The reaction of smectite to illite in shale from the COST 1 well in the south Texas Gulf Coast and from altered ash-fall tuffs from the Morrison Formation in New Mexico was investigated using X-ray powder diffraction in conjunction with transmission electron microscopy. In the COST 1 well, the bulk of the detrital clay was originally a K+-deficient mixed-layer illite/smectite (I/S). As the I/S adsorbed K+ released by the dissolution of K-feldspar during burial, the proportion of expandable layers decreased with depth from ~65% near the top of the well to ~25% at 4500 m depth. In contrast, the proportion of low-charged structural planes [<0.8 eq per (Al,Si)4O10 unit] in the I/S decreased gradually from ~40% near the top of the well to ~15% near the bottom. Authigenic smectite with 100% expandable layers from the Morrison Formation tuffs is an alteration product of vitric ash. Where these tuffs have been buried to ~ 1400 m the smectite has reacted to form I/S with ~ 15% expandable layers.

Direct lattice images of I/S crystallites from both locations reveal a correspondence between edge dislocations and the interface between illite layers and smectite layers. Al, Si, Fe, Ca, Mg, and Na were apparently mobile along these dislocations as the reaction of smectite to illite proceeded. Al was probably retained within the crystallite when illite layers replaced the smectite layers; however, some of the remaining cations were expelled. Lateral replacement of smectite layers by illite appears to have been the principal growth mechanism.

Several hydrates can be synthesized from well-crystallized kaolinites; of importance to the present work are a 10-Å hydrate (called the QS-10 hydrate), an 8.6-Å hydrate, and two kinds of partially dehydrated mixed-layer hydrates. One kind is a series of unstable materials with d(001) varying continuously between 10 and 8.6 Å, and the other kind is stable with d(001) approximately centered at 7.9 Å. The 10- and 7.9-Å phases have been observed in halloysites by many workers using X-ray powder diffraction, and the 8.6-Å phase has been seen by others in selected area electron diffraction photographs. Infrared spectra reveal additional similarities between the synthetic hydrates and both halloysite(10Å) and partially dehydrated halloysites. Because of these similarities, the synthetic hydrates can be used to develop a model for the dehydration of halloysite(10Å).

Previous work on the 10- and 8.6-Å hydrates identified two structural environments for the interlayer water. In one, the water is keyed into the ditrigonal holes of the silicate layer (hole water), and in the other, the water is more mobile (associated water). Both types are found in the QS-10 hydrate and halloysite(10Å), whereas only hole water occurs in the 8.6-Å hydrate. In the QS-10 hydrate, stronger hydrogen bonding between hole water and the clay makes the hole water more stable than the associated water. This difference in stability is responsible for a two-step dehydration process. The first step is the loss of associated water which results in a material with d(001) = 8.6 Å. This stable hydrate must be heated to temperatures near 200°C to drive off the remaining hole water. The less perfect structure of halloysite and its common curvilinear morphology reduce the difference in stability between hole and associated water molecules, so that when halloysite(10Å) dehydrates, loss of hole water and associated water overlaps, and the d-spacing goes directly to 7.2–7.9 Å.

The sorption of Cd, Cu, Pb, and Zn ions by Na-rich bentonite, Al and Zr-pillared Na-rich bentonite (Al-MX80, Zr-MX80), the uncalcined hydroxy-intercalated precursors (HAl, HZr-MX80), and commercial Al-pillared bentonite EXM 534 was investigated. Experiments were conducted in ultrapure water and artificial leachate with varying pH. The experiments were performed over periods to 30 wk. Sorption characteristics were described with one and two-site Langmuir isotherms. The non-exchangeable quantities of heavy metals were determined by fusion of the sorbents after ion exchange with ammonium acetate.

The sorption of Cd, Cu, Pb, and Zn by bentonite was dominated by cation exchange. In artificial leachate, the sorption was reduced due to competition with alkali and alkaline-earth cations. The sorption of Cu, Zn, and Pb at pH 4.9 and Cd at pH 6.9 by Al and Zr-hydroxy-intercalated and pillared MX80 was governed also by cation exchange. In contrast, the sorbed quantities of Zn at pH 6.9 exceeded the cation exchange capacity (CEC) of HAl, HZr, Al, Zr-MX80, and EXM 534 and were partially non-exchangeable. The increase of the sorption of Zn with pH and its independence of the ionic strength of the solution at neutral pH suggest a complexation of Zn ions to surface hydroxyl groups of the intercalated Al and Zr-polyhydroxo cations and pillars. This complexation is the dominating sorption mechanism. Removal of dissolved Zn from solution with time is attributed to surface precipitation. Al-hydroxy and pillared bentonites are considered potential sorbents of Zn ions from neutral pH aqueous solutions, such as waste waters and leachates.

A structure refinement of kaolinite made using the Rietveld neutron profile refinement technique has given non-hydrogen atom positions which were not significantly different from those given by B. B. Zvyagin in 1960. All of the hydrogen atoms have been located; the three inner-surface hydrogen atoms are involved in interlayer hydrogen bonds with lengths of 2.95(4), 2.95(4), and 3.06(4) Å with O-H ... O angles of 168(4)°, 144(4)°, and 146(4)° respectively. The inner hydrogen atom is located in a position consistent with that found earlier in dickite and muscovite which are the only dioctahedral layer silicates studied by neutron diffraction to date. The O-H vector makes an angle of 34° with the (001) plane, away from the octahedral sheet, and the projection of the vector on to (001) is at ~30° to the b axis.

This article emphasizes the influence of crystal-chemical composition on the ease of transformation of micas to vermiculite or more smectite-like minerals. The swelling test, with glycerol, of Mg-saturated mineral is used to characterize the degree of transformation.

The main structural factors of this evolution are tetrahedral substitution of Si by Al, and total charge. There is a relation between these two factors, i.e. the lower the tetrahedral substitution, the greater can be the charge without affecting the smectite swelling behavior.

In this respect there is a contrast between tri and dioctahedral micas. In the first, tetrahedral Al is so high (> 1·20 for Si4O10) that transformation into smectite must imply modification of the tetrahedral layer. For dioctahedral micaceous phyllites (illites, glauconites) where tetrahedral charge is lower, transformation can be easier. Only a lowering of total charge is needed and reduction-oxidation seems to play a very important role in this process.

Olduvai Gorge, Tanzania (East African Rift) exposes a 100 m thick Plio-Pleistocene sequence of dominantly volcaniclastic sediments deposited in a 50 km wide closed basin containing a playa lake. A scanning transmission electron and analytical electron microscopy (STEM/AEM) study of authigenic clay minerals in sediments from representative depositional environments in the basin (pyroclastic fan, fluvial plain, wetland, lake margin and lake basin) was undertaken to determine whether clay compositions and textures could provide unique geochemical fingerprints characteristic of source area (Plio-Pleistocene trachytic volcanics vs. Precambrian quartzose-feldspathic basement) or paleoenvironmental conditions.

Our study shows that compositional signatures obtained by clay minerals during early pedogenesis are inherited from their parent source rocks. Sediments sourced from volcanics contain highly disordered, dioctahedral smectite. Those sourced from Precambrian basement are similar, but are more Al-rich. Subsequent neoformation in the pedogenic (soil) or diagenetic (lake-margin, lake) environments results in the modification of original clay mineralogy, compositions, and textures, and unique paleoenvironmental fingerprints are acquired. Soils developed on the distal pyroclastic fan contain smectite with more Fe(III) and Mg than smectite from the proximal pyroclastic fan sediments. A trend of decreasing Al and increasing Mg content occurs in smectite compositions in samples from the fluvial to lake-margin and lake environments as a result of partial replacement of original dioctahedral Al-rich smectite by neoformed trioctahedral Mg-rich smectite (stevensite). Neoformed celadonite replaces smectite in the most saline lake sediments.

The STEM/AEM data collectively indicate that diagenesis in the saline-alkaline lake results in the replacement of Al-rich dioctahedral smectite by Mg-rich trioctahedral smectite (stevensite) and Mg- and Al-rich celadonite. Thus, determination of clay mineral compositions at a basin-wide scale provides a useful tool for interpreting the spatial distribution of depositional and diagenetic environments.

Aqueous gels of unaltered (oxidized) and chemically reduced ferruginous smectite (SWa-1 from the Source Clays Repository of The Clay Minerals Society) were characterized by transmission electron microscopy, electron diffraction, and energy-dispersive X-ray fluorescence to establish details regarding their texture, inter-layer and inter-particle arrangements, and chemical composition. Micrographs revealed that the reduction of structural Fe(III) to Fe(II) caused a consolidation of smectite particles from an extensive network of small crystals (1–6 layers thick) to distinct particles of limited size in the a-b direction and about 20–40 layers thick. The interlayer distances in the reduced sample appeared to be more uniform than in the oxidized sample, but both exhibited spacings of about 12.6 A. Chemical analysis showed no qualitative differences as a result of oxidation state. Electron diffraction patterns displayed marked differences. The pattern of the oxidized sample consisted of homogeneous rings, indicating that the stacking order in the a-b plane was turbostratic or disordered, whereas the reduced pattern exhibited much more order as evidenced by distinct spots amid low-intensity rings, suggesting that inter-layer attractive forces were stronger if Fe(II) was present in the clay crystal.

There has been limited research on clay mineral transformation in serpentinitic soils under humid tropical conditions. In this study, four soil pedons were selected along a toposequence from the summit (Entisol), shoulder (Vertisol), backslope (Alfisol) to footslope (Ultisol) positions to explore the contributions and the significance of landscape and weathering status of serpentinitic rock with regard to clay mineral transformations in eastern Taiwan. Experimental results indicated that the large amount of dithionite-citrate-bicarbonate-extractable Fe (Fed) and clay in the subsurface horizon were mainly caused by the strong leaching potential from intensive rainfall and weathering of the fine-grained parent rocks. The clay mineralogy reflected the clear weathering trend of the soils along the toposequence: (1) the soils on the summit and shoulder contained smectite and serpentine, which are predominant in the young soils derived from serpentinitic rocks; and (2) vermiculite gradually increased in the relatively old soils on backslope and footslope. The mineralogical transformations observed along the toposequence indicated that chlorite and serpentine, initially present in the Entisol on the summit, weather into smectite and interstratified chlorite-vermiculite in the intermediate soil on the shoulder under strong leaching and oxidizing conditions. Furthermore, vermiculite formed as the major weathering product of chlorite and smectite in the soil developed on the backslope. In addition to vermiculite, kaolinite and quartz formed in the soils on the footslope with the greatest concentration of Fed along the toposequence.

Methods for predicting the volume change and swelling-pressure behavior of expansive clays require detailed understanding of coupled interactions between clay microstructure and macrostructure under hydraulic, thermal, and mechanical loads. In this study a suite of water-vapor sorption experiments was conducted using compacted bentonites hydrated in controlled relative humidity (RH) environments maintained under free and constrained volume-change boundary conditions. Emphasis was placed on examining the influences of compaction and predominant exchange cation on the water uptake, volume change, and swelling pressure response. Densely compacted specimens exhibited greater volume changes under free swelling conditions and greater swelling pressures under fully confined conditions. Water uptake, volume change, and swelling pressure were all more significant for Colorado (Ca2+/Mg2+) bentonite than forWyoming (Na+) bentonite. Plastic yielding, evident as a peak in the relationship between swelling pressure and RH, was more evident and occurred at lower RH for the Colorado bentonite. This observation was interpreted to reflect the limited capacity for interlayer swelling in Ca2+/Mg2+ bentonites and corresponding structural collapse induced by the onset of water uptake in larger intra-aggregate and inter-aggregate pores. A semi-quantitative model for the evolution of clay microstructure resulting from interlayer hydration was considered to attribute the experimental observations to differences in the efficiency with which transitions in basal spacing translate to bulk volume changes and swelling pressure. Results provide additional insight and experimental evidence to more effectively model the mechanical behavior of compacted bentonites used as buffer or barrier materials in waste repository applications.

Scan electron micrographs (SEMs), are shown of three representative types of Georgia kaolin: ‘soft’, Cretaceous-age clay; ‘hard’, fine-grained clay; and ‘flint kaolin’.

Sparta Granite and a thick deposit of its overlying saprolite are taken to serve as examples of probable source rock for Georgia kaolin. SEMs are presented to show sequential argillation of this fresh granite through a transition zone to the saprolite. The saprolite contains abundant books of kaolinite similar to those occurring in Cretaceous kaolin, accompanied by some elongates, whereas in the transition zone elongates are predominant. A morphologic type of fibrous, elongate 1:1 clay mineral, hitherto not illustrated, is shown from kaolinizing Sparta Granite, and from Pre-Cambrian saprolitic gneiss 25 km south of Athens, GA.

Whereas kaolin elongates originated in contact with feldspar in the Sparta transition zone, well-formed books of kaolinite are shown to have been formed in contact with feldspar in the saprolite from a Missouri granite. The arkose above the granite in the Missouri occurrence has altered to loosely packed books of kaolinite. The interpretation is made that book-type kaolinite, where it occurs in large deposits of saprolite or kaolin, was formed under geochemical conditions of relative chemical stability between kaolinite and feldspar, as might prevail in a geological ‘weathering crust’. It is interpreted that elongate kaolin morphology may be formed under conditions of limited or lesser geochemical stability.

SEMs show that the established book-morphology of Sparta saprolite is disarranged during secondary Holocene surface weathering and pedogenic processing.

SEMs show that books of kaolinite, although individually fragile, may be protected from destruction or delamination during artificial blunging and transport when moved within naturally coalesced pellets. Possibly original books of weathered kaolinite might similarly survive natural stream transport during sedimentational processes.

Some ‘flint kaolin’ contains molds of sponge spicules coated with low-cristobalite.

Batch reactor experiments were performed at 150°C, 175°C, and 200°C to determine the effect of high pH KOH solutions on the mineralogy of the Opalinus shale. In these experiments, the change in solution quench pH at 25°C, solution composition, and mineralogy were monitored as a function of time for up to ≈ 50 days. Runs were performed in 50 ml titanium hydrothermal reactor vessels. Each reactor was charged with 0.5–5.0 grams of the 80–200 mesh size fraction of Opalinus shale, and 25 ml of solution (0.08 and 0.008 m KOH). Under these high pH conditions, the general sequence of reaction products observed is the formation of phillipsite, followed by K-feldspar ± K-rectorite. Phillipsite is a metastable intermediate that eventually transforms to K-feldspar. This sequence of mineral reaction products is very different from that found in the NaOH system.

Artificial stacking faults can be created within a well-crystallized kaolinite by intercalating and removing hydrazine. X-ray powder patterns with electron microscopy show that the created defects are ±b/3 translations with a proportion 0.30. The infrared spectrum of the treated kaolinite is not modified with respect to the starting one. On the other hand, a natural kaolinite containing defects by displacement of Al vacancies in a similar proportion shows an infrared spectrum significantly different from that of a well-crystallized kaolinite. The modification of the infrared spectra of natural disordered kaolinites is then related to the presence of defects by change of Al vacancy positions

The mechanism of decolorization of crude maize and sunflower oils was studied by means of adsorption of β-carotene by a low-grade bentonite, containing mixed-layered illite-smectite. Decolorization depends on temperature and the time required for equilibrium decreases with increasing temperature. The study of the kinetics of adsorption showed that decolorization of maize oil is a first-order process which occurs in two steps: a first fast step with higher activation energy (25.6 kJ mol−1), indicating the influence of a chemical interaction between the pigment and the clay surface, followed by a second slow step with low activation energy (12.3 kJ mol−1), characteristic of physical adsorption on the previously adsorbed molecules. Decolorization of sunflower oil is also a first-order process, described by a single mechanism with intermediate activation energy (19.0 kJ mol−1). Adsorption isotherms of decolorization of maize oil follow the Freundlich equation, indicating the existence of heterogeneous adsorption sites on the solid's surface. Heterogeneity is attributed both to different active centers on the smectite surface (Brönsted and Lewis centers) and to the different phases present in bentonite, such as illitic layers and clinoptilolite, which also have active centers on their surfaces.

The 57Fe Mössbauer spectra of an iron-rich montmorillonite, an illite, and two glauconites were measured and computer-fitted with appropriate Fe3+ and Fe2+ doublet resonances. The broad experimental Fe3+ resonance of montmorillonite probably arises from Fe3+ in the octahedral sites and a trans-arrangement of OH groups; however, a large variation in the neighboring environment of these sites exists. In illite this Fe3+ resonance is similar but shows less broadening; it arises from Fe3+ located predominantly in trans-OH octahedral sites, with some Fe3+ being located in cis-OH octahedral sites. Because of the increased iron content less variation exists, compared with montmorillonite, in the neighboring octahedral sites. The Fe3+ resonance is narrower still for the glauconites and represents Fe3+ substituting primarily into cis-OH octahedral sites, similar to that previously reported for nontronite.

The tetrahedral Fe3+ content is very low for montmorillonite and increases progressively for illite and glauconite, suggesting that a higher tetrahedral Fe3+ content directs Fe3+ in the octahedral layer into cis-OH sites. In montmorillonite, the Fe2+ is located only in trans-OH sites; in illite Fe2+ is largely in trans-OH sites and only slightly in cis-OH sites; and in glauconite, Fe2+ is located largely in cis-OH sites and only slightly in trans-OH sites. These assignments suggest that for Fe2-, the doublet with the larger quadrupole interaction arises from Fe2+ in trans-OH sites and the doublet with the smaller quadrupole interaction, from Fe2+ in cis-OH sites.

Three montmorillonite samples, from Wyoming (particle size <0.2 µm, 4.6 wt. % Fe2O3), Greece (<14 µm, 6.5 wt. % Fe2O3), and Poland (0.5 wt. % Fe2O3, two fractions <2 µm and 2–10 µm) were intercalated with hydroxy-Al species. The amount of hydroxy-Al cations exchanged by the clays depended on the particle size: it decreased from 31 wt. % Al2O3 for 0.2 µm in size to 22.6 wt. % for >2 µm. The X-ray powder diffraction (XRD) pattern showed that most of the material was not intercalated after refluxing at 80°C for 2 hr; several redispersions of the pillared clay in hot water were required to obtain a single sharp XRD line at 18.6 Å. The diffusion of the Al cations therefore limited the cation-exchange process. The addition of NH4+ to the pillaring solution decreased the amount of Al fixed by the clay from 31 (no NH4+ to 26 wt. % (NH4+/Al = 10), but increased slightly the microporous volume of the resulting pillared material from 0.13 to 0.14 ml/g. The competition between NH4+ and Al-hydroxy cations for ion-exchange sites forced the Al species into the interlayer space and increased the homogeneity of the resulting pillared material. The influence of particle size on the amount of Al exchanged was then reduced. After calcination in air at 700°C the pillared Wyoming clay retained a microporous volume of 0.09 ml/g if no NH4+ was added, compared with 0.11 ml/g for a NH4+/Al ratio of 10. The Fe content of the clay affected the thermal stability of pillared clays at 800°C: for two samples prepared by competitive ion exchange (NH4+/Al = 10) the microporous volume increased from 0.06 ml/g for 6.4 wt. % Fe2O3 to 0.08 ml/g for 0.5 wt. % Fe2O3. The acidity of the pillared clay was determined by calorimetric adsorption of ammonia at 150°C, on samples calcined at 500°C. Pillared montmorillonite prepared using noncompetitive intercalation showed a weak acid strength: most sites adsorbed ammonia with an enthalpy of 65 kJ/mole. The intercalation of the same original clay using competitive ion exchange yielded a solid that possessed both weak (heat = 60 kJ/mole) and strong (120 kJ/mole) acid sties. Competitive ion exchange appears to be a simple means of preparation of large quantities of pillared clays of reasonable thermal stability and higher acidity than those obtained by conventional methods.

The b-dimensions of the unit cells of six different Na-saturated montmorillonites were determined by X-ray diffraction at water contents ranging from 0 to 20 g per g of montmorillonite. In every case, the b-dimension increased progressively with water content from its initial value, which was characteristic of each dry montmorillonite, to a final value of ~9•0Å, which was common to all montmorillonites. The latter value was reached when the water contents of the respective montmorillonites were equal to those at maximal swelling. When these water contents were plotted against the corresponding changes in b-dimension, a straight line that passed through the origin was obtained.

Different structure-sensitive properties of the water in montmorillonite-water systems (i.e. the partial specific volume, the amount remaining unfrozen at −5°C and the activation energy required for ions to move through it) were available, as functions of the species of exchangeable cation, from previous studies. Relevant b-dimensions were determined in the present study. It was found that all of these water properties were correlated with the b-dimension of the associated montmorillonite.

Our results indicate that epitaxy exists between the crystal lattices of montmorillonite and adsorbed water and that these lattices undergo mutual adjustment with each increment of water. The resulting loss of free energy causes water adsorption, i.e. swelling, to occur spontaneously. Swelling stops when no further adjustment takes place. This does not happen until the adsorbed water is several hundred angstroms thick and has achieved a preferred configuration.

The 0.2–5μ particle size fraction of montmorillonite from three sources was equilibrated with various solutions at room temperature. After 3–4 yr, kaolinite was found in some of the samples that were supersaturated with respect to kaolinite, but not in any of the undersaturated samples or in the original montmorillonite. X-ray diffraction analysis of the precipitated kaolinite showed no inter-layer expansion of glycerated, oriented samples. Random powder samples indicated a poor crystal-Unity. The thermal stability of the precipitated material was indistinguishable from that of crystalline kaolinite. The electron microscope did not reveal any distinctive sizes or shapes.

Equilibration behavior of several samples defined a single kaolinite solubility line at or above which kaolinite apparently begins to precipitate. The solubility line is equivalent to a standard free energy of formation (ΔG) of -904.2 kcal per mole of kaolinite. This represents highly crystalline kaolinite. The stability of kaolinite actually precipitated at room temperature probably depends upon precipitation conditions. Thus kaolinite stability could range from poorly crystalline up to the equivalent of the kaolinite solubility line at which initial precipitation begins.

The lacustrine Sucker Creek Formation (Miocene) of eastern Oregon includes unaltered vitric tuffs as well as tuffs altered to the following diagenetic fades: (1) bentonite, (2) interbedded bentonite and opal-CT, (3) K-clinoptilolite, and (4) Ca-clinoptilolite. Bentonite beds contain Fe-rich smectite (8–10 wt. % Fe2O3), quartz, plagioclase, and Ca-clinoptilolite. Opal-CT-rich layers contain inorganic silica (opal-CT), Fe-rich smectite, and minor diatoms. K-clinoptilolite beds typically contain clinoptilolite that can be extremely K-rich (≤7.6 wt. % K2O), opal-CT, smectite, plagioclase, and K-feldspar. This diagenetic facies also includes smectitic tuff and unaltered tuff. Ca-clinoptilolite beds contain Ca-clinoptilolite, quartz, K-feldspar, smectite, and illite.

Based on its chemistry and mineralogy, the bentonite appears to have been derived from dacitic volcanic ash. Chemical considerations and the close spatial relationship between beds of bentonite and opal-CT suggest that the diagenetic alteration of glass to smectite provided silica to the adjacent opal-CT beds. Based on the presence of late-stage Ca-clinoptilolite, alteration appears to have proceeded in a relatively closed chemical system.

Based on the composition of preserved vitric tuff, the zeolitic tuffs appear to be derived from rhyolitic ash, which diagenetically altered in an open hydrologic system and produced vertical zonations in mineralogy. In this model, bentonite horizons at the top of the K-clinoptilolite diagenetic fades formed by reaction of volcanic glass with dilute fluids that had a relatively low (Na+ + K+ + Ca2+)/H+ activity ratio and ${\mathrm{a}}_{{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_4}$, whereas the underlying K-clinoptilolite beds formed from reactions between glass and dilute fluids having a higher (Na+ + K+ + Ca2+)/H+ activity ratio and ${\mathrm{a}}_{{\mathrm{H}}_4{\mathrm{S}\mathrm{i}\mathrm{O}}_4}$. Unaltered vitric ash between these beds may represent zones of higher permeability that inhibited secondary mineral alteration. Ca-clinoptilo-lite-rich beds appear to have undergone alteration similar to K-clinoptilolite-rich beds as well as to have been subjected to later, low-temperature (perhaps 75°–150°C) hydrothermal alteration which enhanced cation exchange in the zeolite and formed quartz from opal-CT.

TEM characterization of stacking relations in I/S of expanded shale samples from the Gulf Coast and Michigan Basin was carried out to address the issues of the degree of coherency and the nature of layer stacking sequences in smectite, I/S and illite. The two-dimensional lattice fringe images obtained from this study show that cross fringes are commonly observed to be continuous over at least 3–4 layers for smectite, 6–7 layers for ordered I/S and 9–10 layers for illite-rich I/S. This demonstrates that such sequences are coherent, or at least semi-coherent (in smectite) units (MacEwan crystallites). The observations demonstrate that so-called fundamental particles are fragments of MacEwan crystallites formed primarily as a result of disaggregation along weakly-bonded smectite interlayers. However, both 0k1 and h01 reflections may coexist in selected area electron diffraction (SAED) patterns. The frequency of occurrence of the coexistence in SAED patterns decreases in the order smectite, I/S and illite for Gulf Coast samples. This is consistent with the presence of turbostratically-related interfaces in packets of all of these materials. Therefore, any given layer sequence in smectite, ordered I/S and illite may have both turbostratic and coherent interfaces. The proportion of coherently-related layers increases with increasing proportion of illite-like layers. The concept of fundamental or elementary particles is not related to layer sequences in non-disaggregated, original rocks. Indeed, it implies relations that are not valid.

The lattice fringe images, SAED and optical diffraction patterns demonstrate that where layers are coherently-related, 2M1 is the dominant polytypic sequence in all samples. However, this periodic 2M1 stacking is so frequently interrupted by stacking faults in smectite that it gives rise to apparent lMd polytypism. The degree to which the periodic 2M1 sequences are interrupted by stacking faults decreases with increasing proportion of illite-like layers. The SAED patterns of I/S and illite unmodified since its formation are diffuse parallel to c* and have poorly-defined, non-periodic reflections for indices k ≠ 3N as a measure of local ordering superimposed on poorly-ordered coherent sequences with a turbostratic component. X-ray diffraction (XRD) patterns, as integrated over domains with a range of heterogeneous stacking relations, do not represent simple mixtures of discrete IM and 2M1 polytypes.

The observations of this study imply that dissolution-crystallization is a dominant mechanism for the smectite-to-illite transition. The semi-coherent stacking of smectite-like layers in smectite-rich samples implies that either a dissolution-crystallization process took place subsequent to deposition of detrital smectite or that Gulf Coast smectite is an in-situ alteration product of volcanic ash.

Orthopyroxene (En85) weathers initially by vacancy diffusion, and through this process hydration occurs and a sequence of biopyriboles develops, culminating in a talc-like layer silicate whose structure joins coherently to the orthopyroxene structure. Oxidation of Fe2+ to Fe3+ colors the altering pyroxene yellow. The ‘talc’ does not remain in structural coherence with the pyroxene after it has exceeded a few tens of nanometers in size; it is replaced by a mixture of talc and smectite. In some areas the mixture has an epitactic relation to the pyroxene, but commonly it fills faceted solution holes without crystallographic relation to the parent structure. Continued weathering extends the yellow zone at the periphery of the orthopyroxene, and the alteration product increases in smectite and decreases in 'talc’ During this stage of the reaction, MgO and SiO2 are released to form colorless true talc around the altering pyroxene. Eventually, the yellow alteration may become a smectite pseudomorph after orthopyroxene or it may be changed entirely to a mixture of vein talc and iron oxides. The complete conversion of orthopyroxene to talc plus oxides thus takes place through three sequential mineral reactions without the development of a noncrystalline phase.