This paper deals with the dehydroxylation reactions of clay minerals and allied substances such as lamellar hydroxides, serpentine minerals, and amphiboles. Probably in all cases, these reactions occur topotaxially, that is, there is a three-dimensional similarity between the crystal structures of starting material and product, as a result of which each crystal of the starting material is changed into something approaching a single crystal of the product. Many investigators have studied the orientation relationships between starting materials and products, and their results have provided the main experimental foundation for hypotheses about the reaction mechanisms. These hypotheses have, however, rested also on assumptions, of which until recently three have usually been made. These are (i) loss of water occurs more or less uniformly from all parts of the crystal; (ii) the Si-O parts of the structure are more stable than the metal-oxygen parts; and (iii) the silica-rich material often presumed to be formed as a subsidiary product is pure SiO2. Recent work suggests, however, that none of these assumptions is securely based, and the purpose of this paper is to suggest an alternative approach to the problem.

The high temperature reactions of clay minerals are perhaps best understood by treating the latter as assemblies of close packed, or nearly close packed, oxygen ions with cations in the interstices. From this point of view, one might expect that the relatively small cations, such as Si4+, Al3+, Mg2+, and especially H+ would move more easily than the oxide ions. One would not expect the oxygen atoms that are lost as water to be necessarily those present initially as OH- groups; they are more likely to be those that can most easily escape from the structure, e.g. from near external surfaces or grain boundaries. On this hypothesis, a typical dehydroxylation reaction involves three types of process:

1. Loss of oxide ions from exposed or “donor” regions, and their combination with H+ ions from all regions, to form water. The donor regions are changed into pores.

2. Cation migrations. These may be of two kinds. Firstly, all cations other than H+ migrate from the donor regions into the remaining or “acceptor” regions, and H+ ions migrate in the opposite direction. Secondly, within the acceptor regions themselves, further migrations may cause differentiation into Si-rich and Si-poor parts to occur.

3. Changes in the type of oxygen packing in the acceptor regions, to complete the formation of the crystalline products. These last changes may occur throughout the acceptor regions, or only in certain parts of them.

Studies have been made on muscovite single crystals treated under dry and hydro-thermal conditions at temperatures above 800°C. The course of reaction is markedly affected by the amount of water present in the system. In the presence of a large excess of water, appreciable reaction commences at 800°C with the formation of kalsilite and corundum. The kalsilite may bear an epitaxial relationship to the muscovite, but the corundum is quite unoriented. Reaction appears to proceed by solution of part of the muscovite and reprecipitation on unreacted areas. In the presence of low pressures of water the reaction course appears to be similar to that under dry conditions. Above 900°C a new phase has been identified which is similar to the spinel phase reported by Eberhart at temperatures above 1050°C. The phase is monoclinic and probably represents an intermediate stage between dehydrated muscovite and the spinel phase. A tentative mechanism is proposed for the dehydration of muscovite based on an inhomogeneous mechanism.

Single crystal X-ray and electron diffraction studies were carried out to examine the thermal transformation sequences of pyrophyllite and talc, respectively. The results showed that both minerals transformed topotactically. No dehydroxylated talc phase was observed, while an anhydride of pyrophyllite was obtained at about 900°C. The presence of a distorted anhydride lattice of pyrophyllite tends to indicate a homogeneous mechanism for the dehydroxylation of pyrophyllite. In contrast, the diffraction features of heated talc together with the cell volume relationship between talc and enstatite suggest an inhomogeneous mechanism for this reaction.

Direct current resistance measurements were made of kaolinite and serpentine powders to follow the changes in resistances during their dehydroxylation processes in air. The resistance of kaolinite remained almost constant during the dehydroxylation, whereas that of serpentine showed a rapid decrease. The formation of distorted lattice sites where the charge carriers are scattered under an electric field was assumed in the former case. On the other hand, the result in the latter case was interpreted in terms of an increasing cation mobility.

Dehydration and recrystallization reactions of fine and coarse powders and also of massive samples of serpentine under isothermal heating conditions in air are followed by thermobalance measurements and by X-ray diffraction intensities. The rate of recrystallization to forsterite is shown to have an inverse relationship to the rate of dehydration. This result is interpreted in terms of the damage inflicted on the crystal structure of serpentine by the dehydration reaction; the more slowly this reaction occurs, the more readily is forsterite formed as a result of the topotactic relationship between forsterite and serpentine. The surface layers of particles and massive samples of serpentine which dehydrate readily appear to be highly disordered and consequently recrystallize to forsterite very slowly. The corresponding phenomena exhibited by kaolinite are discussed and compared with those by serpentine.

An order of magnitude calculation is made to determine the activation energies involved in the recrystallization of serpentine to forsterite. The so-called “activation energy spectrum” showing the proportion of material with different activation energies is estimated.

Generally, the rate of dehydroxylation of clay minerals may be controlled by reaction at the interface between the decomposed and undecomposed mineral, diffusion through the decomposed phase and/or nucleation of the decomposed phase. Only in rare instances are mineral decomposition reactions homogenous. Some examples are presented to illustrate these principles.

For macrocrystalline muscovite the initial sample weight rather than the external surface area was found to be a parameter of the rate of weight loss. Upon grinding the muscovite to particles smaller than — 65 mesh, an internal surface area factor strongly influenced the rate. This surface area was related to dislocations shown to be present in muscovite.

A rate equation is presented that explains the observed weight loss versus logarithm of time dependence based on a model involving a systematic relief of strain during dehydration.

The rate equation for pyrophyllite was exactly the same as that formulated for muscovite. As with muscovite there was a change in free energy of activation throughout the dehydration process. In contradiction to muscovite, the contribution to the free energy of activation was at least partly enthalpy while with muscovite it was an entropy of activation contribution.

The dehydration of kaolinite was carried out in vacuum. At pressures of one micron the reaction was found to be diffusion controlled with hydroxyl ions as the likely diffusing species.

Several reactions which take place in clay minerals at elevated temperatures were investigated by D.T.A. and continuous high-temperature X-ray diffraction. The combination of these techniques was used to examine the energy changes and phase transformations that occur in kaolinite, halloysite, illite, and montmorillonite at temperatures up to 1450°C.

Diffraction data obtained from these minerals as they were heated are used to interpret the nature of the structural transitions, and new phase developments in two-layer type clay minerals are compared with similar developments in three-layer type clays.

Continuous diffraction is used to detect changes that take place in the firing history of a clay in which small amounts of chemical impurities or other mineralizing substances are present or have been added. Several chemical impurities were added to each of the clays to determine their ability to either enhance or retard structural transition and new mineral development upon firing. The effects which these additives have on predicted structural changes are illustrated and are interpreted in lieu of chemical and structural requirements.

In a natural clay, the occurrence of a high sensitivity, i.e. a high quotient between the shear strength of the undisturbed and remolded soils respectively, under undrained conditions, is connected to thixotropic effects and to “quickness”.

The general composition of the soils in which quick clay occurs is described, and mention is made of the occurrence of non-argillaceous rock fragments in a matrix of clay particles. Mite is the main clay mineral. Quickness occurs both in salt-leached, marine deposited clays and in clays deposited in fresh water. The pore water of these clays is low in electrolytes and undecomposed organic material. It is found that quick clays often occur near peat and similar humic deposits. Also briefly discussed is the internal stress distribution in natural clays under various consolidation conditions, the structure of quick clays and their conditions of formation. The consistency of clays is considered, as is the stability and coagulation conditions of suspensions. This is followed by a commentary on physico-chemical effects contributing to the formation of quick clays under natural conditions, and a discussion of the salt-leaching theory and the theory on the effect of peptizing agents.

In the discussion, the author draws parallels between the coagulation and the thixotropic phenomena, and between the stability and dispersion of suspensions and the quickness.

Recent development at deep levels in the Mt. Con Mine at Butte, Montana, has revealed a structurally early alteration type characterized by K-rich alkali feldspar, brown and green biotite, and 2M muscovite (sericite). This is similar to the K-rich biotite assemblage in numerous porphyry coppers as emphasized by Anderson, Gilluly, Creasey, and Rose. In Butte, and in some of the porphyry coppers, the assemblage is demonstrably earlier than the common feldspar-free, muscovite-quartz-pyrite assemblage which envelops pyrite-rich veinlets. On the other hand, few consistent mineral assemblages or detailed distribution patterns have yet emerged for argillic alteration, where illite, kaolinite, montmorillonite, and locally allophane and pyrophyllite are prominent phases. Probably much of the confusion results from extensive supergene alteration, though even this cannot be proved at present.

In supergene and in hypogene alteration at least below 300°C, supersaturation of SiO2 in the aqueous phase may play an important role in determining alteration assemblages. Some reactions affected ar. (1) the hydrolysis of K-spar to K-mica. (2) albite to montmorillonite. (3) silica-rich illite or montmorillonite to kaolinite, an. (4) pyrophyllite to kaolinite.

The encroachment of the sericitic zone on the argillic zone at Butte may be controlled by changes in ionic species of the veinward migrating SiO2 liberated by argillization of plagioclase. Within the sericitic zone and the veins, silica in solution is probably in equilibrium with quartz.

Argillic alteration is recognized in association with uranium deposits on the Colorado Plateau and along border areas. Alteration and mineralization are geometrically related in sedimentary strata where solution conduits result from lithologic and structural changes. The association also exists in collapse features and in breccia pipes. Occasional diatremes bearing uranium point to hypogene solution activity following igneous invasion as one original source for uranium ions. In several places ascending fluids apparently were confined under sufficient pressure to blast consolidated strata into vertical chimneys of brecciated masses. Pipes and fracture systems which transect porous strata provide paths along which hypogene solutions may ascend from depth and migrate horizontally. Alteration may extend both vertically and horizontally, encircling such areas.

Occasionally silica or carbonate precipitation accompanies uranium. Where found, well-marked masses of carbonate or silica replace sedimentary strata. Field conditions demonstrate upward migration of solutions. Often silica and carbonate may be dissolved from country rock and redeposited at higher levels. The silica minerals involved are quartz, chalcedony, and opal; the carbonates, calcite, dolomite and siderite.

The identification of mica polymorphs close to ore, corresponding to those created at elevated temperatures in the laboratory; the recognition of dickite in gouge zone veinlets associated with primary sulphides; uranorganic complexes; and replacement textures involving multiple pseudomorphs after a single mineral, further indicate alteration by hypogene solution.

Uranium deposition is favored by precipitating agents peculiar to certain strata. Hydrogen sulphide, produced by decaying fossil plant detritus, or petroliferous material, appears to have been an effective precipitating agent. Strata often involved are members of the Chinle or Morrison Formations. These units provide an environment with sustained reducing conditions favorable for uranium precipitation. The reduction of hexavalent uranium in solution occurs to the less soluble tetravalent form. Accompanying bleaching of red beds also suggests reducing conditions.

High-alumina minerals refer, in this paper, to Al-rich, normal sedimentary phases, including gibbsite, boehmite, diaspore, possibly “proto-diaspore” and “proto-alunite”, “Al-chlorite”, and corundum, in association with kaolin minerals. They may be derived from any common Al-containing rock. Processes of origin include direct bauxitization of non-clay silicate minerals and rocks, and the desilication of any of the common clay minerals, particularly of the kaolin group. Apparently aluminous gels were formed within certain marshy basins, and/or were transported into basins, giving rise to concretionary masses of high-alumina minerals. Concretionary deposits were formed by dissolution of Al and reprecipitation.

Alumina is probably more soluble, inherently, than is interpreted from geologic occurrences. Even after Al is dissolved under appropriate pH, or complexing reactions, its solubility is obscured by Al-immobilization through fixation within 3-layer clay minerals, or absorbed by other phases. A 14Å phase, probably aluminous, is not uncommon in certain soils, and in flint and boehmite-diaspore-containing clays. Subtraction of silica by plants, is accompanied by concentration of alumina.

Plastic clay deposits underlie peat beds in several swamps on the island of Kauai, Hawaii. These underclays are appreciably different in mineralogical and chemical composition from the associated well-drained weathered rocks (saprolite). The underclay is much higher in silica and lower in iron oxide than the saprolite. The underclay is composed chiefly of halloysite minerals, with minor amounts of gibbsite, anatase, quartz, goethite, and illite. The saprolite is predominately gibbsite, goethite, and hematite, with minor amounts of titaniferous magnetite, anatase, and halloysite minerals. Apparently both the clay and saprolite have formed by weathering from the same type of parent rock and have undergone similar geologic histories; therefore, differences in drainage and the presence or absence of an overlying peat layer appear to be the principal genetic variables.

The underclay and saprolite are believed to contain amorphous gels and allophane because X-ray intensities for many samples are low, and estimated amounts of identifiable minerals are not adequate to account for the whole samples. Attempts to isolate the amorphous materials in samples from Kauai have not been successful, but both an alumina-silica gel and allophane were found in saprolite at one locality on Maui, Hawaii. These materials were collected from the face of a freshly bull-dozed roadcut at a depth of 2 to 6 ft. They occur as veins and as fillings and coatings on vesicles, vugs, and cracks in a dark-brown saprolite closely associated with hard basalt. The allophane is white, and most of it is covered by a clear or light-colored gel. Some of the gel occurs in clear irregular masses ranging in size from microscopic specks to about one-fourth inch in diameter and is similar to a mass of frog eggs or thick gelatin. Some occurs as light-grayish brown, paper-thin laminae that extend along cracks for several inches.

Both the allophane and dried gel are similar in composition, containing approximately 50 per cent alumina and 22 to 26 per cent silica. The close association and similar composition indicate that allophane forms from the alumina-silica gel. Gibbsite and halloysite minerals occur in a manner very similar to that of the allophane and gel, suggesting that these amorphous materials are intermediate phases in the formation of gibbsite and halloysite. This is supported by the small amount of gibbsite present in the dried gel.

The formation of typical bentonites occurs in two stages. The first stage comprises the development, transportation, and deposition of the volcanic ash, which determines the geometry and structure of the bentonite rock and influences the alteration of the ash during the second stage. The second stage encompasses the alteration of volcanic glass to montmorillonite and other minerals.

The distribution of ash particles in the bentonite beds of Wyoming is satisfactorily explained by a theoretical model based on equations similar to heat flow equations. An extended model, assuming a disk-shaped volcanic cloud containing ash particles, also accounts for the geometrical shapes and geographic variations of the textural parameters of the bentonite beds.

The alteration of Wyoming bentonite ashes to montmorillonite occurred undoubtedly at low temperatures in an aqueous environment. Petrologic evidence indicates that all of the Wyoming ashes and many ashes deposited elsewhere contained sufficient magnesium to form montmorillonite with no addition necessary from without. Also evidence indicates variations of pH of perhaps 5 to 8 made little difference in the alteration products of the ashes.

Laboratory evidence of the processes and products of alteration of ashes to bentonite accord with field observation. With simplifying assumptions, the formation of montmorillonite from volcanic glass may be elucidated from the viewpoint of the stability of montmorillonite with respect to other similar minerals. The results of theoretical calculations of the mineral stabilities will be presented. Three energy parameters will be considered: the Coulomb energy, Van der Waals’ repulsion, and the London energy Thermal motion of the atoms will be taken into account.

Beginning with the work of Ladd in 1898, much has been written about the kaolins of the Southeast, with maximum coverage on the deposits of greatest economic value. Early studies were, of necessity, general in nature but also contain excellent detailed descriptions. In the past 15 years increased demands for kaolins to meet specific requirements have resulted in a large amount of detailed research, much of it unpublished. This paper attempts to review the available information placing emphasis upon the geologic and mineralogical data and the interpretation thereof with respect to the occurrence, detailed character and variability, and probable origin of the clay deposits.

The commercial deposits of kaolin clay discussed herein lie throughout the sands of the Upper Cretaceous Tuscaloosa formation and are localized in a narrow belt along the southeast edge of the “fall line” from Macon, Georgia, to Aiken, South Carolina. The kaolin lenses are irregular in shape, size and purity, ranging from a few feet to a mile in length, and up to 50 ft in thickness. Variability within the lenses is a function of (1) the concentration and localization of non-clay materials (quartz, mica, gibbsite, pyrite, lignite; Fe2O3, TiO2, MnO), (2) the montmorillonite content, (3) the particle size and X-ray crystallinity of the kaolinite, and (4) the texture of the clay aggregate. These mineralogical, chemical and textural variables are not homogeneously distributed but reveal and help to explain differences between samples, layers, pits and the important hard and soft clay types of the region.

The Cretaceous sedimentary kaolin deposits of South Carolina and Georgia occur in a portion of the geologic column which contains abundant evidence of volcanic activity in Texas, Arkansas, Mississippi, and Puerto Rico. It is proposed that volcanic ash from eruptive centers in the Mississippi Embayment and Gulf Coastal region settled on land and was washed into adjacent valleys, lake basins, and possibly lagoons, along with varying amounts of residuum from the erosion surface; alteration of the volcanic ash to montmorillonite, and then to kaolinite, progressed under the influence of subaerial erosion in Cretaceous and later time whenever physical and chemical conditions were favorable.

Several perplexing and previously inadequately explained features of the deposits are clarified if we accept this hypothesis. These features include: (1) The purity of the deposits, being over 95 per cent clay mineral in many deposits 20 ft or more in thickness; (2) General absence of fossils; (3) Lack of bedding; (4) Prevalence of a small but nearly uniform content of TiO2 as minute crystallites of anatase; (5) Presence of elongate bevel-edged particles, now clay, which have the form of glass shards.

Both the “soft” and the “hard” clay are believed to have developed from the same source. At all places where seen in the field the hard clay occurs immediately beneath a prominent unconformity; its different properties are attributed to changes produced by subaerial weathering. Repeated wetting and drying, which resulted in a smaller particle size, and leaching of silica, resulting in a slight bauxitization, are considered important mechanisms.

Kaolinite originating by the low-temperature subaerial transformation of montmoril-lonite occurs extensively in Tertiary clayey sands of central and northern Florida. It occurs in gray iron-stained weathering zones, as much as several feet thick, that transgress primary green clays of the Bone Valley and other formations over hundreds of square miles. Subjacent deposits of opal and chert, clearly post-depositional, are a widespread byproduct of the transformation.

Kaolinization begins as a hexagonal modification of the edges of montmorillonite flakes. X-ray study reveals that montmorillonite degrades and disappears up-section, with the appearance of the 7.2 kaolinite reflection and a very small 20–21 Å reflection. The altered clay loses silica and gains alumina and water.

The clay transformation apparently proceeds by intracrystalline leaching of tetrahedral silica layers by slightly acid ground water. Approximately regular interposition of resultant stripped packets within montmorillonite creates a regular 1:1 mixed-layered montmorillonite-kaolinite, which responds as montmorillonite does to glycolation and heating. The stripped kaoline-like packets develop hexagonal outgrowths of true kaolinite by lateral epitaxy. Nourished by the decomposing montmorillonite, the outgrowths enlarge and nucleate new vertical growth, observed as oriented crystallites, 40–50 Å thick.

Transformation of montmorillonite to kaolinite is a major geochemical process: creating goethite-stained kaolinitic sands of regional extent, liberating much silica for chert deposition and ground-water enrichment, and constituting an eluvial process that leads, in some areas, to residual quartz sand plains.