Stability Of Smectite Research Articles

Fracture smectites in bedrock at the Kivetty and Romuvaara sites, Finland, as natural analogues for radioactive waste repository buffers

Bentonite is used in various geological repository designs for highly radioactive and low- and intermediate-level waste. Stability of such materials has been evaluated in long-term safety assessments, with recent emphasis on potential chemical erosion of these clays in fresh groundwater conditions. The fracture-filling mineralogy at two investigation sites, Kivetty and Romuvaara, in Finland shows the existence of different types of smectites characterized by their respective dominant elements, such as Mg-, Fe- and Na–Ca-rich smectites. Of these, Ca- and Na-smectites are likely to be montmorillonites that provide potential natural analogues for the bentonite buffers and their long-term performance in fractured bedrock environments. The smectite minerals observed reflect the local parent rock mineral composition, suggesting in situ formation during hydrothermal alteration, well predating the current freshwater conditions in the fractured bedrock. These observations support the stability of smectites, including Na- and Ca-montmorillonites, in the fractured bedrock of glaciated terrain with fresh continental groundwater extending to repository-relevant depths. Further, no erosion or sedimentation processes have been observed to have taken place in the fracture systems studied. However, uncertainties remain related to detailed exchangeable cation compositions of the smectite. Current groundwater systems at both sites would alter the composition of montmorillonite towards the Ca-rich form, and more detailed investigations are required to assess the stability of Na-montmorillonite in connection to open fractures. In general, bentonite alteration in fractures to Ca-montmorillonite would be beneficial in repository scenarios that consider potential chemical erosion by dilute groundwater. Supplementary material: Supplementary material 1 (XCT data for fracture smectite samples from Kivetty and Romuvaara) are available at https://doi.org/10.23729/1ce75faa-d52a-43cd-af40-3961b3081f70 [last accessed 5 July 2023] and Supplementary material 2 (XRD diffractogram data) and Supplementary material 3 (SEM-EDS data) are available at https://doi.org/10.6084/m9.figshare.c.7126477 Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Enhanced Identification of Fracture Smectites and Other Alteration Minerals Via Short-Wave Infrared Reflectance at Two Finnish Crystalline Sites, Olkiluoto and Hyrkkölä

Fracture filling minerals and the smectite-group clay minerals in particular play a significant role in the geotechnical stability of the fractured rock mass. Smectites have multiple uses as geomaterials in waste management solutions, of which the most technically demanding is the geological disposal of radioactive waste. Here, the stability and occurrence of smectites as fracture filling minerals were studied with two main objectives: 1) to test the capability of short-wave infrared spectroscopy as a tool to streamline the site assessment where clay mineralogy needs to be characterised in detail, and 2) to provide a first assessment of naturally occurring smectites in various sample types collected from two Finnish sites (Olkiluoto and Hyrkkölä) and their potential as natural analogues of smectite stability for bentonite used in the engineered barrier systems of geological disposal repositories. The short-wave infrared (SWIR) method has been validated to be a useful tool in fracture mineral characterisation, specifically due to its fast acquisition and interpretation time compared to the conventional methods. The study confirms the in situ occurrence of smectites at both sites studied showing that, especially in Hyrkkölä, the smectite-group member is close to montmorillonite, the main constituent of bentonite. At Olkiluoto, the smectites studied show more interstratified features. The differences in the mode of occurrence are related to the history of their formation, especially hydrothermal alteration, and parent rock composition.

A quantitative and mechanistic model for the coupling between chemistry and clay hydration

It is proposed here to describe smectite water vapor desorption isotherms using an exchange formalism that quantitatively accounts for the different hydration states and thus different water contents. This approach makes it possible to reproduce both desorption isotherms and relative proportions of the different hydration states as determined by X-ray diffraction. The method is numerically robust and easy to implement in most reactive transport codes. The formalism is satisfactory from a phenomenological point of view and accounts for the influence of external parameters such as interlayer cation composition and solution cation composition and salinity on clay hydration. Furthermore, in contrast to most solid solution models, this approach focuses on the clay reactivity according to the charge and type of interlayer cation rather than on its solubility and therefore does not require the overall thermodynamic properties of the clay. In addition, such an explicit distinction of the hydration/cation exchange processes from the thermodynamic stability of smectite 2:1 layer allows the use of kinetics driving slow dissolution/precipitation rates if the number of exchange sites is related to the amount of clay minerals.

Thermochemical effects on the smectite alteration of GMZ bentonite for deep geological repository

Gaomiaozi (GMZ) bentonite has been recognized as the first choice for using as buffer/backfill material in deep geological repository for the disposal of high-level nuclear waste in China. High salinity and high pH of the pore solutions may cause changes in the mineralogical composition, and the swelling capacity of the bentonite results in affecting the long-term performance of the engineering barrier system. In the present work, a series of hydrothermal reactivity experiments were performed using batch reactors at 20, 35 and 80 °C for evaluating the chemical and mineralogical responses of GMZ bentonites. The results show that the dissolution of smectite releases Si and Al under the conditions of 1 M KOH at 35 and 80 °C and the released Si and Al may work as a cementing agent in GMZ bentonite. XRD pattern results show that alteration of the GMZ bentonite is expected to be insignificant for reaction with NaCl, KCl and NaOH solutions under low temperatures. However, it is obviously reacted with KOH solutions especially at 80 °C, during which cementitious material was produced leading to kaolinitization. When contacted with KOH solutions, the basal spacing (d001) of the main primary diffraction peaks (001) value of GMZ bentonite increases with increasing temperature. Salinity also can influence the smectite stability and accordingly loss of swelling capacity. This conclusion could be explained by the diffusing double-layer effects induced by salt solutions and ion exchange.

Effects of Chemical Structure on the Stability of Smectites in Short-Term Alteration Experiments

AbstractBecause of their isolating capacity, smectite-rich clays have been proposed as buffer and backfill materials in high-level radioactive waste repositories. These repositories have to guarantee long-term safety for ~1 million years. Thermodynamics and kinetics of possible alteration processes of bentonite determine its long-term performance as a barrier material. Smectites in 25 different clays and bentonites were investigated in order to identify possible differences in their rates of alteration. These samples were saturated for 30 days in 1 M NaCl solution and deionized water, and then overhead rotated at speeds of 20 rpm and 60 rpm. Depending on the octahedral and interlayer composition, each of the smectites studied had specific rate of alteration, a so-called specific dissolution potential of smectite. The bentonites were classed as ‘slow-reacting bentonite’, ‘moderate-reacting bentonite’, or ‘fast-reacting bentonite’ corresponding to a relatively low (ΔP specific dissolution potential — <-5%), moderate (-5% < ΔP < -20%), or high specific dissolution potential (ΔP > -20%), respectively. The larger the amount of octahedral Fe and Mg compared to octahedral Al, the greater the specific dissolution potential. The present study found that the interlayer composition has a discernible impact on the rate of alteration. In experiments with rotation speeds of 60 rpm and a 1 M NaCl solution, Na+ was found to be the stabilizing cation in the interlayers of all the smectites. The Na-stabilizing mechanism was identified in only some of the smectites (type A) in experiments with 20 rpm (1 M NaCl solution). A second stabilization mechanism (by interlayer cations; Ca and Mg) was identified for other smectites (type B). Each bentonite has a specific rate of alteration. ‘Slow-reacting bentonite’ and clay with smectite-illite interstratifications are recommended as potential clay barriers in HLW repositories. The experimental and analytical procedures described here could be applied to potential barrier materials to identify ‘slow-reacting bentonite’.

Effect of the hydrothermal alteration on the surface conductivity of rock matrix: Comparative study between relatively-high and low temperature hydrothermal systems

In volcanic areas, hydrothermal systems are characterized by high bulk electrical conductivity, which is a combination of high-salinity/temperature pore water and hydrothermally-altered rock matrix. The present study focuses on better understanding the influence of the hydrothermal alteration on the surface conductivity associated with generation and stability of smectite. As a case study, the GSB borehole site at Beppu geothermal area in southwest Japan was selected; at this site, hydrothermal fluids with a relatively high temperature (150°C) flow at a shallow depth.The physical properties related to electrical conductivity (formation factor, surface conductivity, and porosity) were estimated on the basis of Revil model using conductivity measurements of the drillcores. Strong hydrothermal alteration at the site GSB was shown by high surface conductivity (10−2–10−1S/m) and high cementation exponent (2.5–4.5). From the comparison between the vertical profiles of the bulk conductivity, pore water conductivity, and surface conductivity, it was shown that the rock matrix makes a non-negligible contribution to the bulk conductivity at the site. This contribution to bulk conductivity is quite different from that of low-temperature hydrothermal systems, where the contribution from the pore water dominates because there is little or no hydrothermal alteration. Furthermore, comparison between the findings of this study and low-temperature hydrothermal systems showed that the surface conductivity could simply reflect temperature to which the rock has been exposed. The surface conductivity maintains the small value at the low temperatures such as <40°C, and significantly increases at the relatively high temperatures (100–150°C). At the higher exposed temperatures >150°C, its value decreases relative to that at the temperatures of 100–150°C. This relationship is consistent with the generation and stability of smectite at active hydrothermal systems, and places strong constraints on the quantitative interpretation of the electrical conductivity structure of a volcano.

Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H 2O activity

We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H 2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The inclusion of anhydrous mica end-members makes it possible to cover, with the same solid-solution model, the entire range of composition from low-charge smectite to mica, through high-charge smectite and illite. Non-ideal Margules parameters were used to describe the non-ideality of the solid solutions between the hydrated and dehydrated smectite end-members. Standard state properties of all smectite end-members as well as Ca- and Mg-muscovite and -phlogopite were initially estimated by oxide summation. These values were then refined and the other non-ideal interactions were estimated on the basis of different experimental data. The stepwise dehydration of smectite, and its stability and compatibility relations were calculated by Gibbs free energy minimising. Our results account for the progressive evolution of smectite to interlayered illite/smectite and then to mica, as observed in nature and experiments, and our model provides an explanation for the thermodynamic stability of smectite and illite/smectite compared to mica + kaolinite or pyrophyllite assemblages. The results suggest that the enthalpic contribution of interlayer water is a function of the ionic potential of the interlayer cation and the number of interlayer water molecules. This evolution makes possible to estimate the standard-state thermodynamic parameters and hydration-temperature behaviour of smectite of virtually all possible compositions. For the four-interlayer cations considered in the study, our model reproduces the 3 → 2 → 1 water-layer transitions that accompany a reduction of water activity or an increase of temperature at ambient pressure. The range of water content and interlayer distance calculated for the 3w, 2w and 1w states are also in fair agreement with the experimental values at ambient pressure.

Smectite suspension structural behaviour

Smectite suspensions, at low solids contents, are known to be naturally high in volume with diverse structural properties. The changing structural properties of smectite aqueous suspensions in the absence and presence of calcium ions were investigated using an acoustosizer and an advanced cryo-SEM technique to further understand and thereby control their environmental impact. In the absence of Ca(II) ions, smectite particles are present as a colloidally stable sol due to electrical double layer repulsion of the negatively charged platelets. The smectite network is observed to be extended throughout the suspension via clay platelets networking with an edge–edge (EE) orientation due to high basal surface repulsion. After the initial addition of Ca(II) ions, the smectite negative zeta potential reduces and the smectite platelets coagulate forming 2 µm aggregates. The platelets are randomly orientated, lettuce-like, coagulated aggregates with a high presence of both edge–edge (EE) and edge–face (EF) orientations. After equilibration, the smectite platelets forming an orientated honeycomb cellular structure comprised of face–face (FF) multiply sheet aggregates. The voids in the cellular structure are larger than prior to Ca(II) addition, measured at 2–8 µm. The changing structural properties of a smectite suspension in the absence and presence of Ca(II) greatly influence smectite stability and in turn, mineral processing and/or environmental management. Adequate time is required to allow suppression of the initial swelling of the smectite, full Ca(II) exchange and platelet aggregation.

Clay as sealing material in nuclear waste repositories

Nuclear waste from thermal plants poses a lasting risk to the biosphere because of its long radioactive life. The planned definitive storage place for it is in deeply buried repositories. Such repositories would need to be both impermeable to water, and plastic during deformation, in order to avoid the formation of cracks that may allow water in. One of the clay minerals, smectite, has these two properties and is an ideal candidate as a sealing material or even host rock for nuclear waste repositories. The chemical stability of smectite in the repository environment is sufficient to maintain good sealing properties during the active life of the relevant radionuclides.

Behavior of Smectite in Strong Salt Brines under Conditions Relevant to the Disposal of Low- to Medium-Grade Nuclear Waste

AbstractTwo industrial bentonites, IBECO SEAL-80 and TIXOTON TE, have been proposed as potential backfill material in the German Asse salt dome, a test field for the disposal of low- to medium-grade active nuclear waste. Considering the unlikely but possible case of a barrier breakdown with infiltration of a highly concentrated salt brine, the physicochemical stability and material behavior of these bentonites in a saturated salt brine (predominantly MgCl2) at 25°C were studied over the time period of 150 days. The results show that no mineral transformations occurred throughout the duration of the experiments and minor dissolution was only active during the first days. Some chemical properties, namely sorption capability and swelling, were reduced during contact with the salt brine, but could be reversed by removing the salt after treatment. Despite restriction of the CEC in the presence of salt solution, interlayer cation exchange reactions are still active in this environment. The long-term chemical stability of smectite in salt brine is predicted under these low-temperature conditions, but the increased permeability during aggregate formation could lead to physical breakdown of the backfill component.

Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance

Mineralogical and chemical changes during weathering of basalts from central and southern Portugal were studied. Both regions have a temperate Mediterranean type climate, the Algarve being more arid. The areas concerned are the Lisbon (72 Ma) and the Runa Volcanic Complexes (100–70 Ma), and Rio Maior (160–130 Ma) in central Portugal, and Budens (170–180 Ma) in the south. The basalts are generally rich in olivine phenocrysts. Clay minerals are in general well crystallised and comprise smectites, associated with 1:1 minerals (principally halloysite), illite and palygorskite. Kaolin minerals prevail in topographic highs, whilst smectites predominate in lower areas. Smectites were analysed from the <2 μm fraction and from weathered microsites on olivine crystals. Both approaches identified dioctaedral smectites. Smectites from the south (Budens) have higher Na contents and lower Mg concentrations than those from central Portugal. This, together with different climatic or microclimatic conditions, may explain the smectite–analcime association in the first case, and smectite–palygorskite in the second. Smectites can thus generate one or the other of these minerals. In weathered microsites on olivine crystals, iron oxides were noted as well as the possible replacement of smectites and Fe oxides by calcite. The general weathering features appear to be similar to those found on basalts in other temperate climates, i.e. the formation of secondary minerals with a predominance of smectites or 1:1 minerals according to geomorphic location. However, the degree of aridity may affect the stability of smectites, which seem to evolve either to palygorskite or to analcime.

Smectite stability in acid salt solutions and the fate of Eu, Th and U in solution

AbstractThe alteration and transformation behaviour of Ceca smectite in two acid and saline solutions (NaCl and KCl) was studied in batch experiments. This type of smectite is proposed as potential backfill material for nuclear waste storage sites. The initial solution was enriched with 500 ppm of U, Th and Eu. The evolution of pH and solution concentrations were measured over a period of 25 months. The mineralogical and chemical evolution of the clays was also studied. X-ray diffraction revealed a significant loss of diffraction intensity and the clays became amorphous to Xrays with increasing reaction time. Deconvolution of the XRD data indicated a continuous collapse of the smectite layers but no illitization. Infrared spectroscopy revealed unchanged smectite spectra at the end of the experiment. Thorium precipitated as amorphous ThO2, but showed an incipient crystallization after 500 days. The Th, Eu and U were neither adsorbed onto the clays nor incorporated into a secondary phase.

Synthesis of Smectite Clay Minerals: A Critical Review

AbstractSmectites are one of the most important groups of phyllosilicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (&lt;100°C), (2) under moderate hydrothermal conditions (100–1000°C, pressures to several kbars), (3) under extreme hydrothermal conditions (&gt;1000°C or pressures &gt;10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most successful in terms of smectite yield and phase-purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The presence of Mg2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temperatures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under-and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody-namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.

Water–rock interaction and hydrothermal mineral equilibria in the Tendaho geothermal system

The Tendaho geothermal system occurs within a NW–SE-trending rift basin filled with Quaternary volcanics (mainly basalts) and fluvio-lacustrine sediments. Three deep (TD-1, TD-2 and TD-3) and one shallow (TD-4) geothermal wells have been drilled. The waters of productive wells TD-1, TD-2 and TD-4 are typical Na–Cl geothermal waters with reservoir temperature ranging between 220 and 270°C. Chemical analysis of core samples (altered basalts) shows increased Ca, Fe, Mg, Al content (owing to the dissolution of plagioclase and femics and the precipitation of wairakite, laumontite, epidote, garnet, calcite and clay minerals) and decreased Na, K, Si and Ti (owing mostly to the dissolution of glass matrix). Petrographic study of cuttings and core samples shows: (1) evidence of an early stage of calcite, zeolite (wairakite or laumontite) and quartz crystallization, while calcite underwent different stages of dissolution/precipitation, possibly due to abrupt changes in pH and CO 2 partial pressure; (2) that epidote, garnet, prehnite, pyroxene and amphibole crystallization occurred after wairakite or laumontite. Chlorite is the main layered silicate in the basaltic rocks in wells TD-1 and TD-2, temperatures beyond the stability of smectite and chlorite/smectite (C/S) interlayers must have been reached. The smectites and C/S interlayers in association with chlorite in well TD-3 indicate that this well has had a more complex thermal history, with variable temperatures. The smectites occur at temperatures above 120°C, which is considered to be the threshold for the transformation of smectite to illite. No evidence of disequilibrium conditions of smectites has been found at reservoir temperatures currently present at Tendaho. Fluid inclusion data indicate heating in the well TD-1, and thermal conditions similar to the ones measured in the deepest part of well TD-2, while the uppermost part of this latter has undergone cooling. Intense cooling has affected well TD-3, drilled far from the upflow zone of the field, probably in an area characterized by self-sealing, cooling and very low permeability.

Mineralogical responses of siliciclastic carbonate-cemented reservoirs to steamflood enhanced oil recovery

Rock–fluid interactions induced by steamflood enhanced oil recovery were investigated in laboratory simulations to determine the geochemical reactions and the effects of these reactions on reservoir permeability. Flow-through laboratory experiments using mixtures of quartz, kaolinite, and siderite were performed in a high temperature/high pressure permeameter at a confining pressure of 1200 psi and temperatures between 150–250°C. Fluid compositions used in the experiments simulated the vapor and residual liquid phases encountered in steamflood operations as well as an intermediate fluid composition. Effects of fluid pH, fluid salinity and flow rate were systematically investigated in the experiments. The most extensive fluid–rock interactions were observed in the vapor phase simulations and high temperature/high pH condition simulations. Smectite, chlorite, illite, mixed-layer clays, greenalite, analcime, and K-feldspar were all identified as products of rock fluid interaction in the experiments. Smectite was the dominant authigenic phase to reduce permeability in the experiments. The experiments showed that the formation of smectite in Fe-rich environments does not require a clay precursor. Smectite is likely the most damaging neoformed mineral to reservoir permeability under different hydrogeochemical conditions for several reasons including: (1) its relatively high surface area (including microporosity in the “honeycomb texture”), (2) its propensity to migrate and block pore throats during fluid flow in porous media because of its small particle size, pore-lining texture, and electrochemical surface properties, and (3) the wide range of stability of smectites in the physical and chemical conditions that exist in reservoirs undergoing steamflood EOR. The rapid precipitation of authigenic minerals in these experiments suggests that the period required for fluids and rock to reach equilibrium in diagenetic environments are extremely short when considering geologic time scales. The armoring of pre-existing minerals by grain-coating authigenic minerals appears to result in the attainment of local equilibrium conditions prior to when one would predict assuming a continuous supply of reactant minerals was present.

Role of permeability versus temperature in the distribution of the fine (< 0.2 μm) clay fraction in the Chipilapa geothermal system (El Salvador, Central America)

Detailed X-ray diffraction of clay minerals from the less than 0.2 μm fraction of altered rocks has been carried out on cuttings and cores from three boreholes (CH7, CH9 and CH7B) from the geothermal field of Chipilapa (El Salvador, Central America). In most of the low-permeable horizons, the vertical distribution of I/S and C/S mixed-layers fairly agrees with the general scheme of di-smectite to illite and tri-smectite to chlorite conversion series with increasing depth (max. 2600 m) and temperature (max. 220 °C). The deepest parts of the wells are characterized by chlorite and illite-rich illite/smectite mixed-layers (I/SR ≥ 1) in close agreement with the measured temperature (170–220°C). Within fracture-controlled permeable horizons, the distribution of clay minerals strongly disagrees with the “classical pattern” of di-smectite to illite and tri-smectite to chlorite conversions observed in low-permeable horizons. Great amounts of smectites and/or smectite-rich mixed-layers occur at temperatures which strongly exceed the current range of smectite stability in natural systems (up to 205 °C). Similar discrepancies may also be viewed from chlorite crystallinity in well CH9 and smectite content in smectite-rich chlorite/smectite mixed-layers in well CH7B. These high-temperature smectites are considered as present or very recent metastable phases representing the first step of kinetically controlled conversion series toward illite and/or chlorite. Such phenomena are restricted to the levels in which the present hydrodynamic conditions promote the crystallization process: boiling and mixing of fluids with contrasting temperatures. High-temperature smectites and smectite-rich mixed-layers may be of great interest for the understanding of the very recent evolution of geothermal reservoirs. They do not only indicate the present horizons of active fluid circulations in geothermal wells (smectite coatings at the wall of fractures), but they are also potential guides in the determination of the horizons in which permeability has recently decreased.

Longevity of Bentonite as Buffer Material in a Nuclear-Waste Repository

Longevity of bentonite as a nuclear waste barrier depends on mineralogical stability of the smectite component of the clay as well as on its ability to perform as a buffer in the repository for long periods of time. Processes affecting the stability of smectites under the geochemical and thermal conditions in a high-level nuclear waste repository can be construed by examining the geological analogues and the results of the hydrothermal experiments. The physical and chemical properties of bentonite seem to be related to the nature of smectites in them. Smectites represent a range of expandable layer silicates of Al, Mg, Fe and other transition metals. Their thermal, hydrothermal, or solution stabilities vary from one smectite species to another. This is also true to a certain extent for the physical properties of bentonites, i.e. swelling, permeability, plasticity and adsorption. Swelling ability and permeability of naturally altered (metabentonites) and steam-treated bentonites are badly damaged. Temperature distribution in a clay barrier that was simulated in situ under the repository conditions showed a sharp drop from 175°C at the clay/canister contact to 81°C at a distance 65 mm away. Microstructures of bentonites were noticeably altered after hydrothermal treatment at 200°C.

Clay Mineralogical Relationships in Florida Haplaquods1

The clay mineralogy of 59 Alfic and Ultic Haplaquod pedons was studied with respect to distributional relationships involving horizons, chemical parameters, and geomorphic surfaces. Relative estimates of clay-sized minerals were based on x-ray diffraction (XRD) peak height ratios after calibration with standards and normalization. Mineralogical differences based on horizon, base status, or elevational groupings were tested statistically. Quartz dominated the clay fractions of upper, sandier horizons for all pedons. Kaolinite, smectite, and gibbsite were usually more abundant in deeper, finer textured horizons. Hydroxy-interlayered minerals, relative to other phyllosilicates, were significantly concentrated (p <0.0001) in Bh horizons. Kaolinite tended to be more abundant than other phyllosilicates in A and E horizons. Alfic subgroup pedons usually contained less kaolinite and more smectite than ultic pedons. Most pedons on older (>30 m) geomorphic surfaces: (i) were virtually depleted of bases, (ii) contained little or no smectite, and (iii) were dominated by kaolinite in lower horizons. Soils on younger (>30 m) surfaces contained significantly (p = 0.0054) more smectite than older surfaces on the average, and differences were even greater when only ultic subgroups were considered. The mineralogical-chemical-geomorphic relationships collectively support the concept that smectite stability decreases with the loss of bases which has occurred most thoroughly on the oldest surfaces.

Evaluation of Aluminum‐Smectite Stability Equilibria in Naturally Acid Scoils

AbstractStability characteristics of six Al‐smectites in naturally acid (pH &lt; 5.0) montmorillonitic soils were determined by using structural ionic constituents and in situ soil solution compositions. Approximate solubility products (pk) for each smectite were calculated from δG0t values of hydroxide counterparts of cation structural components. Calculated ion‐activity products (pQ) of soil solutions were approximately equal (within the range of analytical uncertainties) to extimated pks of smectites in three of six soils. The other three solutions were either supersaturated (C horizon) or undersaturated (Bt horizons) relative to smectite. Solutions obtained from C horizons were supersaturated with respect to amorphous silica due to dissolution of biogenic opal and albite. In these borizons, where large quantities of smectite have been preserved, smectite stability appeared to be enhanced by high solution levels of Si and Mg. Low ratios of solution Si/Al in A and B horizons, where not compensated by relatively high pH, promoted weathering of smectite to kaolinite. Standard free energies of formation (δGot for Al‐smectites, computed from estimated pk values, ranged from −10 161 ± 1021 to −10 345 ± 1040 kJ/mol. The highest δof values were encountered in smectites with the most structural OH‐.

Stability of Soil Smectite From a Houston Black Clay

AbstractThe stability of smectite separated from a Houston Black clay soil was studied by solubility methods in an acid environment. High Silicon levels (supersaturated with respect to amorphous Si) probably were due to dissolution of the smectite and slow precipitation of amorphous Silicon. Also, mica and vermiculite impurities may have contributed to high solution Si values. Solubility data from equilibrium solutions of various treatments and chemical structural analyses permitted the formulation of a solubility equation. The ΔG°f for the Houston Black smectite computed from pK values was —2433.9 ± 0.8 kcal/mole. The stability of this clay could then be determined by calculations for any desired solution environment. It was found that under some conditions this soil smectite could be more stable than Belle Fourche and Aberdeen montmorillonites. Therefore, it appears that this soil clay has the required stability area in which it can form in nature.