Clay minerals occur most commonly as constituents of sedimentary rocks and nearly all the clay mineral species, with the possible exception of nacrite, have been found in sediments. Different clay minerals occur in different kinds of sediments and may be used as one criterion for classifying sediments and interpreting their genesis.

The determination of the clay minerals present in mixtures by x-ray diffraction examination of oriented aggregates on glass slides becomes more reproducible and more conclusive when the relative humidity of the air in contact with the specimen is controlled during the recording of the diffraction pattern. The basal diffraction peaks of expanding and partially expanding clay minerals vary considerably in spacing and intensity with normal variations of relative humidity. These variations may be eliminated by the application of a simple modification to standard x-ray equipment. This modification is useful in maintaining a constant relative humidity at values between <1 and 100 percent. Dry air is used to maintain the effects of dehydration by heat and, in many cases, may be used as a substitute for heat treatment. This is especially true for mixed-layer minerals, which give more intense and more reproducible diffraction effects in dry air. Wet air is better, in many instances, than glycerol or ethylene glycol in resolving the basal reflections of montmorillonite and illite in mixtures. More information can be obtained from preparations of mixed-layer minerals and mixtures of clay minerals by recording the diffraction pattern at various stages of hydration.

Several kaolinite samples were selected for x-ray diffraction studies on the basis of their paper-coating ability. The evaluation of the kaolin clay for paper coating was based on a series of empirical tests including viscosity, resistivity, brightness, and particle-size distribution. The crystallinity of the kaolinites was determined by use of x-ray powder diffraction patterns. In all samples that were good paper coaters, the crystallinity was well developed. Other samples which were poor paper coaters either had poor crystallinity, or, if they had good crystallinity, contained montmorillonite. Crystallinity is not a function of the size fraction. Generally, if the crystallinity is good in the raw sample, then the crystallinity is also good in the < 0.25 micron fraction.

Suitability of kaolinite clay for paper coating varies directly with the perfection of crystallinity and indirectly with the montmorillonite content. Both of these variables can be determined by use of x-ray diffraction data.

Cored specimens of several commercial bentonite beds were preserved at field moisture levels for laboratory examination. Green strength, water content, and x-ray diffraction character of undisturbed material were found to conform substantially to the relation that strength is proportional to the solid fraction.

A detailed explanation of the most important properties of natural and synthetic silica and silica gels is given. Specific reference is made to the differences between the formation of synthetic silica gels and of quartz and cristobalite. The effect exerted by the synthetic products is explained and the importance of their amorphous condition on their reactivity is also referred to. On the basis of ultra- and electron-microscopic studies it is demonstrated that these synthetic products are not crystalline, but amorphous aggregates, and it is also shown how their reactivity can be increased by appropriate dispersion before use.

The colloidal properties of silicone resins are then pointed out and their production explained. Their advantages over purely organic matter and the reasons for this difference are also referred to.

Finally, reasons are given why more attention must be paid to colloid science in any field where silica or silicones are studied or put to use.

Numerous attempts have shown that it seems to be impossible to crystallize co-precipitates of silica and hydroxides; we therefore attempted to prepare clay minerals starting from solutions of their constituents. By allowing these dilute solutions (tenths of milligrams per liter) to pass slowly into a flask containing distilled water, it has been possible to prepare substances similar to clay minerals. These have been identified by x-rays, chemical analysis, dehydration curves, and electron micrographs.

Using as cations Mg, Fe2+, Fe3+, and Ni, minerals of the montmorillonite type have been prepared for various pH values (generally greater than 7). Below this, value, depending on the cations; oxides, hydroxides, or amorphous materials are obtained.

Under certain conditions in the absence of silica the glass vessel was attacked, and anti-gorites of Mg and Ni were then obtained. Always when the supply of silica was small, and with the cation introduced in the form of a very dilute solution, the process gave rise to montmorillonite. In the presence of K, illites were sometimes formed.

The method seems to be of very general application. Only the purely aluminum minerals offer difficulties of preparation. The significance of these results is discussed.

It is shown by x-ray analysis and electron micrographs that the clay from Les Eyzies is composed of about equal amounts of platy material (kaolinite), and rolled or tubular material (halioysite). Detailed examination of the electron micrographs shows oriented tubes or rolled sheets which may have originated from well-aligned platy material which has fractured and subsequently curled into rolled forms. Hexagonal features on the surfaces of the rolled forms are described. The laboratory results are correlated with geological data on the formation of the clay.

Hydrated halloysite clays altered from glassy tuff during weathering were examined in detail by x-ray, thermal, and electron micrographic methods. The electron micrographs show fibrous crystals of hydrated halloysite associated with many rounded grains, the former frequently jutting out from the latter. X-ray powder diffraction data and thermal curves show clearly the presence of hydrated halloysite, and on closer examination dehydrated halloysite and a small amount of montmorillonite can be detected. The presence of allophane as an accessory mineral is suggested in certain specimens. Furthermore the degree of crystallinity of hydrated halloysite varies slightly from specimen to specimen. The variation of the degree of crystallinity of the hydrated halloysite and of the kinds and amounts of accessory clay minerals, and the variation of the shapes of the particles observed in the electron micrographs are generally related to each other. Hydrated halloysite of a relatively well-crystallized sort generally co-exists with halloysite (dehydrated) and consists of elongated crystals having sharp edges; rounded grains are composed entirely of these fibrous crystals. On the other hand, a poorly crystallized sort tends to be associated with allophane and to show extremely fine hairlike, twisted fibers; these are associated with well-defined rounded grains. A genetic process of clay-mineral formation from volcanic glass is considered.

The dehydration of sodium, potassium, lithium, hydrogen, calcium, magnesium, and manganese monoionic montmorillonites (prepared with ion-exchange columns) and a ver-miculite, was examined by the x-ray powder diffraction oscillating-heating method. The position and intensity of each of the first-order basal spacings were measured at intervals of 5° to 10° C from room temperature to 900°C. Position and intensity measurements were made for the first five orders of the basal spacing of the calcium montmorillonite up to 300° C and compared with computed intensities at these spacings. These data are presented as oscillating-heating x-ray diffraction diagrams showing the intensity change with temperature, with important spacing shifts indicated, and as graphs showing the change in spacing with change in temperature.

Like vermiculite, calcium, magnesium, manganese, lithium, and hydrogen montmorillonite have an octahedral coordination of their exchange cation, and upon heating they also pass through two stable hydrates (ca. 14.5 A and 11.5 A). Montmorillonites of the two larger ions, sodium and potassium, have a regular one-water-layer configuration at 12.4 A, which is distinctly different from the 11.5 A hydrate of the smaller ions.

The foreshortened oxygen-oxygen approaches of the lower hydrate (11.5 A) may be accounted for by assuming that the exchange ions take up coordinate positions between silica-oxygen and water levels or, alternatively, that some of the silica tetrahedra are inverted.

Comparison was made between a sample of Wyoming bentonite and its synthetic analogue. Equivalent decomposition temperatures and mineral phases were obtained in hydrothermal environments and by calcinations.

The hydrated Mg-vermiculite lattice contains double sheets of water molecules interleaved with silicate layers. In the fully hydrated condition, each water sheet is arranged in a regular hexagonal pattern and the interlayer cations are located midway between water sheets in octahedral coordination.

Release of water molecules from interlayer positions leads at first to a gradual contraction of the basal spacing from 14.81 A, during which the water network becomes increasingly distorted. The contraction ceases temporarily at 14.36 A while further water is withdrawn. When the probability that a cation has a complete octahedral “shell” of water molecules at a given instant falls below a certain value, an abrupt contraction to 13.82 A takes place during which the cations are displaced to sites near the silicate layer surfaces.

Release of further water causes a contraction to a lattice (11.59 A) in which single sheets of water molecules are interleaved with the silicate layers. The 11.59 A phase is stable until only a small proportion of the interlayer water remains, when an approximately regular interstratification of 11.59 A and 9.02 A (dehydrated) layers develops. The final traces of interlayer water are retained in the interior of the crystal with great tenacity owing to the sealing of the crystal near the edges.

The Middle Paleozoic shales of northern Ohio contain a well-crystallized variety of illite as their main constituent. Powder diagrams relate the illite to the two-layer monoclinic muscovite crystallization. The low asymmetry of the 10 A reflection indicates that inter-layering of non-mica or hydrated-mica units is small. The relative intensities of the basal reflections show that the illites approach the ideal muscovite composition but have about one-fourth the octahedral aluminum replaced by iron and are deficient about one-half or less the interlayer potassium. The interlayer positions are essentially anhydrous. One-dimensional Fourier syntheses made before and after thermal treatment of the illites indicate that intensity changes of the basal reflections due to thermal treatment are related to electron density changes confined to the octahedral layer. Dehydroxylization begins at moderately low temperatures and continues until an anhydride structure is formed.

The progressive reconstitution of the thermally decomposed members of the kaolin group (the so-called “meta” phases) has been studied as a function of time, temperature and water vapor pressure of reconstitution, with a view to determining the extent of “pseudomorphism” after the parent structure. The nature of the dehydroxylated dickite which shows a 14A reflection has been investigated further but no additional x-ray data characteristic of a 14A-type structure can be found.

The sequence of rehydration shows that all the meta phases tend to form kaolinite, but the actual sequence of events depends on the parent material. Under the proper conditions well-crystallized kaolinite is obtained from all the meta phases. However, the original minerals themselves could not be converted directly to kaolinite within the time limits of the experiments.

A preliminary x-ray diffraction study of the effect of treatment of typical layer-lattice silicates with molten lithium, sodium, and potassium nitrates has been made. Treatment of Wyoming bentonite (Upton, Wyoming) with molten lithium nitrate produced random interstratification of spacings of 9.6 and 17.8 A when specimens were glycerol-solvated, with approximately 20 percent expanded layers. Treatment with sodium nitrate at 320° G produced a very high degree of preferred orientation in film specimens dried from water, whereas treatment with molten potassium nitrate produced a more random orientation in the specimen. The bentonite treated with molten sodium nitrate expanded completely to 17.8 A with glycerol; treatment with potassium nitrate resulted in a small amount of random interstratification, the proportion of expanded layers being about 80 percent.

Treatment of the <5μ fraction of Delamica (muscovite) with molten lithium nitrate removed a large portion of the potassium from the mica, and lithium was fixed in a not readily exchangeable manner. When barium-saturated and glycerol-solvated, the treated material gave an 001 spacing of 17.8 A, the basal spacing for glycerol-montmorillonoids. The amount of potassium removed from the muscovite varied with the muscovite/lithium-nitrate ratio and length of treatment.

The studies undertaken by the Instituto de Edafologia y Fisiologfa Vegetal on clay minerals and related silicates can be classified into three main groups: (A) Mineralogical identification and crystalline structure: (B) physico-chemical behavior; and (C) technical applications. They are summarized briefly.

From 1945 to 1949 clay-mineral studies in Belgium were connected with the Comité Belge pour l’Etude des Argiles, In 1947 this group organized the C. I. P. E. A., which in turn helped to form active national groups of clay workers. Clay-mineral research is now pursued in five laboratories in Belgium. Projects in Brussels include industrial problems, study of reactions of silicates and the oxides of Al, Ca, and Mg; quantitative determinations by x-ray spectrometry; and the use of a differential thermobalance and electron microscopy. Basic clay-mineral problems involving agriculture are being studied at Heverlee, where investigations include studies of fixation of iron on kaolinites and examination of the topography of kaolinite minerals attacked by different reagents. A group at Tervuren has been concerned with formation, deferrization, and determination of oxides by D. T. A. The Laboratoire National des Silicates at Mons is investigating creep phenomena in refractories. The activities of the Ghent group are described in a separate paper.