Showing papers on "Clay minerals published in 1990"

X-ray Diffraction and the Identification and Analysis of Clay Minerals

Abstract: Instructor: Dr. W. Crawford Elliott, Associate Professor, Department Chair Office: 331 Kell Hall Office Phone: (404) 413-5756 E-mail: wcelliott@gsu.edu Primary Texts: X-ray Diffraction and the Identification and Analysis of Clay Minerals, by D.M. Moore and Robert C. Reynolds, Jr. (2nd Edition), 1997, Oxford University Press. Office Hours: 4-5 pm M, anytime the door is open, or by appointment. Lectures: M, W, 5:30pm – 7:10 pm, Kell 314. Laboratory: By arrangement, see below.

Ostwald ripening of clays and metamorphic minerals.

Abstract: Analyses of particle size distributions indicate that clay minerals and other diagenetic and metamorphic minerals commonly undergo recrystallization by Ostwald ripening. The shapes of their particle size distributions can yield the rate law for this process. One consequence of Ostwald ripening is that a record of the recrystallization process is preserved in the various particle sizes. Therefore, one can determine the detailed geologic history of clays and other recrystallized minerals by separating, from a single sample, the various particle sizes for independent chemical, structural, and isotopic analyses.

Soil colloids and their associations in aggregates.

Abstract: I: Soil Inorganic Colloids.- 1 Structure, Crystal Chemistry, and Origin of the Phyllosilicate Minerals Common in Soil Clays.- 2 Some Properties of Clays and of other Soil Colloids and their Influences on Soils.- 3 Some Properties of Soil and Synthetic Iron Oxides.- 4 Some Observations on the Formation and Transformation of Iron Oxides.- 5 Interaction of Stable and Metastable Monomeric Iron(III) Species with a Kaolinitic Soil Clay.- 6 The Use of Moessbauer Spectroscopy in the Study of Soil Colloidal Materials.- 7 Soil Manganese Oxides.- 8 Structures and Genesis of Allophanes and Imogolite and their Distribution in Non-Volcanic Soils.- 9 Organo-alumino Polymer Associations and their Significance in Soil and Environmental Sciences.- II: Soil Organic Colloids.- 10 Genesis, Isolation, Composition and Structures of Soil Humic Substances.- 11 Composition, Origins, Structures, and Reactivities of Soil Polysaccharides.- 12 Microorganisms, Enzymes and Soil Colloid Surfaces.- III: Water and Clays.- 13 Structure and Dynamics of Water at Clay Surfaces. Inferences from Neutron Scattering Studies.- 14 Behaviour and Microstructure of Clay Minerals.- IV: Soil Aggregates.- 15 Characterisation of the Sand, Silt, and Clay Fractions of some Mollisols.- 16 Interparticle Forces in Relation to the Stability of Soil Aggregates.- 17 Associations of Colloids in Soil Aggregates.- 18 Soil Aggregates - Formation and Stability.- V: Soil Conditioners and Soil Aggregates.- 19 Applications of Polymeric Substances as Physical Soil Conditioners.- 20 Applications of Soil Conditioners for Agriculture and Engineering.- 21 Erosion Control in the Tropics.- Author Index.

Stabilizing Compacted Clay against Chemical Attack

Abstract: Concentrated organic chemicals have been shown to cause large increases in the hydraulic conductivity of compacted clay. Mechanical and chemical methods of stabilizing four different types of compacted clay against chemical attack are investigated. Mechanical stabilization using large compactive effort (modified Proctor compaction) or application of a compressive stress ≥\N 10 psi (70 kPa) is found to render a compacted clay invulnerable to attack by concentrated organic chemicals under laboratory-test conditions. Attapulgite, a clay mineral having little electrical charge, was found to be relatively unaffected (compared to more common clay minerals such as kaolinite, illite, and smectite) by concentrated organic chemicals. Addition of approximately 7% (by weight) of lime, portland cement, or lime plus sodium silicate greatly improved the ability of compacted clay to resist attack by concentrated organic chemicals; in some cases the amended soils were less permeable to concentrated organic chemicals than the unamended soils were to water.

Buffer capacity and lead retention in some clay materials.

Abstract: The link between buffer capacity and the ability of clays to retain Pb in interactions between Pb contaminant leachate and the clays, has been studied, using a natural clay from Quebec and laboratory-prepared clays (kaolinite, illite, and smectite). The retention of Pb in the clay suspensions as they received increasing amounts of acid was investigated, and the results examined in terms of the buffer capacity of the clays. As the clays receive increasing amounts of acid (e.g. akin to addition of acid leachate), high amounts of Pb can be retained if the buffer capacity prevents the pH from dropping to values where precipitation mechanisms are not operative. The high carbonate content in the illite gave it a higher buffer capacity than the smectite and natural clay, and permitted it to retain high amounts of Ph. High Pb uptake by the clays can affect the buffer capacity and the Pb retention capacity.

Clay mineral authigenesis in coal and shale from the anthracite region, Pennsylvania

Abstract: Textural, chemical, and mineralogical analyses of authigenic clays in anthracite-rank coal and associated shale from eastern Pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region. Minerals in anthracite occur in distinct assemblages associated with the following microenvironments: coal matrix, two orthogonal joint sets (termed systematic and nonsystematic cleat), and a third joint set. Mineralogical differences among microenvironments allow inferences about clay-mineral origins and the parameters that controlled mineral authigenesis. Kaolinite occurs in the shale, and all microenvironments within the coal seam. Authigenic minerals that replaced preexisting kaolinite during the latest stage ofcoalification (anthracitization) at T > 200 'C include NHu-rich illite, pyrophyllite, and the following minerals, which are primarily restricted to the systematic cleat set: tosudite (Rl-ordered, dioctahedral, mixed-layer chlorite/smectite), sudoite (di, trioctahedral chlorite) and rectorite (Rlordered, mixedJayer paragonite/smectite). Alteration of smectite to illite may be responsible for formation of authigenic illite and Na-bearing illite, which are present only in the shale and coal matrix, and Fe-, Al-rich chlorite and quartz in the third joint set. The chemical components for authigenesis appear to have multiple sources: AI and Si from preexisting kaolinite and quartz, N from local organic matter, Mg and Na (for tosudite, sudoite, and rectorite) largely from metasomatic hydrothermal fluids, Fe and Mg for Fe-, Al-rich chlorite from smectite illitization. Minerals in the shale, the coal matrix, and the nonsystematic cleat set are interpreted to represent authigenesis in a low-permeability environment (closed-system alteration); however, the assemblage sudoite * tosudite * rectorite in the systematic cleat set is interpreted to be the result ofone or two stages ofhydrothermal alteration (open-system alteration). We suggest hat differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the Alleghanian orogeny. Hydrothermal alteration may be related to large-scale basinal flow induced by Alleghanianage uplift; such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank ofcoalification in this region.

Wettability of Clay Minerals

Abstract: Clay minerals, hydrated oxides, and hydroxides, mainly of Si, Al, Fe, and Mn, and some organic macromolecules are responsible for most wettability properties of geologic systems that constitute the sedimentary cycle. They are the soil components that exert the dominant influence on chemical and physical properties of a soil, and a detailed examination of the nature of their wettability by water is essential for a complete understanding of a soil’s potential with respect to agricultural and engineering applications.(1–3) The clay minerals, which generally constitute the greatest part of the colloid fraction in these systems, are named after the term used by sedimentologists and soil scientists for the fraction of particles having a very small size, with an equivalent diameter smaller than 2 μm, the “clay fraction.” Although most clay minerals occur as particles too small to be resolved by the ordinary microscope, x-ray diffraction analysis shows that most of them, even in their finest size fraction, are composed of crystalline particles and that the number of crystalline minerals likely to be found is limited. Furthermore, a wide distribution of particle size is frequently present and certain clay deposits contain well-defined crystalline particles with diameter greater than 2 μm.

Flocculation of Reference Clays and Arid-Zone Soil Clays

Abstract: The effect of electrolyte concentration, exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR), and pH on the flocculation-dispersion behavior of reference kaolinite, reference montmorillonite, reference illite, and the clay fraction of three arid-zone soils was investigated. The clay mineralogy of the soils was dominated by either montmorillonite, kaolinite, or illite. The clays were Na- or Ca-saturated and freeze-dried before use. Critical coagulation concentrations (CCCs) were investigated in the range of pH 5.5 to 9.5, ESP of (here equated with percent Na clay) 0, 5, 10, 20, 40, 60, 80, and 100, SAR of 0, 5, 10, 20, 40, 60, 80, and «, and using electrolyte concentrations of 0 to 500 mmolc L-'. The CCC values increased with increasing ESP, increasing SAR, and increasing pH. The pH dependence of kaolinite and illite was greater than that of montmorillonite at all ESPs and SARs. The CCC behavior of the soil clays resembled that of illite, especially in its pH dependence at ESP 100 and SAR ». The CCC values of the soil clays, kaolinite, and illite showed a sharp increase at high ESP and SAR, whereas montmorillonite showed a sharp increase in CCC at low ESP and SAR. The CCCs for all soil clays were similar in magnitude, despite their differing clay mineralogies. The CCCs for the reference clays were much lower than those for the soil clays, indicating that extrapolation from reference-clay results is not possible, and that additional factors such as organic-matter content and Al- and Fe-oxide content may influence the dispersion of soil clays.

Behaviour and microstructure of clay minerals.

Abstract: Clays have several important influences on the properties and the performance of soils, and one of the most important of these is associated with their effects on the water holding capacity of the soil For soils with appreciable contents of clays, the uptake and the release of water gives rise to changes in bulk volume (swelling and shrinking), and this alters the clay microstructure at different levels of organisation The nature and the quantities of the clays in any soil are of fundamental importance to the behaviour of the soil, but this behaviour is strongly influenced also by the climatic conditions, the soil organisms, the topography, and the length of time for soil genesis

Aspects of climatic influence on the clay mineralogy and geochemistry of soils, palaeosols and clastic sedimentary rocks.

Abstract: Climate strongly influences soil clay mineralogy and bulk chemical composition. In humid climates chemical weathering is at its most intense and virtually all ‘primary’ bedrock minerals suffer complete breakdown. They are replaced by authigenic hydrated oxides of Fe and Al together with halloysite and kaolinite in deep profiles. These minerals develop where the residence time of water in the soil is slight. Longer residence times lead to higher solute concentrations and the stabilization of smectite. Authigenic palygorskite and sepiolite may form in highly saline, alkaline soils. The stability of these precipitated minerals is problematic: they may be metastable or even unstable. All other clays found in soils are unstable, being incomplete, solid-state hydrolysis products of substrate minerals. The balance between stable, metastable and unstable minerals and the rate of profile development are both much influenced by temperature, rainfall and activities of soil biota; themselves reflecting climate. Bedrock influence is greatest under conditions of low to moderate precipitation where unstable minerals persist. Local topography, however, also strongly influences both the rate and products of chemical weathering, quite independently of climate. On the grand scale, tectonic factors may constrain the time available for profiles to develop, hence the extent to which bedrock minerals survive or are altered. Mountain soils reflect local climate and bedrock composition much more than latitude. The sediments delivered to major basins today do not reflect global soil distribution patterns. The bulk composition of suspended sediment entering the Indian Ocean, for example, is rather similar to that entering the Arctic Ocean. Sediment dispersal patterns can also confuse climatic signals: a problem not encountered with palaeosols. Authigenic clays formed within sedimentary basins and post-depositional (burial) alteration of both sediments and buried soil profiles must lead to interpretational ambiguities. Climatic signals are likely to survive best in bulk compositional data (although diagenetic reactions at depths in excess of about 2 km do not appear to be isochemical) and in the distribution of more stable yet climatically sensitive minerals (kaolinite as opposed to illite, chlorite).

Lateritic weathering of pyroxenites at Niquelandia, Goias, Brazil: the supergene behavior of nickel.

Abstract: Two typical lateritic weathering profiles (Jacuba ahd Angiquinho) from the Niquelandia Ni deposits, Brazil, were studied in order to establish the petrological relationship between the supergene Ni products and the parental pyroxenes. Frbm the base to the top of the profiles, pyroxenes are replaced by goethite and kaolinite through a series of transitional Ni-bearing phyllosilicates. The mineralogy and the chemical composition (especially the Ni content) of these clay minerals depends on the degree of fracturing and serpentinization of the pyroxenite and the location of the pyroxenite with respect to neighboring dunite. Within the Jacuba profile, smectite and pimelite pseudomorphs after pyroxene are especially Ni rich, and in fact, are the most Ni-enriched clay minerals now known in lateritic weathering profiles.

Mineralogy of the rhizosphere in forest soils of the eastern United States

Abstract: Chemical and mineralogical studies of forest soils from six sites in the northeastern and southeastern United States indicate that soil in the immediate vicinity of roots and fine root masses may show marked differences in physical characteristics, mineralogy and weathering compared to the bulk of the forest soil. Examination of rhizosphere and rhizoplane soils revealed that mineral grains within these zones are affected mechanically, chemically and mineralogically by the invading root bodies. In SEM/EDS analyses, phyllosilicate grains adjacent to roots commonly aligned with their long axis tangential to the root surface. Numerous mineral grains were also observed for which the edge abutting a root surface was significantly more fractured than the rest of the grain. Both the alignment and fracturing of mineral grains by growing roots may influence pedogenic processes within the rhizosphere by exposing more mineral surface to weathering in the root-zone microenvironment. Chemical interactions between roots and rhizosphere minerals included precipitation of amorphous aluminium oxides, opaline and amorphous silica, and calcium oxalate within the cells of mature roots and possible preferential dissolution of mineral grains adjacent to root bodies. Mineralogical analyses using X-ray diffraction (XRD) techniques indicated that kaolin minerals in some rhizosphere samples had a higher thermal stability than kaolin in the surrounding bulk forest soil. In addition, XRD analyses of clay minerals from one of the southeastern sites showed abundant muscovite in rhizoplane soil adhering to root surfaces whereas both muscovite and degraded mica were present in the immediately surrounding rhizosphere soil. This difference in mineral assemblages may be due to either K-enrichment in rhizoplane soil solutions or the preferential dissolution of biotite at the root-soil interface

Carbon dioxide in clastic rocks and silicate hydrolysis

Abstract: The amount of carbon dioxide in clastic rocks in sedimentary basins increases with depth. Organic matter, dissolved organic species, and dissolution of carbonate minerals have been suggested as sources of CO{sub 2}, which increases in abundance with depth and temperature. Isotopic compositions of aqueous HCO{sub 3}{sup {minus}}, gaseous CO{sub 2}, and calcite from conventional hydrocarbon wells and steam-assisted recovery of heavy oil suggest that calcite, or other carbonate minerals, are the source of CO{sub 2}, particularly at higher temperatures. Detailed examination of the stability of diagenetic minerals such as kaolinite, smectite, and analcime, among others, relative to the coexisting waters, indicates that silicate hydrolysis is the driving force for dissolution of carbonate minerals and the ultimate source of abundant CO{sub 2} in clastic rocks in diagenetic environments at temperatures over approximately 100 C.

Spectroscopic characterization of minerals and their surfaces

Abstract: Spectroscopic Characterization of Minerals and Their Surfaces: An Overview Thin-Film Elemental Analyses for Precise Characterization of Minerals High-Resolution Transmission Electron Microscopy Applied to Clay Minerals Skeletal Versus Nonbiogenic Carbonates: VN-IR (0.3-2.7- m) Reflectance Properties Photoluminescence, Candoluminescence, and Radical Recombination Luminescence of Minerals Three-Dimensional Thermoluminescence in Minerals Induced Thermoluminescence and Cathodoluminescence Studies of Meteorites: Relevance to Structure and Active Sites in Feldspar Emission of Particles and Photons from the Fracture of Minerals and Inorganic Materials What Excites Triboluminescence? ]5]7Fe-Bearing Oxide, Silicate, and Aluminosilicate Minerals: Crystal Structure Trends in Mossbauer Spectra Crystal Chemistry, Electronic Structures, and Spectra of Fe Sites in Clay Minerals: Applications to Photochemistry and Electron Transport Dissociation and Recombination of Positive Holes in Minerals Variable Oxidation States of Iron in the Crystal Structure of Smectite Clay Minerals Surface Activities of Clays Probing the Surface of Clays in Aqueous Suspension by Fluorescence Spectroscopy of Proflavine Multinuclear Magnetic Resonance Studies of Structure and Dynamics at the Interface of Clay Materials Near-Infrared Correlation Spectroscopy: Quantifying Iron and Surface Water in a Series of Variable Cation-Exchanged Montmorillonite Clays Raman and FT-IR Spectra of the Kaolinite-Hydrazine Intercalate Reactive Cr-O Sites: Catalytic Properties of Chromia-Pillared Montmorillonite and Preliminary Study Results

Hydrothermal alteration in anthracite from eastern Pennsylvania: Implications for mechanisms of anthracite formation

Abstract: Orthogonal joint sets (cleat) in anthracite-rank coal beds from eastern Pennsylvania contain two mineralogically and chemically different authigenic clay mineral assemblages In localities from all four anthracite fields, the systematic cleat contains significant quantities (20 to 95 wt%) of authigenic rectorite, sudoite, or tosudite, which are clay minerals primarily associated with hydrothermal veins and ore deposits The strike of systematic cleat in this region roughly parallels the inferred direction of lateral compressive stress during the Alleghany orogeny The opposing nonsystematic cleat contains mostly authigenic NH 4 -illite, a clay that also occurs in the coal matrix All of these authigenic clay minerals formed during anthracitization (T> 200 °C) by replacement of kaolinite and quartz, both of which formed during an earlier stage of coalification and are present in the joints and matrix of nearly all coal samples These distinct mineralogical-structural relations are observed throughout the Anthracite region and suggest that (1) mineralogical variations in the coal joints are related primarily to permeability variations that were controlled by the orientation of Alleghanian stress fields, (2) hydrothermal fluids passed through the systematic cleat during anthracitization, and (3) hydrothermal alteration influenced diagenesis in this region Hydrothermal alteration may be related to basin-wide fluid migrations that were driven by Alleghanian-age uplift Basinal fluid flow, concentrated along permeable joints and detachment zones, could have efficiently transported heat from depth and thereby increased the rate of coalification and decreased the postulated minimum depths at which anthracitization occurred (perhaps ≤5km vs 6-10 km)

Influence of climate and eolian dust on the major-element chemistry and clay mineralogy of soils in the northern Bighorn basin, U.S.A.

Abstract: Soil chronosequences in the northern Bighorn basin permit the study of chronologic changes in the major-element chemistry and clay mineralogy of soils formed in different climates. Two chronosequences along Rock Creek in south-central Montana formed on granitic alluvium in humid and semiarid climates over the past two million years. A chronosequence at the Kane fans in north-central Wyoming formed on calcareous alluvium in an arid climate over the past 600,000 years. Detailed analyses of elemental chemistry indicate that the soils in all three areas gradually incorporated eolian dust that contained less zirconium, considered to be chemically immobile during weathering, than did the alluvium. B and C horizons of soils in the wettest of the chronosequences developed mainly at logarithmic rates, suggesting that leaching, initially rapid but decelerating, dominated the dust additions. In contrast, soils in the most arid of the chronosequences developed at linear rates that reflect progressive dust additions that were little affected by leaching. Both weathering and erosion may cause changes with time to appear logarithmic in A horizons of soils under the moist and semiarid climatic regimes. Clay minerals form with time in the basal B and C horizons and reflect climatic differences in the three areas. Vermiculite, mixed-layer illite-smectite, and smectite form in the soils of the moist-climate chronosequence; smectite forms in the semiarid-climate chronosequence; and smectite and palygorskite form in the arid-climate chronosequence.

Distribution and geochemical characteristics of metal enrichment in the New Albany Shale (Devonian-Mississippian), Indiana

Abstract: Black shale beds in the uppermost part of the Clegg Creek Member of the New Albany Shale (Devonian-Mississippian) in southern Indiana have organic C contents from approximately 12 to 25 wt percent and are enriched in Cu, Pb, Zn, Ni, Cd, Mo, and V. Brownish-black to olive-black shale located lower in the formation has normal metal contents and organic C abundances up to 12 wt percent. Shale units lower in the section were deposited under anaerobic conditions, but C-S-Fe relationships and sulfur isotope values suggest that the H 2 S-O 2 boundary occurred near or at the sediment-water interface. Conversely, the carbon- and metal-rich beds near the top of the formation were deposited under strongly euxinic conditions with the chemocline located within the water column. Metal sulfide formation was limited by the availability of metals. A sharp increase in slope at 12 to 14 wt percent C organic is observed in organic C-metal plots and is related to organic carbon preservation and metal adsorption due to the presence of an H 2 S-bearing water column.Sequential chemical analyses and electron microprobe studies indicate that Zn, Cu, and Cd are held in sulfide minerals, Pb as a selenide, Mo both as a sulfide ( approximately 56-85%) and within kerogen (20%), Ni within kerogen (50%) and as a sulfide (45%), and V primarily within clay minerals. Petrographic studies also indicate that the beds in the upper part of the Clegg Creek Member are characterized by the highest proportion of inertinite and vitrinite found in the New Albany Shale.A conceptual model is developed whereby trace metals adsorbed on surfaces of organic material were preserved from release by oxidative degradation in the water column due to the high elevation of the H 2 S-O 2 boundary. Transition metal contents of the Clegg Creek Member may have been locally high due to an increase in the influx of terrestrial organic matter. Once delivered to the sea floor, metals took part in diagenetic reactions that produced metal sulfides and vanadiferous clays. Only Mo and Ni remained significantly enriched in kerogen. Although Ni and V contents are high in the soluble organic material from the shale, their abundance constitutes less than 1 percent each of the whole-rock metal content.

Chemical and biochemical weathering of pyritic mudrocks in a shale embankment

Abstract: Following the partial collapse of a shale embankment in Derbysllire, a detailed mineralogical, chemical and electron microscopical investigation was carried out to establish whether the crushed mudstone fill had been significantly affected by chemical weathering after emplacement. Samples of unweathered and weathered Namurian mudrocks from the area were also analysed for comparison and laboratory experiments carried out to simulate abiotic and biochemical shale weathering. It is concluded that chemical alteration of the shale is a rapid process and that significant post-emplacement chemical and mineralogical changes occurred in the embankment fill. The main reactions involve oxidation of framboidal pyrite, leaching of carbonates by sulphuric acid, formation of authigenic sulphate and iron hydroxide minerals, and leaching of cations from clay minerals. In parts of the fill forming the outer shoulders of the embankment, up to 10lume loss may have occurred due to acid leaching.

Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements : implications for the adsorption of organic solutes from aqueous media

Abstract: The apolar and the polar (electron-acceptor and electron-donor, or Lewis acid-base) surface tension components and parameters of solid surfaces can be determined by contact angle measurements using at least three different liquids, of which two must be polar. With swelling clay minerals (e.g. smectite clay minerals), smooth contiguous membranes can be fabricated, upon which contact angles can be measured directly. With non-swelling clay minerals (e.g. talc), contact angles can be determined by wicking, i.e. by the measurement of the rate of capillary rise of the liquids in question through thin layers of clay powder adhering to glass plates. The apolar and polar (acid-base) surface tension components and parameters thus found for various untreated and quaternary ammonium base-treated clays allowed the determination of the net interfacial free energy of adhesion of human serum albumin onto the various clay particle surfaces immersed in water. The free energies of adhesion, thus found, correlate well with t...

Clay mineral formation at the continent-ocean boundary; the verdine facies

Abstract: The verdine facies, previously confused with components of ancient ironstones, has been studied in relation to its distribution, mineralogy, genesis, environment, and geological significance. Several chemically specific components are present including (i) a ferric and magnesian dioctahedral-trioctahedral 1 : 1 clay mineral, (ii) a chemically equivalent chlorite, (iii) a component intermediate between smectite and swelling chlorite, and (iv) a problematic low-temperature pyrophyllite-like clay mineral. The genetic mechanism is a series of crystallogenetic reactions using cations from the previous phase; this model, where a mineral phase is shown to be a temporary state of arrangement of cations, suggests that authigenic “marine” minerals may reflect diagenetic geochemical factors different from those in the original sea.

Surface Properties and Clay Mineralogy of Hydrated Halloysitic Soil Clays. II: Evidence for the Presence of Halloysite/Smectite (H/Sm) Mixed-Layer Clays

Abstract: A B S TRACT: Six clays from volcanic ash soils at different stages of weathering differ in their relative halloysite content with respect to kaolinite and several surface properties, namely CEC, and exchange selectivity for K +. These three parameters are related to each other in that they all decrease with increasing soil weathering stage. XRD data show that the hydrated 1 : 1 layer-silicates in these clays combine with smectite to form interstratified H/Sm clay minerals. In these mixed-layers, the content and layer charge of smectitic units decrease as the relative halloysite content in the clay decreases. These clays thus depict a weathering sequence that is parallel to the weathering sequence of the soils from which they originate. It is also shown that the smectites in the H/Sm minerals have the distinctive composition and ESR spectrum of Fe-rich 2:1 clay minerals belonging to the beidellite-nontronite series. The information obtained explains why these clays have high CEC and distinct affinities for K + . It is hoped that this study will help to clarify the controversy concerning the CEC and related surface properties attributed to hydrated halloysite.

Weathering of biotite into dioctahedral clay minerals

Abstract: A B S T R A C T : Changes which occur during the natural weathering of biotite in granite gneiss and associated soils are measured by microanalysis and illustrated by SEM. Biotite weathers through a series of interstratified minerals to vermiculite and/or smectite phases which decompose rapidly to kaolinite. Both vermiculite and smectite phases appear to be dioctahedral, on the basis of chemical compositions derived from microprobe data. Weathering products are first apparent on the edges of laminae, where interstratified minerals are formed at right angles to both the edge face and the cleavage. Weathering soon develops along cleavage planes, initially most strongly near the edges of flakes, but then permeating extensively into the body of flakes and subdivided segments. The orientation of interstratified minerals and kaolinite within cleavages is parallel to the cleavage. Oxidation of Fe in biotite causes internal stresses which are relieved by physical deformation of the crystals. This accelerates chemical decomposition, particularly along cleavage planes. At an advanced stage of weathering when decomposition is active at many cleavages, biotite deteriorates to very finely divided, wafery remnants consisting of thin laminae separated by more open layers of particulate clay. Parts of the thin laminae remain relatively unweathered and have the same chemical composition as the original biotite immediately after oxidation. These relatively unweathered layers within the laminae have X-ray diffraction (XRD) characteristics of trioctahedral illite and they persist when the biotite remnants are broken up into clay (Fordham, 1989b). Other decomposition products are also formed. Some occur in sufficient bulk (at the microanalytical scale) to be identified directly by microprobe, but the chemical composition of others must be estimated by graphical procedures (Fordham, 1989a). These additional decomposition products of biotite are the subject of the present paper. Their formation in different parts of biotite flakes taken from different locations within the soil profile is described, and the weathering sequence as a whole is discussed. Microanalyses were performed on both thin sections and whole specimens, and the products are illustrated by scanning electron microscopy (SEM).