Dacitic andesitic calc-alkaline lavas and their pyroclastic rocks of Upper Cretaceous age are widespread in the Ordu area, eastern Black Sea Coast of Turkey. Ca-bentonite and Ca/Nabentonite deposits with significant economic potential formed in the broader region. The mineralogy and geochemistry of these deposits were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analyses. The parent rocks (PR) and bentonite samples were analysed for the major and trace element contents, including rare-earth elements, to understand the effect of differential alterations on element mobility. The bentonites contain mainly di-octahedral Ca-montmorillonite with minor amounts of illite, quartz, calcite, dolomite and feldspar. Primary K-feldspar and plagioclase phenocrysts were altered in the bentonite horizons, and were completely leached at greater depth. For this reason, the higher quality bentonites are found at deeper zones. Bentonite horizons are characterized by relatively high loss on ignition (LOI: 9.8–20.8%) and MgO content (3–5%) and low K2O (<0.5%) and Na2O (<2%) contents compared with the least altered parent rocks. Silicon, Ca, Mg, Mn and P are markedly enriched whereas Fe, Na, K and Ti are depleted in the bentonite samples. The chondrite-normalized rare earth element (REE) patterns show identical trends characterized by strong light rare earth element (LREE) enrichment in both the PR and bentonite samples. The LREE contents are higher than those of their heavy counterparts (HREE) in the samples. Alteration of the PR to bentonite is associated with intense chemical leaching and subsequent removal of K and Na in open-system conditions.

The work reported herein involves the characterization of natural zeolites from two different mines (San Andrés and Tasajeras; Cuba) using XRD, SEM, TG-DTA, ICP and surface area measurements. In addition, the chemical composition of zeolites, the heavy metal and environmental organic toxins content, the ion exchange rates, stability under biological conditions using simulated body fluids as well as the binding capacity for histamine have been investigated with a view to using them as medical products. The investigated zeolites contain clinoptilolite and mordenite as major phases. Furthermore, the samples are apparently free from fibrous materials according to SEM observations. In particular, the San Andrés zeolite binds remarkable amounts of histamine which are nearly irreversible under acidic (pH = 1; 12.4 mg/g) and neutral conditions (pH = 7; 15.7 mg/g), respectively. Thus, the San Andrés sample may well be applied as a medical product due to its excellent binding capacity for histamine along with its remaining optimum physico-chemical characteristics.

To a mineralogist, minerals are defined primarily by their crystal structure and chemical composition. Many minerals of industrial importance retain this identity in their final application. For the producer and end user, the physical properties of these industrial mineral products such as size and shape distribution, refractive index, density, hardness, refractoriness and colour are of primary importance, since it is the combination of these, together with its surface chemistry, which gives the mineral its functionality. As a result, minerals which are very different in structure and origin but have many similar physical properties, such as kaolin and calcium carbonate, are often used in very similar roles and in similar applications.

Examples of mineral use in diverse applications such as paper, paints and coatings, polymers and films show how the physical properties of different minerals are exploited and engineered to provide optical, mechanical and rheological performance and to minimise cost. Industrial mineral producers have been manipulating these properties for many decades and continue to do so in search of improved performance and new applications. As new high performance materials become more commonplace, the challenge is to find new ways of transforming minerals to provide the necessary functionality for them.

This paper describes a study of the hydraulic reactions between metakaolin (MK) and air lime using electrochemical impedance spectroscopy (EIS) and nuclear magnetic resonance spectroscopy (NMR). Tests were carried out at 20, 25 and 30°C on lime-MK pastes with 10:1 w/w ratio. Tests over 28 days allowed identification of relevant changes in the EIS signals and characterization of pastes using thermal analysis (TGA/DSC), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and uni-axial compressive tests. Tests over shorter periods of time (up to 42 h) allowed more detailed studies of the hydraulic phases formed at the very beginning of the reactions.

Results of thermal analyses demonstrate formation of hydraulic compounds such as CSH, C4AH13 and C3ASH6 and show their evolution over time. MIP analysis demonstrates changes in pore size distribution related to the formation and trasformation of hydraulic phases. Variations of impedance response with time are shown to be associated with reaction kinetics. Changes in the NMR signal within the first 42 h of reaction are shown to be associated with the dissolution of calcium hydroxide in the pore solution. Overall, this paper demonstrates the importance of NMR in the study of hydraulic reactions in lime based materials and the ability of EIS to detect the formation of hydraulic compounds and the end of the calcium hydroxide dissolution process.

The Cambridge Diagenesis Conferences (1981–1998) were set up to act as a conduit for the interchange of clay mineral expertise between universities and research institutes on one hand, and the hydrocarbon industry on the other. At the time, oil companies were dealing with the development of the North Sea Oil Province which was turning out to be a natural laboratory for the fundamental study of authigenic clay minerals and their relationship to lithofacies, burial, overpressure, reservoir quality and hydrocarbon emplacement. This symbiosis between industry and academia flourished for nearly two decades. Each conference was followed by a special issue of Clay Minerals dealing with topics relevant to, or discussed at the particular meeting. By the late 1990s the North Sea had become a mature province and the major oil companies were looking to other parts of the world to replenish their reserves.

This study focuses on the effects of both thermal treatment (between 80 and 700°C) and chemical activation (concentration of sulfuric acid between 0.3 and 2 M) of natural Ranong kaolins (ground or not) from Thailand to remove the undesirable colour of rice bran oil. The mineralogical, physical and physicochemical properties of the initial and activated kaolins are discussed in relation with the bleaching effectiveness of the activated sample investigated. Generally, the greater the temperature used before the activation step and the concentration of sulfuric acid used during activation, the greater the structural degradation of the kaolinite; Al is removed from the octahedral sheet of kaolinite and amorphous SiO2 dominates the samples. The measured maximum bleaching capacity is not necessarily obtained when using the activated kaolin having the highest specific surface area and pore volume; rather, the bleaching capacity is dependent on both alumina contents and proportion of kaolinite in the samples. Indeed, the partial preservation of the kaolinite structure is crucial to obtain a good bleaching capacity of kaolin in relation to the preservation of the aluminol sites which are likely to be involved in the adsorption of unsaturated molecules present in the rice bran oil. Moreover, as previously demonstrated, a partial leaching of Al from octahedral sheets of kaolin is also an important factor in order to obtain good bleaching capacities. Finally, the optimal preheating temperature and concentration of sulfuric acid which permit the best bleaching capacity of kaolin are reported.

It has been established that disagreements between different methods of particle size determination of clay minerals can be ascribed to the non-spherical shape of the clay particles. However, by having aspect ratios available, particle sizes can be harmonized. One frequently used approach to obtain aspect ratios is to compare particle sizes originating from at least two devices operating on the basis of different physical principles. In this contribution aspect ratios of nine kaolinite-dominated and one dickite-dominated sample were determined by conductometric titrations. The aspect ratios obtained were then successfully used to correlate particle size distributions from dynamic laser scattering and acoustic spectroscopy.

Redox activation (reduction of structural Fe) of smectites greatly alters their chemical reactivity and physical properties, which may be exploited for various environmental, agricultural or industrial purposes. Their re-oxidation during preparation, characterization, and use is, however, a significant risk to their utility. In this study, methods and apparatus were developed and described which mitigated reoxidation. Ferruginous smectite (sample SWa-1, Na saturated) was used as the model smectite. It was reduced with sodium dithionite in a citrate-bicarbonate buffer solution at 70°C for 4 h, which achieved a maximum Fe(II)/total Fe ratio of 0.9113 ± 0.0048. The first step in rendering reduced samples useful is to remove from them the reducing agents and other solutes present during reduction. This was accomplished in the present study by reducing the sample in an inert-atmosphere reaction tube (IRT) (a 50 mL centrifuge tube equipped with a removable septum cap), then removing solutes from the suspension by centrifuge washing. The washing steps were performed with the aid of a controlled-atmosphere liquid exchanger (CALE) which provided connections between the sample suspension and deoxygenated solutions. The reduced state was measured by 1, 10-phenanthroline or by Mössbauer spectroscopy at 77 K to give Fe(II)/total Fe ratios. Some samples were freeze dried after washing. Results revealed that if reduced smectites are washed without protection from atmospheric O2, the extent of reoxidation is on the order of 40 to 60%. If the sample is subsequently dried, reoxidation increases to more than 76%. If the sample is protected using the IRT and the CALE, however, reoxidation is decreased to less than 2%. Freeze drying in a glove box allowed reoxidaton to increase to slightly more than 10%. These results indicate that more reoxidation occurred during the drying stage than during the washing stage. These observations lead to the conclusions that (1) protection of reduced samples from atmospheric O2 is essential if extensive reoxidation is to be prevented, and (2) the methods and apparatus described herein are effective for accomplishing that purpose in abiotically reduced smectites. They may also be effective if applied to microbially reduced smectites.

Sudoite, an aluminium-rich di-trioctahedral chlorite, is known primarily from highgrade diagenetic/ low-temperature metamorphic Al-rich rocks and from hydrothermal deposits in Alrich terrains. This contribution reports a rare occurrence of pore-lining sudoite in Permian red beds from the eastern part of the Southern Permian Basin. Sudoite is a ubiquitous mineral in aeolian sandstones of the Eastern Erg, a large dune field buried in the subsurface of the Fore-Sudetic Monocline, SW Poland. Sudoite crystals are arranged in a honeycomb texture and often associated with high-porosity intervals. It is concluded that sudoite crystallized at the expense of early diagenetic dioctahedral smectite at a temperature lower than 180°C under the influence of hot, Mgrich and K-poor fluids, which migrated from a basement and/or deep part of the basin upward through permeable aeolian sandstones and mixed with colder pore solutions. Tosudite was a likely intermediate phase in the smectite to sudoite conversion.

The instability of shales in drilled formations leads to serious operational problems with major economic consequences for petroleum exploration and production. It is generally agreed that the nature of the clay minerals in shale formations is a primary causative factor leading to their instability, although the exact mechanism involved is more debateable. Currently, the principal cause of shale instability is considered to be volume expansion following the osmotic swelling of Nasmectite. However, illitic and kaolinitic shales may also be unstable, so that interlayer expansion cannot therefore be considered as a universal causative mechanism of shale instability. This review considers alternative scenarios of shale instability where the major clay minerals are smectite, illite, mixed-layer illite-smectite (I/S) and kaolinite respectively. The influence of interacting factors that relate to shale clay mineralogy such as texture, structure and fabric are discussed, as are the pore size distribution and the nature of water in clays and shales and how these change with increasing depth of burial. It is found from the literature that the thickness of the diffuse double layer (DDL) of the aqueous solutions associated with the charged external surfaces of clay minerals is probably of the same order or even thicker than the sizes of a significant proportion of the pores found in shales. In these circumstances, overlap of the DDLs associated with exposed outer surfaces of clay minerals on opposing sides of micropores (<2 nm in diameter) and mesopores (2–50 nm in diameter) in a lithostatically compressed shale would bring about electrostatic repulsion and lead to increased pore/ hydration pressure in smectitic, illitic and even kaolinitic shales. This pressure would be inhibited by the use of more concentrated K-based fluids which effectively shrink the thickness of the DDL towards the clay mineral surfaces in the pore walls. The use of soluble polymers would also encapsulate these clay mineral surfaces and so inhibit their hydration. In this scenario, the locus of action with respect to shale instability and its inhibition is moved from the interlamellar space of the smectitic clays to the charged external surfaces of the various clay minerals bounding the walls of the shale pores.

The distribution of Al3+ and Fe3+ between octahedral and tetrahedral sites of dioctahedral smectites was shown to be controlled by a partition coefficient Kd(4/6) = [(Fe3+)4×(Al3+)6]/[(Fe3+)6×(Al3+)4]. The Kd(4/6) value is near 0.006 for natural dioctahedral smectites, formed between 2 and ~100°C, and near 0.0174 for smectites synthesized at 200°C. These Kd(4/6) values, obtained from both chemical and spectroscopic data, were consistent with those calculated using the ionic radii of cations and Brice’s model (Brice, 1975). The partition coefficient approach explained well why for natural and synthetic dioctahedral smectites no tetrahedral Fe3+ is detected when the total Fe3+ content is below 3 atoms per unit cell (24 oxygen atoms).

The chemical and sorption properties of clay minerals from the Mako area, Senegal, were investigated using FTIR spectroscopy, X-ray diffraction, scanning electron microscopy equipped with an X-ray energy dispersion spectrometer, thermal analysis and chemical analysis. The clay sample is essentially dominated by kaolinite and quartz as also shown by treatment with ethylene glycol and dimethylsulfoxide (DMSO). The clay fraction of this natural clay was organically modified by grafting with 3-aminopropyltriethoxysilane (APTES) in order to improve significantly its retention ability of heavy metals. The silane groups of the APTES reagent were partly grafted on the surface of platy kaolinite particles and the remaining ethoxy groups could be hydrolysed by aqueous treatment. The natural clay, its clay fraction and the organo-functionalized clay (with APTES) were investigated as adsorbents for the removal of Pb(II) from aqueous solutions. Evidence for an organic grafting has been demonstrated by comparing the spectroscopic characteristics of the natural clay and those of its chemically modified derivatives. The effects of different parameters (i.e. initial Pb(II) concentration and contact time) on the adsorption efficiency were studied. For an initial concentration of 10 mg L−1 Pb(II), the adsorption was maximized after 30 min contact time both for the raw material and its clay fraction and after 90 min for the APTES grafted clay. Although the maximum of sorption for the APTES grafted clay is reached with slower kinetics, this maximum amount of Pb(II) uptake at room temperature (Xmax) is significantly higher since it is 0.99 mg g−1 for the raw clay, 1.46 mg g−1 for its clay fraction and 3.02 mg g−1 for the organically modified clay, i.e. three times greater than the raw clay.

Pedogenic alteration of illite from red earth sediments in Jiujiang in subtropical China was investigated using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Illite, hydroxy-interlayered vermiculite (HIV), kaolinite and mixed-layer illite-HIV (I-HIV) are present in the soils. The characteristic reflections of the clay phases were 14 Å, 10–14 Å, 10 Å, and 7 Å, respectively. After Mg-glycerol saturations, the 14 Å peak of the samples did not expand, and after heating at 350°C and 550°C it shifted to 13.8 Å and 12 Å respectively, with no residual 14 Å reflection, suggesting the occurrence of hydroxy-interlayered vermiculite. The randomly interstratified I-HIV clays were characterized by a broad peak at 10–14 Å, which did not change its position after Mg-glycerol saturation, but collapsed to 10 Å after heating at 350°C and 550°C. HRTEM analysis showed different lattice fringes of 12 Å, 10 Å and 7 Å . Mixed-layer I-HIV, HIV-K and illite-kaolinite (I-K) were observed in the HRTEM images which represented the intermediate phases during illite alteration. The merging of two 10 Å illite layers into a 12 Å HIV layer, lateral transformation of one HIV layer into one kaolinite layer and alteration of one illite layer into two kaolinite layers illustrated the mechanisms of illite-to-HIV, HIV-to-kaolinite and illite-tokaolinite transformation, respectively. The proposed pedogenic alteration of illite and the weathering sequence of the clay minerals in Jiujiang is illite → I-HIV → HIV → HIV-K → kaolinite. In addition, illite may transform directly to kaolinite.

The influence of individual clay minerals on formation damage of reservoir sandstones is reviewed, mainly through the mechanism of fine particle dispersion and migration leading to the accumulation and blockage of pore throats and significant reduction of permeability. The minerals discussed belong to the smectite, kaolinite, illite and chlorite groups respectively. These minerals usually occur in an aggregate form in reservoir sandstones and the physicochemical properties of these aggregates are reviewed in order to reach a better understanding of the factors that lead to their dispersion in aqueous pore fluids. Particularly significant properties include the surface charge on both basal and edge faces of the clay minerals and how this varies with pH, external surface area of both swelling and non-swelling clays, porosity and pore size distribution in the micro- and meso-pore size range and overall aggregate morphology. For non-swelling clays, and perhaps even for swelling clays, dispersion is thought to be initiated at the micro- or meso-pore level, where the interaction between the pore solution and the charged clay surfaces exposed on adjacent sides of slit- or wedge-shaped pores brings about expansion of the diffuse double electric layer (DDL) and an increase in hydration pressure. Such expansion occurs only in dilute electrolyte solutions in contrast to the effect of concentrated solutions which would shrink the thickness of the DDL and so inhibit dispersion. Stable dispersions are formed, particularly where the solution pH exceeds the isoelectric pH of the mineral, which is often at alkali pH values, so that both basal face and edge surfaces are negatively charged and the particles repel each other. The osmotic swelling of smectitic clays to a gel-like form, so effectively blocking pores in situ, is often invoked as an explanation of formation damage in reservoir sandstones. Such swelling certainly occurs in dilute aqueous solutions under earth surface conditions but it is uncertain that stable smectitic gels could form at the temperatures and pressures associated with deeply buried reservoir sandstones.

The unique properties of clays make them suitable for a large number of applications. Among the procedures that can be used to characterize clayey materials, the assessment of the abrasion and the abrasiveness index can be employed to predict the mechanical behaviour of clay samples. However, only a limited number of references to these testing procedures are available in the literature. Several Portuguese clayey samples were collected and the <63 μm fraction was analysed. The main clay minerals present are kaolinite, illite and smectite. The abrasiveness index depends on the granulometric and mineralogical compositions. A lower abrasiveness is induced by the presence of a high abundance of the <2 μm fraction, small mean particle size and a large phyllosilicate content . Kaolinite and illite show good correlation with the abrasiveness index, while smectite decreases abrasion.

Mg-silicate minerals (e.g., stevensite, kerolite, talc, sepiolite) play an important role in the construction of facies models in lacustrine and peri-marine environments because they are sensitive to changes in solution chemistry. However, the response of Mg-silicate mineralogy to changing aqueous chemistry is only broadly understood because the mechanisms underpinning the coprecipitation of Mg2+ and SiO2(aq) from surface water, and subsequent Mg-silicate crystallization, are unclear. Here we describe the results of experiments designed to systematically examine the effects of pH, Mg/Si and salinity of the parent solution on the nature of initially precipitated products. Structural interrogation of the products with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis (TGA/DTA) allow comparison of synthetic products with naturally occurring crystalline counterparts. In general, Mg2+ and SiO2(aq) co-precipitation and nucleation of Mg-silicate layer structures first involves the rapid formation of 2:1 layers with trioctahedral occupancy and a mean coherent X-ray scattering domain between 1–2 unit cells with respect to the c axis. Well defined but diffuse hk reflections indicate two-dimensional growth, turbostratic stacking and highly variable interlayer hydration. Diffuse reflectance FTIR shows numerous structural similarities with stevensite, kerolite and sepiolite. However, TGA/DTA analysis indicates the presence of variable kerolite/stevensite interstratification not readily detectable through XRD analyses, as well as a significant degree of surface and interlayer hydration (e.g. 15–20 wt.%).

We observe a number of clear trends in the products with respect to solution chemistry. For example, at low salinity, kerolite-like products dominate at high Mg/Si and high pH, whereas sepiolite-like products are formed at lower pH and lower Mg/Si. At high salinity and high Mg/Si, stevensite-like products are favoured at high pH and kerolite-like products dominate at lower pH, whereas a decrease in Mg/Si of the solution leads to sepiolite-like products at low pH and only stevensite-like products at high pH. Higher pH leads to an increase in octahedral vacancies which favour stevensite-like products; this may result from a higher rate of two-dimensional tetrahedral sheet expansion relative to the octahedral sheet, as inferred from studies of silica oligomerization and brucite growth kinetics.

Together, our results indicate that the neoformation of Mg-rich silicates from solution may often begin with the rapid nucleation of hydrated 2:1 layers. Subsequent dehydration leads to progressive layer stacking order and could occur in response to wetting/drying cycles, prolonged exposure to high salinity solutions, or burial and heating. The surface and interlayer water associated with these products is undoubtedly an important source of diagenetic water in Mg-silicate-bearing successions, and the chemistry of this water upon later diagenesis should be a focus of future investigation.

A kaolin clay was activated with 1.0 M H2SO4 solution at minimum liquid to solid ratio (L/S) using microwave heating. The optimum experimental conditions for activation were L/S ratio 3.0 mL 1 M H2SO4 per gram kaolin, microwave input power 500–600 W, and heating time 5–10 min. Activation at L/S < 3.0 mL/g using 1.0 M H2SO4 was not efficient, indicating the influence of solvent for absorbing microwaves more intensively and thus improving activation. Significant physicochemical changes were observed by the proposed procedure with smaller volumes of activator compared to the conventional heating method. Microwave input power and heating time have a strong influence on the quality of the final material; activation at high input power (>700 W) and longer heating times (>10 min.) are not recommended since they cause dissolution of kaolinite structure. Microwave-heated kaolin manifested better adsorption for tartrazine dye due to improvements in textural and chemical properties of kaolinite. Moreover, irradiation of used kaolinite has significantly improved dye desorption, increasing the importance of microwaves in regeneration/recycling studies. Detailed dielectric measurements of kaolin-acid mixtures recorded at frequencies much lower than 2.45 GHz revealed that absorption of radiation is highly dependent on the activator solution in the mixture. For 3.0 mL/g mixtures, high dielectric constant ε’ 5223, dielectric loss factor ε” 5083, tangent loss tan d 1.30, penetration depth dp 0.57 cm at (103 Hz), and AC-conductivity σ 0.032 Om–1 were determined at 105 Hz. Filling the pores of kaolin by acid solution increased the microwave absorption and hence de-alumination of kaolinite.

A bentonite from Milos, Greece, was activated with oxalic acid and the effect of acid activation conditions on bleaching of sunflower oil was investigated. The activation parameters studied were temperature, retention time and oxalic acid to bentonite mass ratio. The activated materials produced had good bleaching properties and were suitable for industrial use as bleaching earths. Optimum bleaching properties could be achieved using a variety of combinations of acid to bentonite ratios and activation times. Bleaching efficiency tests indicated that 24 h activation at 100°C with 1m oxalic acid and 25% pulp density (w/v) gave results equivalent to that of a commercial bleaching earth (Tonsil Optimum 210 FF). The combination that is likely to be more preferable on an industrial scale was 100°C, 25% pulp density (w/v), 1m initial oxalic acid concentration, 60% recycling of the oxalate solution (making up an acid to bentonite ratio 0.2 w/w) and 6 h activation time. The materials produced under these conditions have acceptable bleaching properties, corresponding to bleaching capacity greater than 78% compared to commercial Tonsil, and colour index (in red and yellow units) equal or even better than Tonsil.

This study focuses on the mineralogical and geotechnical characterization of northern Moroccan clays from the Tangier and Tetouan areas and compares them with the main clay deposits used in the Moroccan ceramic industry (from Meknes, Fes, Salé and Safi regions). Sampled clays were analysed by X-ray diffraction on bulk and clay (<2 μm) fractions to identify the mineralogical assemblages of the clay outcrops. Further analyses were conducted to determine the particle size distribution (laser diffraction particle analyser), the total organic matter content (Loss- On-Ignition measurements) and the Atterberg limits of the raw clays. The study aims at investigating the spatial variability of the clay samples and at evaluating their potential application as raw materials in the ceramic industry.

Tetouan and Tangier clays are characterized by diversified mineralogical assemblages (in particular a variable proportion of clay, quartz and calcite) compared with the Meknes, Fes, Salé and Safi clays (high clay content, quartz and calcite). The clay fraction of the Tetouan and Tangier samples is dominated by illite and kaolinite with variable amounts of chlorite, smectite and/or vermiculite. Illite is the dominant phase in the Meknes, Fes, Safi and Salé clays, but is associated with kaolinite. No direct relationship between the mineral assemblage composition and the lithology of the series was found.

The clays materials studied consist generally of fine particles with medium to high plasticity and low organic matter content. Due to their mineralogy, grain-size distribution and plasticity the clays appear to be suitable as raw material for the growing Morocco ceramic industry.

The boundary conditions of saponite formation are generally considered to be well known, but significant gaps in our knowledge persist in respect to the influence of solution chemistry, temperature, and reaction time on the mineralogy, structure, stability, and chemical composition of laboratory-grown ferrous saponite. In the present study, ferrous saponite and Mgsaponite were synthesized in Teflon-lined, stainless steel autoclaves at 60, 120 and 180°C, alkaline pH, reducing conditions, and initial solutions with molar Si:Fe:Mg ratios of 4:0:2, 4:1:1, 4:1.5:0.5, 4:1.75:0.25, and 4:1.82:0.18. The experimental solutions were prepared by dissolution of sodium orthosilicate (Na4SiO4), iron(II)sulfate (FeSO4·6H2O) and magnesium chloride salts (MgCl2·6H2O with ≤ 0.005 mass% of K and Ca) in 50 mL ultrapure water that contained 0.05% sodium dithionite as the reducing agent. The precipitates obtained at two, five and seven days of reaction time were investigated by X-ray diffraction techniques, transmission electron microscopy analysis, infra-red spectroscopy, and thermo-analytical methods.

The precipitates were composed mainly of trioctahedral ferrous saponite, with small admixtures of co-precipitated brucite, opal-CT, and 2-line ferrihydrite, and nontronite as the probable alteration product of ferrous saponite. The compositions of the obtained ferrous saponites were highly variable, (Na0.44−0.59 K0.00−0.05 Ca0.00−0.02) (Fe2+0.37−2.41 Mg0.24−2.44 Fe3+0.00−0.28)Σ2.65−2.85 [(Fe3+0.00−0.37 Si3.63−4.00)O10](OH)2, but show similarities with naturally occurring trioctahedral Fe and Mg end members, except for the Al content. This suggests that a complete solid solution may exist in the Fe-Mg-saponite series.

A conceptual reaction sequence for the formation of ferrous saponite is developed based on the experimental solution and solid compositions. Initially, at pH ≥ 10.4, brucite-type octahedral template sheets are formed, where dissolved Si-O tetrahedra are condensed. Subsequent reorganization of the octahedra and tetrahedra via multiple dissolution-precipitation processes finally results in the formation of saponite structures, together with brucite and partly amorphous silica. The extent of Fe2+ incorporation in the octahedral template sheets via isomorphic substitution is suggested to stabilize the saponite structure, explaining (i) the abundance of saponite enriched in VIFe2+ at elevated Fe supply and (ii) the effect of structural Fe on controlling the net formation rates of ferrous saponite.