The active clay material in soil, particularly in combination with small amounts of organic matter, exerts a tremendous effect on soil properties. This effect may be on structure (the arrangement of soil particles), or on consistence (the response of the soil to mechanical manipulation). Where structure is favorable soil grains are clumped together into effectively larger aggregates so that soils have a more open arrangement and water and air can move freely and roots function normally. Where structure is unfavorable, soils tend to be heavy and impervious, and both the physical and chemical properties of the soil become unfavorable for plant growth. Soils which are low in clay, such as sands and silts, exhibit a rather narrow range over which physical properties can change and may be unfavorable for plants, being droughty and lacking in fertility. Structure of soil may change through action of natural forces, management practices, or cropping systems, and it is of great importance that we understand how structure affects plants, and how it is formed and stabilized by reactions involving clay.

Combination of clay with relatively small amounts of certain organic compounds greatly changes the physical properties of the system and the nature of the combination and the mechanics of the soil structure-forming process are but little understood. Such changes greatly affect soil consistence and soil-water relationships as well as soil structure. The problem is made difficult since the results of any particular combination or change in clay characteristics must be interpreted not in terms of the clay system alone but in light of the resultant effect on the complex and dynamic system which constitutes a soil. Solution of such problems, however, may be of great importance to our future ability to produce food and fiber abundantly and efficiently from our limited soil resource.

Interparticle friction is the principal property which permits soils and granular materials to resist load without formation. Dry soils of virtually all types are highly stable. The suitability of soils for engineering purposes depends largely upon their ability to remain in place and support whatever loads may be placed upon them, either by a permanent engineering structure or by transient vehicle loads. A study of the properties which distinguish the more satisfactory from the less satisfactory soils indicates that in the majority of eases clays are detrimental to stability. It is apparent that wet clay has the effect of a lubricant in diminishing the natural resistance due to friction that would otherwise exist. It is necessary that the civil engineer responsible for construction of any form of earth work should be informed not only concerning the Quantity of clay minerals that are present but also about their nature and their potential influence on the engineering properties of the soil.