Despite recent advances in clay mineralogy there is still no generally accepted definition of the terms clays and clay minerals. This problem is discussed from the historical aspect: the similarities of definitions given from the 16th century to the present are remarkable and lead one to consider whether possible simple modifications might not eventuate in some measure of universal agreement. Comparison of nomenclature and classification schemes shows that from the time of Theophrastus to that of Agricola there appears to have been little advance, but a change becomes noticeable by the end of the 18th century, and by the end of the 19th old names, such as porcelain clay, etc., have given place to what are now regarded as names of mineral species or varieties. Only after the 1920’s have satisfactory groupings been obtained, but no internationally acceptable classification has yet been devised. Finally, the various factors operative during clay formation by weathering and diagenesis are discussed, and it is stressed that field and laboratory studies are complementary.

Interlayering of 2: 1 layer silicates varies as a function of chemical weathering from the simple, homogeneous K or Na interlayers of micas to the heterogeneous systems of mica intercalated with expanded 2: 1 layer silicates. “Frayed edge” type of weathering at dislocation planes of mica is collated with K release and preferential cation-exchange uptake of K relative to Ca by such expansible layer silicate systems; mica islands maintain alignment of the silica sheet cavities, which facilitates recapture of lattice K. Intercalation of the expanded 2: 1 layer silicates with alumina interlayers appears to be a characteristic function of chemical weathering in soils, with the formation of 2: 1–2: 2 intergrades not only of 14 Å spacing but also of swelling 18 Å types that give small 12, 14, 18 Å and higher spacing peaks (along with the 10 Å peak) at 550°C. Interlayer precipitates appear to be characteristic of soil clays, contrasting with “pure” minerals of deposits developed in less “open” environments than those of soils. The “2: 2 lattice building” phenomenon in expansible 2: 1 layer silicates relates to layer charge density and crystal size, and frequently tends to inhibit the formation of free gibbsite in soil chemical weathering so long as there are expansible layer silicates present to become intercalated with aluminum hydroxide—a weathering phenomenon that may be called an “antigibbsite effect”. Accumulation of alumina (possibly with some iron, magnesium, and allophane) as interlayers in 2: 1 minerals of soils is seen as a genetic stage in the 2: 2 → 1: 1 weathering sequence through which kaolinite and halloysite develop in soils.

Negative charge and related characteristics of dioctahedral micas have been investigated by conducting artificial weathering studies on a muscovite from Ontario, Canada, and saprolitic micas from Virginia. Emphasis is given to the relationship between loss of potassium, increase in net negative charge, or cation exchange capacity, and expansion characteristics.

Boiling solutions of acetic acid, sodium chloride and magnesium chloride removed potassium and sodium from a saprolitic mica but produced a negligible increase in cation exchange capacity. Expansion of the mica structure occurred following treatment with salt solutions but not with acid solutions. Boiling solutions of sodium citrate removed potassium effectively, produced marked expansion, and caused a large increase in cation exchange capacity. The small increase in cation exchange capacity resulting from acid and salt treatments is attributed to the hydrolysis of exchangeable aluminum to form hydroxy-aluminum polymeric groups, which can occupy exchange sites but remain non-exchangeable. The results of the sodium citrate tratement are attributed to the complexing of aluminum by citrate, thus releasing the hydroxy-aluminum polymeric groups from exchange positions and permitting an increase in cation exchange capacity.

Molten lithium nitrate treatment of specimen muscovite produced a greater increase in cation exchange capacity than in the case of muscovite separated from soil-forming mica phyllite. The observed difference in charge characteristics, concomitant with the greater expansion of the muscovite, suggest that soil micas and specimen-type micas differ in their alteration tendencies.

When interlayer hydroxy groups are eliminated and account is taken of residual sodium, potassium, and water, dioctahedral micas apparently lose no negative charge on expansion to vermiculite-like minerals.

The weathering of Stone Mountain Granite (adamellite) forms kaolinite, endellite, allophane and gibbsite of which kaolinite is the most stable. Bulk density ranges from 2.65 in fresh rock to a minimum of 1.48 in saprolite. It is a good index of weathering. Abrasion pH ranges from 5.0 in saprolite to 9.3 in fresh rock, and is directly related to bulk density and the amount of clay mineral. Among the original minerals, biotite is least stable, followed by oligoclase. Residual microcline fragments occur at the base of the B horizon indicating that it is not completely weathered. Muscovite weathers slightly and quartz appears quite stable.

Evidence shows that solution and reconstitution is necessary in the formation of some if not all clay minerals. This is particularly evident in the formation of endellite veins.

Expansion of the rock in the early phases of weathering occurs but does not produce a significant increase in volume.

Recent investigations of soils from widely distributed sites reveal the presence of clay minerals with 14 Å d spacings in the B horizons of podzolized soils in Alberta. The minerals appear to be concentrated in the 2.0–0.2 micron clay fraction, the fraction less than 0.2 micron being comprised primarily of material amorphous to X-ray. Despite the presence of montmorillonite in the eluvial horizon and soil parent material only interstratified minerals of this group appear in association with the 14 Å mineral. Smaller amounts of quartz, kaolinite and illite are also noted in these B horizons.

The data available reveal that the 14 Å mineral is derived from montmorillonite and that formation occurs in situ in the B horizons.

It is apparent from the investigations conducted that weathering of associated feldspars under mildly acid conditions, releases hydrated alumina ions. These ions not only move into the interlayer regions of montmorillonite of fine clay size, where they exist in a discontinuous manner, but are also adsorbed onto the surfaces of these minerals binding adjacent particles into aggregates of coarse clay size. Adsorbed hydrated alumina hydroxy ions in the interlayer region are responsible for the observed 14 Å d spacings.

Most weathering reactions have been interpreted in terms of the solid-liquid interface. In dry climates or at low humidity, weathering might be initiated or controlled by a solid-gas reaction. The stability of gibbsite and some hydrous borates involves the activity of water vapor; and the stability of azurite depends on a ratio of the partial pressures of both water vapor and carbon dioxide. Much additional work is required on the rates of equilibrium dehydration to contribute to our knowledge of the solid-gas interface in weathering.

The oxidation and reduction of clay minerals, particularly chamosite and nontronite, are compared with those of the amphibole crocidolite. Oxidation is very similar, but differences in behaviour on reduction are described. These differences are interpreted in terms of the different environments of the oxygen atoms in the various structures. Mechanisms of the various processes are discussed in detail.

Oscillating-heating X-ray diffraction was used to determine d-spacings and intensity changes of the (001) diffraction maximum for various combinations of 3 clays, 6 cations and 7 polyalcohols from 20°C to 1000°C under non-equilibrium conditions.

The temperature of collapse of the one layer complex was measured. Analysis of variance of this parameter for a model containing 3 clays, 6 cations and 7 polyalcohols showed that there is a significant difference among cations and polyalcohols but not among clays.

Additional work on one clay, 6 cations and 18 polyalcohols shows that ΔT—the difference in temperature between the collapse of the clay-organic complex and the boiling point of the polyalcohol—is proportional to the length of the carbon chain ; ΔT is also a function of the number and position of the OH groups.

The mean temperatures of final collapse for all polyalcohols are related to the valence of the interlayer cations and inversely related to their ionic radii.

There are two groups of d-spacings for the one-layer complexes. Complexes with polyalcohols having OH groups at the ends of the chain have mean d-spacings between 13.6–13.7 Å, while complexes with polyalcohols with interior OH groups have mean d-spacings of 13.9–14.0 Å.

The hydrogen content of dioctahedral and trioctahedral micas has been investigated by means of infrared spectroscopy and radioactivo measurements. Dioctahedral and trioctahedral micas have been treated with deuterium oxide and tritium-bearing water. The introduced isotopes of hydrogen have been determined after various thermal treatments.

It is concluded that dioctahedral hydrous micas may represent mixed crystals between K(Si3Al)Al2O10(OH)2 and H2O(Si3Al)Al2O9(OH)3, whereas trioctahedral micas seem to represent mixed layer minerals between phlogopite and vermiculite.

Diagenesis refers to the process, and the changes (usually excluding cation exchange) that take place in sediments after deposition, but definitions of the process vary widely over which changes, and what part (or all) of the time between the transportation of the sediment and the time that the sediment(ary rock) is studied, should be included in diagenesis. Furthermore, Weaver, Griffin, and others, have favored alternative explanations to account for many clay mineral distributions that previously have been interpreted as representing changes in clay minerals in both recent and ancient sediments, and which were referred to diagenesis.

Although agreeing that such alternative explanations account best for many distributions of clay minerals, additional evidence that presumably is convincing of diagenesis is presented herein to account for the formation of certain other occurrences of glauconitic mica, glauconite, illite, corrensite, kaolinite, and vanadiferous micaceous clay. A two-stage mechanism is proposed for a possible process by which illite and probably corrensite are formed: (1) cation sorption driven by energy (activities) of the ions in solution, and binding energy of the clay minerals, and (2) subsequent rearrangement of ions in the clay mineral to produce illite. It is finally proposed that diagenesis of clay minerals is characterized by the addition to and incorporation into them of Me ions, such as Mg, K, Ca, Na, Al, Fe, V, SiO2, and others, driven mainly by chemical energy and aided by mechanical and thermal energy, but as the latter two become more effective the process is designated metamorphism (anamorphism).

Long-time, diagenetic reactions after burial of sediments are probably less important, volume-wise, than provenance and terrestrial weathering in determining the mineralogy of argillaceous sedimentary rocks in the geologic column.

X-ray diffraction, differential thermal, chemical analysis and cation exchange data indicate a progressive genesis upon hydrothermal alteration of rhyolitic materials in an extensive fault zone. Kaolinite, a Ca-Mg beidellite, mixed-layer beidellite-hydrous mica and beidellite-vermiculite clay minerals, quartz, cristobalite and feldspar commonly occur in the deposit.

Detailed studies show that the sequence of the 2:1 clay mineral alteration is beidellite-hydrous mica mixed-layer → beidellite-vermiculite mixed-layer → beidellite. This mineralogical change parallels a progressive decrease in the layer charge and amount of fixed K+ present and an increase in exchangeable Ca+ and Mg+ respectively.

Although the wide range of minerals resulting from alteration of tuffaceous rocks under hydrothermal and weathering conditions is well known, the extent of variation in composition of discrete bentonite beds subjected only to normal diagenesis and compaction is perhaps less well known.

Three bentonite samples from the Atoka Formation of Pennsylvanian age are predominantly mixed-layer illite-montmorillonite with subordinate kaolinite. Most bentonites in the Fierre Shale of Late Cretaceous age are entirely montmorillonite, but a few contain some kaolinite; one is predominantly kaolinite; some also contain appreciable interlayer Al(OH)3 or Mg(OH)2 forming a mixed-layer montmorillonite-chlorite clay; several are composed of considerable clinoptilolite, and one is largely phillipsite. Bentonite beds in the Carmel Formation of Middle and Late Jurassic age differ among themselves in composition; in a few, illite is the dominant clay; in others, mixed-layer chlorite-montmorillonite-illite in different proportions is the dominant clay; kaolinite is common and makes up nearly half of one sample. A rhyolite tuff near Denver, Colorado, has been altered to halloysite. Reasons for variation in the composition of these bentonites are only partly understood.

The physical process of compaction of clay sediments and the inverse process of swelling can be discussed in terms of particle interaction forces. The swelling pressure in a given situation equals the nct repulsive force per unit area between the particles. The net repulsive force is the resultant of various repulsive and attractive forces. Two stages of swelling or compaction can be distinguished, one in which the particles are relatively far apart, and a second in which they arc separated by only a few monomolecular layers of water.

In the first stage the swelling pressure is primarily due to double-layer repulsion (“osmotic swelling”). The pressures range from a fraction of one atmosphere to several times ten atmospheres. These pressures are of particular interest for foundation engineering problems.

In the second stage the swelling pressures are dominated by the work of adsorption of the water layers on the clay surfaces. Tile pressures may reach several thousand atmospheres. Quantitative data on the swelling pressures in this range are derived from water vapor sorption isotherms combined with X-ray data on the c-spacing of expanding clays during dehydration.

It is concluded that overburden pressures in nature will seldom be high enough to remove all adsorbed water from between parallel grain surfaces.

Magnesium-bearing hydrothermal solutions moving along channelways created by faulting in the Willsboro, New York, wollastonite deposit have effected the conversion of wollastonite to layer-lattice silicates. Alteration products sheath faults in wollastonite and may be divided into three gradational but regular zones: (1) least altered (fresh) country rock composed of wollastonite with minor diopsidic clinopyroxene and grossularite-andradite garnet; (2) moderately altered rock rich in montmorillonite; and (3) strongly altered rock composed predominantly of talc.

X-ray diffraction, oscillating-heating X-ray diffraction, electron microscopy, infrared absorption, base exchange, and chemical analyses substantiate identification of the montmorillonitic alteration product, pseudomorphous after wollastonite, as stevensite. Stevensite, a trioctahedral magnesium montmorillonite, has been reported heretofore only in basaltic rocks where it has been formed by the action of magnesium-bearing solutions on pectolite. Pectolite and wollastonite have similar compositions and structures and may be similarly converted to stevensite in a magnesium-rich hydrothermal environment.

Talc, the dominant mineral in the most intensely altered zone bordering the solution channelway, was formed by the continuing action of magnesium-bearing solutions on stevensite. Stevensite has a defect structure, i.e., a deficiency in the total number of ions in octahedral coordination. The addition of magnesium from hydrothermal solutions to the octahedral layer of stevensite converts this mineral to talc, concomitantly freeing interlayer cations and water.

Stevensite is unstable and is converted to talc above a temperature of 275°C at water pressures of 690 bars. It is believed that hydrothermal alteration of wollastonite took place at temperatures near or below 275°C with stevensite forming in the cooler part of the reaction zone, and talc forming from stevensite at slightly higher temperatures nearer the solution channelway.

Preliminary results of investigations of Trenton-Black River (Ordovician) K-bentonites from well cores in northern Kentucky and from outcrops along the Kentucky River illustrate the variations in some physical and chemical properties. It is hoped that some features may eventually be useful in stratigraphic correlation, considering that the very nature of these altered ash beds makes them ideal time-lines.

That these beds are derived from volcanic ash is evidenced by significant amounts of biotite, magnetite, leucoxene, apatite, and zircon.

The following means of investigation have been employed: X-ray diffraction, DTA, cation exchange, heavy mineral studies, and insoluble residue studies of adjacent limestones. X-ray analyses of untreated and glycolated clay samples indicate a mixed-layer, dioctahedral, illite-montmorillonite type structure with an I-M ratio varying from 4:1 to 3:2. The proportion of expandable layers in many samples is higher than that reported for Ordovician K-bentonites elsewhere. No evidence of chlorite is detected.

DTA data show a primary hydroxyl peak near 725°C suggesting a basic montmorillonite-type of hydroxyl bonding. Treatment with 1M solutions of various cations produces a slight expansion of the expandable layers with Ca2+, and contraction with Na+, K+, and NH4+ giving some indication of original environmental composition. The evidence supports suggestions by earlier authors that the clays are basically montmorillonites which have undergone partial contraction due to the adsorption of potassium in the interlayer positions.

While euhedral zircons are most likely non-detrital, recent work on zircons in igneous rocks suggests the same may also be true for some of the sub-rounded and fractured species.

Insoluble residues of limestones contain abundant chert and mixed-layer illite-montmorillonite clay directly adjacent to the ash beds, but farther away illite and chlorite dominate and are probably detrital in origin.

The clay minerals were studied from eleven soil profiles representing three Great Groups of eastern and central Oklahoma. Montmorillonite and illite type clay minerals were found to be the dominant minerals; montmorillonite dominates the fine clays; illite is the chief constituent of the coarse clays. The fine clays dominated the clay fraction of most horizons, including the parent material. The results of the investigation indicate that the present régime of soil formation has had little effect in altering the clay minerals of the soil from those present in the original sediments.