Showing papers on "Clay minerals published in 1995"

Rapid Clay Mineral Formation in Amazon Delta Sediments: Reverse Weathering and Oceanic Elemental Cycles

Abstract: Formation of aluminosilicate minerals in marine sediments was proposed over 30 years ago as a potentially important control on the chemistry of the oceans. Until now, this reverse weathering process has been largely discounted because of insufficient direct evidence for its existence. Experiments with unaltered, anoxic, Amazon delta sediments showed that substantial quantities of K-Fe-Mg clay minerals precipitated on naturally occurring solid substrates over times of ∼12 to 36 months at ∼28°C. A range of pore-water, solute-flux, and solid-phase criteria indicates that comparable clay mineral precipitation processes occur throughout Amazon shelf sediments, contributing ≳3 percent of the weight of the deposits and consuming ∼10 percent of the global riverine K+ flux.

Clay mineral thermometry : a critical perspective

Abstract: Diagenetic clay minerals usually occur as heterogeneous assemblages of submicroscopic layers consisting of different structure types such as illite, smectite and chlorite, with variable composition within a given structure type, and with highly variable concentrations of imperfections. The dimensions of mixed-layering, the semi-coherent to coherent nature of the structures across the layering, and compo- sitional heterogeneity occur at a scale well below that of an individual thermodynamic phase. These relations imply that most clays are not distinct minerals or phases, and that assemblages of clays in shales and mudstones are incompatible with the phase rule. Such relations are better evaluated in terms of the formation of metastable materials with each small unit having unique chemical properties, rather than as a small number of stable homogeneous phases. Consequently, treatment of most clay minerals in terms of equilibrium stability with either a thermodynamic or experimental approach is subject to error. Chemical reactions involving most clay minerals are best understood with kinetic models. These involve a great variety of parameters such as time, fluid/rock ratio, deformation history, nature of starting materials and transformation mechanisms, as well as the variables, such as temperature, pressure and composition, that are commonly used to define equilibrium. Solubility experiments on the stabilities of clay minerals are unlikely to attain equilibrium at low temperatures. Moreover, the activity of soluble species may be controlled by surface equilibria, or by absorbed or exchangeable cations. Interpretations of available experiments on the solubility of illite vs. other mineral assemblages are in violation of Schreinemakers' rules and indicate lack of equilibrium. Predictable sequences of clay minerals as a function of temperature are best understood through the Ostwald step rule, in which clay mineral assemblages undergo reactions in response to kinetic factors that represent reaction progress rather than an approach to equilibrium. Currently used clay mineral ther- mometers (illite crystallinity, smectite/illite reaction, chlorite composition) are not based on equilibrium reactions. Such systems are not accurate thermometers and therefore have questionable utility.

The effects of cultivation on the composition of organic matter and structural stability of soils

Abstract: Soil samples were obtained from the surface horizons of five untilled sites and adjacent sites under short- and long-term cultivation. The soil samples were fractionated based on density and organic materials were concentrated in various fractions which enabled comparative chemical composition of the organic materials in cultivated and uncultivated sites by solid-state C-13 CP/MAS NMR spectroscopy. Changes in the nature of organic carbon with cultivation were different in different soils and resulted from variations in the chemistry of carbon inputs to the soils and a greater extent of decomposition of organic materials in cultivated soils. Differences in the chemical composition of organic carbon between cultivated and uncultivated soils resided mostly in organic materials occluded within aggregates, whereas the chemistry of organic matter associated with clay particles showed only small changes. The results indicate a faster decomposition of O-alkyl C in the cultivated soils. Wet aggregate stability, mechanically dispersible clay and modulus of rupture tests were used to assess the effects of cultivation on structural stability of soils. In four of five soils, the virgin sites and sites which had been under long-term pasture had a greater aggregate stability than the cultivated sites. Neither total organic matter nor total O-alkyl C content was closely correlated with aggregate stability, suggesting that only a part of soil carbon or carbohydrate is involved in aggregate stability. The fractions of carbon and O-alkyl C present in the form of particulate organic matter occluded within aggregates were better correlated with aggregate stability (r = 0.86** and 0.88**, respectively). Cultivation was not the dominant factor influencing water-dispersible clay across the range of soil types used in this study. The amount of dispersible clay was a function of total clay content and the percentage of clay dispersed was controlled by factors such as clay mineralogy, CaCO3 and organic matter content of soils. The tendency of different soils for hard-setting and crusting, as a result of structural collapse, was reflected in the modulus of rupture (MOR). The cultivated sites had significantly higher MOR than their non-tilled counterparts. The soils studied had different MOR due to differences in their physical and chemical properties.

Formation of Clay Minerals in Hydrothermal Environments

Abstract: Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type

Abstract: Summary The effect of changing pH and electrolyte concentration on the dispersion and zeta potential of Na-and Ca-forms of kaolinite, illite and smectite was investigated in relation to changes in their net negative charge The percentage of dispersible Na-clay and the percentage increase in net negative charge was positively correlated with pH, but the slopes varied from clay to clay In general, the net negative charge was the primary factor in clay dispersion, and the pH affected clay dispersion by changing the net charge on clay particles Na-smectite had larger net charge at all pHs than Na-illite and Na-kaolinite, and it always had larger flocculation values The role of electrolyte concentration could be due to its effect both on flocculation and variable charge component of the clay minerals The zeta potential at different pHs also reflected the same trend of clay dispersion with net particle charge In Ca-clays the trends were similar to Na-clays up to pH 70 In more alkaline solution CaCO3 formation led to charge reduction on clay particles, resulting in flocculation and reduction of zeta potential At similar pHs the electrophoretic mobilities of all the clays showed constant potential behaviour However, the zeta potentials of Ca-clays were always smaller than those of sodic clays because the clays were more aggregated Net particle charge was the most important factor in controlling clay dispersion for the whole range of pH and ionic strength and for all types of cations

Contribution of organic matter and clay minerals to the cation exchange capacity of soils.

Abstract: The cation exchange capacity (CEC) at pH 7 was measured for samples of 347 A horizons and 696 B horizons of New Zealand soils. The mean CEC was 22.1 cmolc/kg for the A horizons and 15.2 cmolc/kg for the B horizons. Multiple regressions were carried out for CEC against organic carbon (C), clay content, and the content of seven groups of clay minerals. The results, significant at p <0.001, were consistent with most of the CEC arising from soil organic matter. For the samples of A horizon, the calculated CEC was 221 cmolc/kg per unit C and for the B horizons was 330 cmolc/kg C. There was also a contribution from sites on clay minerals. Multiple regression indicated that smectite had a higher CEC (70 cmolc/kg) than other minerals but it was not as high as that of type smectites; kaolin minerals had the lowest CEC. There was a significant effect of interaction between organic matter and some clay minerals on the CEC. Samples from B horizons containing allophane had lower CEC than those not containing ...

Fourier Transform Raman Spectroscopy of Kaolinite, Dickite and Halloysite

Abstract: The vibrational modes of clay minerals are uniquely accessible to FT Raman spectroscopy, but this potentially powerful technique has found limited application to the study of clay mineral structure. Raman spectra in the 50 to 3800 cm-t region were obtained for a number of kandite clays. The kandite clay minerals are characterised by relatively intense bands centred at 142.7 cm -I for kaolinite, 143 cm -~ for halloysite and 131.2 cm-I for dickite with prominent shoulders at 129, 127, and 120 cm- 1 respectively. These vibrational modes are attributed to the O-A1-O and O-Si-O symmetric bends. Differences in the lattice modes for the kandite clay minerals in the 200 to t200 cm -I were obtained. Four OH bands were obtained for kaolinite 3621, 3652, 3668, and 3695 cm-1; three OH bands were found for a selection of dickites and halloysites. The San Juan Dickite and the Eureka Halloysite show further resolution of the low frequency 3620 cm -1 hydroxyl band. This splitting is attributed to variation in the position of the inner hydroxyls. Variation in band intensity and position was found to be sample dependent.

Origin of Clays by Rock Weathering and Soil Formation

Abstract: It is a fact that mankind’s domain of influence at the surface of the planet is roughly that of clay mineral formation: soils, weathered rocks, diagenetic series, continental and marine sediments, geothermal fields. These clay resources have been exploited since the discovery of fire. It is now important, for environmental studies, to know as well as possible, how and where these minerals form. Curiously, among the numerous works published until now, only a few are devoted to the mechanisms of clay formation at the scale of a soil profile, i.e. the metric scale in temperate zones. Indeed, more is known at the scale of a country (km) or the mineral-fluid interface (nm). For example, at the scale of a country, weathering can be considered as a homogeneous process. As a consequence, it is possible to model chemical transfers and clay-mineral stability fields using calculation codes. On the other extreme, the intimate dissolution-recrystallization mechanisms at the fluid-mineral interface scale are studied on isolated pure crystals in order to simplify the chemical system.

Clays in Crustal Environments: Isotope Dating and Tracing

Abstract: 1 An Introduction to Clay Minerals and Isotope Geochemistry.- 1 Fundamentals of Clay Mineralogy.- 1.1 Definition.- 1.2 Basic Structural Units and Layer Types.- 1.3 Classification of Clay Minerals.- 1.4 Thermodynamic Considerations for Clay Minerals.- 1.5 Clay Separation and Characterization.- 2 Principles of Isotope Geochemistry.- 2.1 Fundamentals of Isotope Geochemistry.- 2.2 Radiogenic Isotope Geochemistry.- 2.3 Stable Isotope Geochemistry.- 3 Specific Aspects of Clay Isotope Geochemistry.- 3.1 Retentivity of Radiogenic Argon.- 3.2 Effects of Mechanical Treatments.- 3.3 Effects of Chemical Treatments.- 3.4 Significance of the Leachates.- 4 Summary.- 2 Isotope Geochemistry of Clay Minerals in Continental Weathering Environments.- 1 Clay Authigenesis in Soil Profiles.- 1.1 Isotope Redistribution in Silicate Precursors During Weathering.- 1.2 Sr Isotopic Compositions of Clay Minerals Derived from Biotite and Feldspars.- 1.3 Sr Isotopic Characteristics of Fluids in Weathering Profiles.- 1.4 A Sr Isotopic Model for Clay Authigenesis in Soil Profiles.- 1.5 Stable Isotope Geochemistry of Clay Minerals from Soil Profiles.- 1.6 Experimental Clay Authigenesis and Evaluation of Mass Transfers in Soil Profiles.- 1.7 Isotopic Dating of Clay Authigenesis in Soil Profiles.- 2 Clay Weathering and Alteration in Soils.- 3. Summary.- 3 Isotope Geochemistry of Clay Minerals in Young Continental and Oceanic Sediments.- 1 Recent Continental Erosional Debris and Clay Sediments.- 1.1 Sr Isotope Geochemistry.- 1.2 Nd Isotope Geochemistry.- 1.3 Ar Isotope Geochemistry.- 1.4 Stable Isotope Geochemistry.- 2 Terrigenous Clays in Young Ocean Basins.- 2.1 Sr Isotope Geochemistry.- 2.2 Ar Isotope Geochemistry.- 2.3 Nd Isotope Geochemistry.- 2.4 Stable Isotope Geochemistry.- 3 Authigenic Clays in Young Deep-Ocean Basins.- 3.1 Isotopic Characteristics of the Present-Day Ocean Waters.- 3.2 Clay Authigenesis and Modification at Ocean Floor Conditions.- 3.3 Evolution and Paleogeography of Deep-Sea Red Clays.- 4 Isotopic Evolution of Clays in Buried Deep-Ocean Sediments.- 4.1 Isotopic Characterization of Sediment Pore Waters.- 4.2 Isotope Composition of Clays in Shallow Buried Ocean Sediments.- 5 Summary.- 4 Isotope Geochemistry of Clays and Clay Minerals from Sedimentary Rocks.- 1 Syndepositional Evolution of Clay Minerals in Argillaceous Sedimentary Rocks.- 1.1 Analyses of Whole-Rock Samples.- 1.2 Dating Sedimentation Times by Analysis of Clay Fractions.- 2 Diagenetic Evolution of Clay Minerals in Deeply Buried Shales and Sandstones.- 2.1 Diagenetic Clay Fractions in Buried Shales.- 2.2 Diagenetic Clay Fractions in Buried Sandstones...- 2.3 Comparative Evolution of Diagenetic Clay Minerals of Buried Shale-Sandstone Associations.- 2.4 Duration of Diagenetic Illite Formation.- 2.5 Reconstruction of Thermal Histories of Sedimentary Basins.- 3 Isotopic Records of Clay Diagenesis in Formation Waters.- 4 Summary.- 5 Isotope Geochemistry of Mica-Type Minerals from Low-Temperature Metamorphic Rocks.- 1 Isotope Geochemistry of Mica-Type Minerals in Different Metamorphic Environments.- 1.1 Regional Thermal Metamorphism.- 1.2 Dynamothermal Metamorphism.- 1.3 Cataclastic Metamorphism.- 1.4 Contact Metamorphism.- 1.5 Retrograde Metamorphism.- 1.6 Multiple Metamorphism.- 2 Isotope Geochemistry of Mica-Type Minerals Related to Metamorphic Rock Lithology.- 2.1 Metapelite-Metavolcanic Association.- 2.2 Metaarkose-Slate Association.- 2.3 Coal Bed-Slate Association.- 3 Isotope Geochemistry of Mica-Type Minerals Under Hydrothermal Conditions.- 3.1 Isotopic Dating of Hydrothermal Activities.- 3.2 Indirect Isotopic Dating of Hydrothermal Ore Deposits.- 4 Summary.- 6 The Frontiers of Clay Isotope Geochemistry.- 1 Isotope Geochemistry as a Dating Tool for Clay Minerals.- 1.1 Depositional Time Indicator.- 1.2 Diagenetic Time Marker.- 1.3 Provenance Tracer.- 1.4 Isotope Dates and Particle Size.- 2 Isotope Geochemistry as a Record of the Physical and Chemical Conditions of Clay Formation.- 2.1 Isotope Geochemistry and the Process of Clay Formation.- 2.2 Isotope Geochemistry as a Record of Closed Versus Open System Behavior.- 3 Can Isotope Geochemistry Elucidate the Concept of Clay Genesis?.- References.

Composition and Mineralogy of Clay Minerals

Abstract: Clay minerals were initially defined on the basis of their crystal size. They were determined as the minerals whose particle diameters were less than 2 μm. This limit was imposed by the use of the petrographic microscope where the smallest particle which could be distinguished optically was of this size. Clays were essentially those minerals which could not be dealt with in a conventional nineteenth century manner. Chemical analyses were nevertheless made of fine grain size materials, most often with good results. However, the crystal structure and mineralogical family were only poorly understood. This was mainly due to the impurities present in clay aggregates, either as other phases or in multiphase assemblages. Slow progress was made in the early twentieth century, but the advent of reliable X-ray diffractometers allowed one to distinguish between the different mineral species found in the <2 μm grain size fraction. Today we know much more about clay mineral XRD (X-ray diffraction) properties; perhaps too much at times.

Mobilization of Heavy Metals as Affected by pH and Redox Conditions

Abstract: Water is a major vector of transport of heavy metals in the lithosphere. The solids present in soils, aquifers and surface water bodies (i.e., suspended and deposited sediments in rivers, lakes, oceans) can trap significant quantities of toxic heavy metals and act as reservoirs in the various hydrocycles taking place at the earth’s surface. The geochemical phenomena controlling the retention of heavy metals are adsorption and precipitation, while dissolved complexation influences advective and dispersive transport. For all of these, pH and redox conditions are master variables controlling the potential release of stored pollutants to the aqueous phase and therefore their dispersion in the environment and their availability to biota. While the redox status of natural environments is usually expressed as Eh (potential measured at an electrode), by analogy with pH, it is sometimes useful to express redox potentials in terms of pe (-log of electron activity). In this way electrons can be treated like classical reactants and products so that both chemical and electrochemical reactions can be expressed by a single equilibrium constant. Various solids control the fixation of heavy metals: clay minerals, organic matter, oxides and hydroxides of Fe, Mn and Al for adsorption, and low solubility sulfide, carbonate and phosphate minerals for precipitation. Complexing by organic matter, also very important, is addressed in Chapter 3 (Geiger et al.).

Clay Microporosity in Reservoir Sandstones: An Application of Quantitative Electron Microscopy in Petrophysical Evaluation

Abstract: Clay mineral microporosity in sandstones is measured using computer-assisted image analysis of back-scattered electron micrographs of petrographic sections. Diagenetic kaolinite has a variety of textures with microporosity values ranging from 15 to 61%. Diagenetic chlorite has a generally uniform grain-coating texture and microporosity of about 50%. Fibrous illitic clays are difficult to characterize by the same method (an average value of 63% microporosity was recorded), but analysis of stereo-pair micrographs from scanning-electron microscopy analyses reveals that illite commonly has microporosity of approximately 90%. Clay microporosity data are used to calculate effective pore volumes and volumes of clay-bound water for clay minerals in sandstones. Converting from wei ht percent clay to volume percent clay is important. Microporosity data are valuable input to Vshale evaluation where water saturation is associated with clay mineral type, texture, and volume.

Depositional and post-depositional geochemistry, and its correlation with the geotechnical properties of marine clays in Ariake Bay, Japan

Abstract: The mineralogy, geochemistry and geotechnical properties of clay samples in Ariake Bay, Japan are presented. The mineralogy and geochemistry are discussed in terms of depositional and post-depositional processes, and the correlations of the clay mineralogy and geochemistry with the geotechnical properties were examined by multiple regression analysis. The most predominant clay mineral in the deposits was found to be smectite. The vertical distribution of trace elements in the deposits was indicative of the periodical changes in the parent materials involved in the depositional processes. The chloride concentration in the pore water varied, according to a change in the sea level, from brackish to marine, and then brackish towards the surface. The higher iron oxide contents near the surface and in the deepest zones were the results of pyrite oxidation in the deposits due to weathering under subaerial conditions. Smectite content was the most predominant governing factor for the consistency limits and activi...

Hematite, pyroxene, and phyllosilicates on Mars: Implications from oxidized impact melt rocks from Manicouagan Crater, Quebec, Canada

Abstract: Visible and near-IR refectivity, Moessbauer, and X ray diffraction data were obtained on powders of impact melt rock from the Manicouagan Impact Crater located in Quebec, Canada. The iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples. Phyllosilicate (smectite) contents up to approximately 15 wt % were found in some heavily oxidized samples. Nanophase hematite and/or paramagnetic ferric iron is observed in all samples. No hydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250 C. Oxidative alteration is thought to have occurred predominantly during late-stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near-IR band minimum correlated with the extent of aleration Fe(3+)/Fe(sub tot) and ranged from approximately 1000 nm (high-Ca pyroxene) to approximately 850 nm (bulk, well-crystalline hematite) for least and most oxidized samples, respectively. Intermediate band positions (900-920 nm) are attributed to low-Ca pyroxene and/or a composite band from hematite-pyroxene assemblages. Manicouagan data are consistent with previous assignments of hematite and pyroxene to the approximately 850 and approximately 1000nm bands observed in Martian reflectivity spectra. Manicouagan data also show that possible assignments for intermediate band positions (900-920 nm) in Martian spectra are pyroxene and/or hematite-pyroxene assemblages. By analogy with impact melt sheets and in agreement with observables for Mars, oxidative alteration of Martian impact melt sheets above 250 C and subsequent erosion could produce rocks and soils with variable proportions of hematite (both bulk and nanophase), pyroxene, and phyllosilicates as iron-bearing mineralogies. If this process is dominant, these phases on Mars were formed rapidly at relativly high temperatures on a sporadic basis throughout the history of the planet. The Manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition, which is also indicated for Mars from the analyses of soil at the two Viking landing sites.

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley, Juan de Fuca)

Abstract: We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24 A stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14 A with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300° C±30° for chlorite-quartz at 32 m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.

Sepiolite-palygorskite from the Hekimhan region (Turkey)

Abstract: Upper Cretaceous-Tertiary marine clayey-calcareous rocks of the Hekimhan basin contain fibrous clay minerals in significant amounts. Ophiolitic rocks in the provenance area have contributed the elements to form the clay minerals. XRD, SEM, major, trace and REE analyses were applied to samples taken from several stratigraphic sections. Diagenetic minerals such as smectite, dolomite, calcite, gypsum, celestite and quartz/chalcedony are associated with sepiolite-palygorskite group clays. Trace and rare earth elements (REE) are more abundant in palygorskite than sepiolite. REE abundances in the sepiolite-palygorskite are characterized by negative Eu and positive Nd anomalies when normalized with respect to chondrite and shale. Sepiolites with sharp XRD peaks are formed by diagenetic replacement of dolomite and diagenetic transformation of palygorskite, or by direct crystallization from solution. The average structural formula of the sepiolite is: (Mg7.15Al0.13Fe0.31Cr0.06Ni0.04)(Si11.98Al0.02)O30(OH)4(OH2)4Cao0.03Na0.02K0.02.8H2O Palygorskite appears to be authigenic by direct precipitation from solution. It exists in both monoclinic and orthorhombic forms with the mean structural formula given below (Mg2.22Al1.00Ti0.04Fe0.77Cr0.01Ni0.02)(Si7.68Al0.32)O20(OH)2(OH2)4Cao0.07Na0.05K0.10.4H2O

Clay Stability in Clay-Dominated Soil Systems

Abstract: Seven samples from a chronosequence of soils developed in historically created polders on the Atlantic coast (Marais Poitevin, Vendre, France) were investigated in order to illustrate the rate of mineralogical change in a clay-dominated system. The oldest polder was constructed in 1665, the last one in 1912; thus the time span of soil evolution is from 80 to 330 years. All the samples had more than 50% clay (<2 ~tm). The most reactive, fine clay sub-fraction (<0.1 ~tm) was investigated in detail by X-ray diffraction and chemical analysis. The observed mineralogical changes with increasing age followed the schematic reaction: smectite + mica = illite + mixed-layer minerals. The progress of reaction in time appears to be non-linear. This reaction seems to occur in a chemically constant system, and the mineralogical change can be seen as a readjustment of species to a given chemical composition. The rate at which soil clays form is difficult to assess. In soil formation from glacial tills or volcanic materials some data can be found to show the increase of the content of clay forming from non-clay phases (Ugolini, 1968; Protz et al., 1984; Lowe, 1986), but the evolution of clay minerals as a function of time in clay-rich soils has not yet been investigated. The interest in such a problem lies mainly in the rate at which clay minerals can respond to new chemical conditions. It is ultimately related to the fertility of soils. The reaction rates in soils with a high clay content should be the lowest due to the similarity of the phases involved. This paper describes some historically created soils from the west of France in an attempt to illustrate the rates of change in a clay dominated system.