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The understanding of the structural formula of smectite minerals is basic to predicting their physicochemical properties, which depend on the location of the cation substitutions within their 2:1 layer. This implies knowing the correct distribution and structural positions of the cations, which allows assigning the source of the layer charge of the tetrahedral or octahedral sheet, determining the total number of octahedral cations and, consequently, knowing the type of smectite. However, sometimes the structural formula obtained is not accurate. A key reason for the complexity of obtaining the correct structural formula is the presence of different exchangeable cations, especially Mg. Most smectites, to some extent, contain Mg2+ that can be on both octahedral and interlayer positions. This indeterminacy can lead to errors when constructing the structural formula. To estimate the correct position of the Mg2+ ions, that is their distribution over the octahedral and interlayer positions, it is necessary to substitute the interlayer Mg2+ and work with samples saturated with a known cation (homoionic samples). Seven smectites of the dioctahedral and trioctahedral types were homoionized with Ca2+, substituting the natural exchangeable cations. Several differences were found between the formulae obtained for the natural and Ca2+ homoionic samples. Both layer and interlayer charges increased, and the calculated numbers of octahedral cations in the homoionic samples were closer to four and six in the dioctahedral and trioctahedral smectites, respectively, with respect to the values calculated in the non-homoionic samples. This change was not limited to the octahedral sheet and interlayer, because the tetrahedral content also changed. For both dioctahedral and trioctahedral samples, the structural formulae improved considerably after homoionization of the samples, although higher accuracy was obtained the more magnesic and trioctahedral the smectites were. Additionally, the c, Ministerio de Economía y Competitividad (MINECO), Fundação para a Ciência e a Tecnologia, Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

A review and a synthesis of the geological, mineralogical, and crystal chemical data available in the literature on active Spanish bentonitic exploitations were done, and at the same time, new data are provided from a set of representative samples from these deposits. They were located in three different areas with different geological origins: (1) Miocene sedimentary deposits from the Tajo Basin (Madrid–Toledo provinces) in the center of the Iberian Peninsula, where bentonites appear in two different units named for their colors (Green Clays and Pink Clays); (2) samples from Tamame de Sayago (Zamora province) originating from the hydrothermal alteration of granitic Variscan rocks; and 3) Miocene deposits originating from the hydrothermal alteration of volcanic or subvolcanic rocks from the Cabo de Gata volcanic area (Almería Province) in the southern part of Spain, where the three main deposits (Cortijo de Archidona, Los Trancos, and Morrón de Mateo) were studied. The bentonites from the Tajo Basin were formed mainly by trioctahedral smectites, and there were significant mineralogical differences between the Green and Pink Clays, both in terms of the contents of impurities and in terms of smectite crystallochemistry and crystallinity. The smectites from Tamame de Sayago were dioctahedral (montmorillonite–beidellite series), and they appeared with kaolinite, quartz, and mica in all possible proportions, from almost pure bentonite to kaolin. Finally, the compositions of the bentonites from the three studied deposits in Cabo de Gata were quite similar, and zeolites and plagioclases were the main impurities. The structural formulae of the smectites from Cortijo de Archidona and Los Trancos showed a continuous compositional variation in beidellite–montmorillonite, while in Morrón de Mateo, the smectites were mainly montmorillonite, although there was continuous compositional variation from Al montmorillonites to Fe–Mg-rich saponites. The variation in the smectite compos, Ministerio de Economía y Competitividad (MINECO), Portuguese Science and Technology Foundation (FCT), Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

In this study, beidellite, paragonite and other 2:1 type layered aluminosilicates with high layer charges (2:1 type LA-HLC) were synthesized under hydrothermal conditions by using boehmite as a precursor. The concentrations of dissolved Al from boehmite precursor had important effects on the mineral components of the hydrothermal products and the layer charge density of the resultant minerals. A combination of 2:1 type LA-HLC, paragonite, and minor beidellite formed at a relatively high concentration of dissolved Al, while beidellite with a small amount of 2:1 type LA-HLC was produced at a low concentration of dissolved Al. The resultant 2:1 type LA-HLC exhibited poor swelling ability when treated with ethylene glycol. A solid-state transformation mechanism was proposed for the transformation based on the HRTEM images and EDS analyses, which showed that lattice fringes representing (001) faces of 2:1 type layered aluminosilicates (2:1 type LA) were parallel to the (020) direction of boehmite. The successful transformation of boehmite into 2:1 type LA provides insights to well understand the formation of 2:1 type LA in geological processes and offer a facile preparation approach for the synthesis of 2:1 type LA with different layer charges.

Chitosan (CS) and purified sodium beidellite (Na-Bd) were used to obtain a nanocomposite able to carry anionic pharmaceutical ingredients. Diclofenac sodium (DS) was chosen as a model drug and was loaded by intercalation solution technique. Solid state characterization of the resultant hybrids was performed, including X-ray diffraction, Fourier transformed infrared spectroscopy, Thermal analysis and high resolution transmission electron microscopy coupled with energy dispersive X-Ray analysis. Drug loading capacity and encapsulation efficiency were quantified by measuring equilibrium concentration by UV–Vis spectroscopy at 276 nm. Solid-state characterization of the samples confirmed both chemical and physical interaction of DS with the nanocomposites. High drug loading along with a modified cumulative release of the drug in simulated intestinal fluid was obtained. The developed clay/polymer hybrids can act as potential candidates for the design of modified dosage forms of anionic drugs.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The surfaces of natural beidellite clay were modified with cationic surfactant, tetradecyltrimethylammonium bromide, at different concentrations. The organo-beidellites were analysed using thermogravimetric analysis which shows four thermal oxidation/decomposition steps. The first step of mass loss is observed from room temperature to 130 °C due to the dehydration of adsorbed water. The second step of mass loss between 130 and 400 °C is attributed to the oxidation step of the intercalated organic surfactant with the formation of charcoal. The third mass loss happens between 400 and 500 °C which is assigned to the loss of hydroxyl groups on the edge of clays and the further oxidation step of charcoal. The fourth step is ascribed to the loss of structural OH units as well as the final oxidation/decomposition step of charcoal which takes place between 500 and 700 °C. Thermogravimetric analysis has proven to be a useful tool for estimating loaded surfactant amount.

The surfaces of natural beidellite were modified with cationic surfactant octadecyl trimethylammonium bromide at different concentrations. The organo-beidellite adsorbent materials were then used for the removal of atrazine with the goal of investigating the mechanism for the adsorption of organic triazine herbicide from contaminated water. Changes on the surfaces and structure of beidellite were characterised by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and BET surface analysis. Kinetics of the adsorption studies were also carried out which show that the adsorption capacity of the organoclays increases with increasing surfactant concentration up until 1.0 CEC surfactant loading, after which the adsorption capacity greatly decreases. TG analysis reveals that although the 2.0 CEC sample has the greatest percentage of surfactant by mass, most of it is present on external sites. The 0.5 CEC sample has the highest proportion of surfactant exchanged into the internal active sites and the 1.0 CEC sample accounts for the highest adsorption capacity. The goodness of fit of the pseudo-second order kinetic confirms that chemical adsorption, rather than physical adsorption, controls the adsorption rate of atrazine.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

The external invertase isoform 1 (EINV1) was immobilised on eight differently modified beidellite nanoclays, Modifications were composed of organo-modification with different amounts of surfactant - hexadecyl trimethylammonium cation (HDTMA), pillaring with Al/Fe containing polyhydroxy cations and acid modification of Na-enriched and pillared clays. The modified nanoclays were characterised by XRD, N-2-physisorption, SEM and FT-IR spectroscopy. The amount of bound enzyme activity was significantly influenced by the modification of beidellite ranging from 50 to remarkable 2200 U/g. Biochemical characterization was performed for five modified nanoclays showing the highest enzyme activity after invertase immobilisation. The investigation demonstrated that after immobilisation the structure and the catalytic properties of invertase were preserved, while Km values were slightly increased from 26 to 37 mM. immobilisation significantly improved thermal and storage stability of EINV1. Results indicate that beidellite nanoclays obtained by low cost modifications can be applied as a suitable support for the immobilisation of invertase. The immobilizate can be efficiently engaged in sucrose hydrolysis in batch reactor.

Pillared clays (PILCs) based on bentonite clay from "Bogovina" (Serbia) were obtained using Al3+ and Fe3+ pillaring cations in different ratios. Thorough characterization of the starting material and PILCs was performed for better understanding of the catalytic activity that they had exibited in the catalytic wet peroxide oxidation (CWPO) of organic pollutants in water. The characterization consisted of the following techniques: XRD, XRD of oriented samples (swelling test), Greene-Kelley test (smectite type determination), TGA/DTA followed by XRD analysis of samples heated at characteristic temperatures, elemental analysis using ICP-OES and microwave digestion. FTIR, DR UV-Vis, Mossbauer spectroscopy, nitrogen physisorption at -196 degrees C and SEM imaging. Smectite in admixture with quartz was identified, together with impurities like feldspar, illite, chlorite and X-ray amorphous matter. Smectite mineral was dominantly of the beidellite type. Pillaring led to changes in the d(001) smectite basal spacing and stabilization of the smectite structure. Increasing Fe3+ content of the pillaring solutions resulted in the increased incorporation of iron into the PILCs as isolated octahedrally coordinated Fe3+ cations, smaller and bigger oligonuclear octahedrally coordinated Fe3+ species and octahedrally coordinated Fe3+ particles. The structure of incorporated pillars was similar to that of naturally occurring minerals akageneite and lepidocrocite. Micro- and mesoporosity were improved due to pillaring and the layered smectite structure was retained.

Pillared clays (PILCs) based on bentonite clay from Bogovina (Serbia) were obtained using Al3+ and Fe3+ pillaring cations in different ratios. Thorough characterization of the starting material and PILCs was performed for better understanding of the catalytic activity that they had exibited in the catalytic wet peroxide oxidation (CWPO) of organic pollutants in water. The characterization consisted of the following techniques: XRD, XRD of oriented samples (swelling test), Greene-Kelley test (smectite type determination), TGA/DTA followed by XRD analysis of samples heated at characteristic temperatures, elemental analysis using ICP-OES and microwave digestion. FTIR, DR UV-Vis, Mossbauer spectroscopy, nitrogen physisorption at -196 degrees C and SEM imaging. Smectite in admixture with quartz was identified, together with impurities like feldspar, illite, chlorite and X-ray amorphous matter. Smectite mineral was dominantly of the beidellite type. Pillaring led to changes in the d(001) smectite basal spacing and stabilization of the smectite structure. Increasing Fe3+ content of the pillaring solutions resulted in the increased incorporation of iron into the PILCs as isolated octahedrally coordinated Fe3+ cations, smaller and bigger oligonuclear octahedrally coordinated Fe3+ species and octahedrally coordinated Fe3+ particles. The structure of incorporated pillars was similar to that of naturally occurring minerals akageneite and lepidocrocite. Micro- and mesoporosity were improved due to pillaring and the layered smectite structure was retained. (c) 2012 Elsevier Inc. All rights reserved.

Pillared clays (PILCs) based on bentonite clay from Bogovina (Serbia) were obtained using Al3+ and Fe3+ pillaring cations in different ratios. Thorough characterization of the starting material and PILCs was performed for better understanding of the catalytic activity that they had exibited in the catalytic wet peroxide oxidation (CWPO) of organic pollutants in water. The characterization consisted of the following techniques: XRD, XRD of oriented samples (swelling test), Greene-Kelley test (smectite type determination), TGA/DTA followed by XRD analysis of samples heated at characteristic temperatures, elemental analysis using ICP-OES and microwave digestion. FTIR, DR UV-Vis, Mossbauer spectroscopy, nitrogen physisorption at -196 degrees C and SEM imaging. Smectite in admixture with quartz was identified, together with impurities like feldspar, illite, chlorite and X-ray amorphous matter. Smectite mineral was dominantly of the beidellite type. Pillaring led to changes in the d(001) smectite basal spacing and stabilization of the smectite structure. Increasing Fe3+ content of the pillaring solutions resulted in the increased incorporation of iron into the PILCs as isolated octahedrally coordinated Fe3+ cations, smaller and bigger oligonuclear octahedrally coordinated Fe3+ species and octahedrally coordinated Fe3+ particles. The structure of incorporated pillars was similar to that of naturally occurring minerals akageneite and lepidocrocite. Micro- and mesoporosity were improved due to pillaring and the layered smectite structure was retained. (c) 2012 Elsevier Inc. All rights reserved.

Pillared clays (PILCs) based on bentonite clay from "Bogovina" (Serbia) were obtained using Al3+ and Fe3+ pillaring cations in different ratios. Thorough characterization of the starting material and PILCs was performed for better understanding of the catalytic activity that they had exibited in the catalytic wet peroxide oxidation (CWPO) of organic pollutants in water. The characterization consisted of the following techniques: XRD, XRD of oriented samples (swelling test), Greene-Kelley test (smectite type determination), TGA/DTA followed by XRD analysis of samples heated at characteristic temperatures, elemental analysis using ICP-OES and microwave digestion. FTIR, DR UV-Vis, Mossbauer spectroscopy, nitrogen physisorption at -196 degrees C and SEM imaging. Smectite in admixture with quartz was identified, together with impurities like feldspar, illite, chlorite and X-ray amorphous matter. Smectite mineral was dominantly of the beidellite type. Pillaring led to changes in the d(001) smectite basal spacing and stabilization of the smectite structure. Increasing Fe3+ content of the pillaring solutions resulted in the increased incorporation of iron into the PILCs as isolated octahedrally coordinated Fe3+ cations, smaller and bigger oligonuclear octahedrally coordinated Fe3+ species and octahedrally coordinated Fe3+ particles. The structure of incorporated pillars was similar to that of naturally occurring minerals akageneite and lepidocrocite. Micro- and mesoporosity were improved due to pillaring and the layered smectite structure was retained.

A mineralogical and microtextural study of Somosaguas Miocene deposits, located in the Madrid Basin (western Madrid, Spain), was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and optical microscopy, whereas crystal chemistry data were obtained by analytical electron microscopy-transmission electron microscopy and electron icroprobe analysis. Four stratigraphic sections were studied, compising detrital rocks representing intermediate and distal facies from alluvial fan deposits. The predominant source area of these sediments was the granitic rocks of the Spanish Central System with a lesser contribution of metamorphic rocks. Clayey arkoses are the most abundant rocks of these sections, typical of granite alteration under warm, semi-arid climates. The mineralogy is characterized by phyllosilicates, followed by feldspars and quartz. The data obtained reveal mineral mixtures of detrital (quartz, feldspars, kaolinite, micas and chlorite), transformed (illite and beidellite) and neoformed (montmorillonite) origin. Clay minerals resulted from interactions between detrital minerals and meteoric waters. Two trends of degradation of micas are detected. The first shows a transition from muscovites and dioctahedral illites, to beidellites. The other trend is defined by the biotite degradation to beidellites with different layer charge and octahedral Fe content. Montmorillonites were neoformed from the hydrolysis and weathering of primary minerals (feldspars and muscovite). Magnesian clay minerals such as sepiolite, palygorskite and trioctahedral smectites, extremely abundant in the centre of the basin, were not detected in Somosaguas sediments., Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

This paper represents an overview of the spectroscopic studies of both synthetic and naturally occurring beidellites performed as part of my research over the past 16 years. It shows that detailed information on the local structure of beidellite and changes in this local structure upon heating can be obtained by combining a range of spectroscopic techniques such as mid-infrared, near-infrared, infrared emission, Raman, nuclear magnetic resonance and X-ray photoelectron spectroscopy.

The effects of a subvolcanic intrusion on its host rocks in the Priego de Córdoba area (SE Spain) was studied by optical microscopy, X-ray diffraction and electron microscopy. The intrusion of a laccolith of stratiform dolerite in partially consolidated marly sediments at quite shallow depths below the ocean floor took place during the intracontinental rifting phase of the Subbetic zone of the Betic Cordilleras. In the first stage, contact metamorphism caused the crystallization of calc-silicate consisting of grossular andradite garnet, diopside, vesuvianite and titanite for which the upper temperature limit was 500°C. Phyllosilicates are found in a network of veins cross-cutting the carbonate and calc-silicate matrix, indicating their formation during a process of hydrothermal alteration superimposed on the contact aureole. In the area closest to the subvolcanic rocks, saponite is the main phyllosilicate although some dispersed chlorite is found. In the zone furthest from the contact, corrensite together with saponite and beidellitic smectites were observed. The presence of low-temperature phases (saponite) in the area closest to the laccolith reveals the evolution of the hydrothermal system toward cooler conditions. In the early stage, the circulation of a hot hydrothermal fluid caused the crystallization of chlorite in the area surrounding the subvolcanic body and corrensite in the more distal area, which might have begun even during the contact metamorphism stage. A cooling phase followed, resulting in the crystallization of saponite in the host rocks, and the crystallization of dioctahedral and trioctahedral smectites inside the subvolcanic body.

A number of smectitic samples from Almeria (SE Spain) were studied by chemical analysis, DTA, TG, XRD (oriented aggregates with ethylene glycol treatment and Greene-Kelly test), FTIR and MAS NMR. Chemically they resembled a beidellite-montmorillonite series, displaying DTA/TG characteristics already quoted in the literature in beidellite descriptions. They did not swell after the Greene- Kelly test, as it has also been reported for some beidellites. Nevertheless XRD of the oriented, glycolated samples, FTIR, MAS NMR and revision of the chemical analysis demonstrated that they were mixedlayered kaolinite/montmorillonite. It is possible that some of the reported beidellites in the literature are kaolinite/montmorillonite. Beidellite characterization must be supported by several different techniques.

[ES] Se han estudiado tres perfiles de alteración de rocas graníticas y dos muestras de arcosas en la zona NO de la provincia de Madrid. Las rocas, clasificadas como granodioritas y adamellitas, presentan albitas con sericitización variable, y escasos feldespatos potásicos muy poco alterados. La fracción arcilla de los horizontes de alteración de estas rocas presenta un alto contenido en beidellita, así como porcentajes menores de ilita y haloisita-7Ǻ. Por otra parte, los minerales de la arcilla presentes en las arcosas son ilita, beidellita y caolinita bien cristalizada. El estudio de la microfábrica muestra la presencia de albitas alteradas con superficies limpias y beidellita de neoformación en cualquier zona de la muestra sin ningún tipo de relación espacial con la albita. En la alteración edafoquímica de la albita, el sodio de este silicato se intercambiaría con protones procedentes de la fase acuosa, con una cinética muy rápida. Esta alcalinización inicial daría lugar a un aumento en la solubilidad de la sílice liberada a partir de la red de las albitas. En aquellos microambientes en los que se produce un prolongado contacto entre la solución de alteración y la albita, la primera puede enriquecerse suficientemente en sílice y alúmina como para alcanzar la precipitación de beidellita o haloisita-7Ǻ. Las variaciones en las condiciones microclimáticas del medio de alteración deben producir fluctuaciones en las actividades de sílice y alúmina, con cambios de pH suficientes como para dar Jugar a la neoformación de beidellita cuando las actividades de sílice sean elevadas y, de haloisita-7Ǻ en caso contrario., [EN] Three weathering profiles of granitic rocks and two arkoses in the NW of Madrid Province have been studied. Rocks are granodiorites and adamellites with albites showing variable degrees of sericitization, and no-weathered potassic feldspars. Clay fraction in the weather horizons from these rocks display~ high content in beidellite, and minor amounts of illite and 7A halloysite. On the other hand, beideltite, illite and well crystallized kaolinite are clay minerals in the arkosic rocks. Microfabric characteristics from SEM studies show abundant weathered albites with clean surfaces and formation of beidellite in al1 areas of the samples. Moreover, spatial relations between albite and beidelliíe are not observed. In pedochemical weathering of albite, sodium from this mineral should be exchanged with protons from soil solution with very quick kinetics. This initial alkaiinization produces an increase in the silica solubility released from albite network. In these environments, a continued contact between weathering solution and albite causes a silica and alumina enrichment and beidellite or 71-halloysite precipitation. Microclimatic variations in weathering environment may cause íluctuations in silica and alumina activities with resultant pH changes. So, when silica activities are high, beidellite nwformation is favoured; on the contrary, 7A -halloysite is precipitated.

The Samos bentonites of Upper Miocene age are medium-low grade (40-70% smectite), formed at the expense of acidic pyroclastic flows mainly in a subaerial-lacustrine environment, and consist mainly of Ca-rich medium-high-charge montmorillonite, opal-CT and sanidine with minor quartz and locally mordenite. The Chios bentonites of Lower-Middle Miocene age were formed at the expense of trachyandesitic tuffs in a lacustrine environment and consist mainly of Ca,Mg-rich high-charge montmorillonite (33-75%), similar to the SAz-1, and opal-CT, minor plagioclase, chlorite, plagioclase, carbonates and traces of talc and serpentine. In Thrace, Lower Oligocene pyroclastic flows, tuffs and lavas of acidic-intermediate composition were altered to Ca-rich and in places, Na-rich, dioctahedral montmorillonite or Fe-rich beidellite (Sappes area) and with minor opal-CT, mordenite, quartz, plagioclase and, in places, illite. The bentonites are medium-high grade (50-58% smectite in Feres-Pefka, 74-86% in Skaloma and 29-40% in Sappes). The bentonites have moderate cation exchange capacity and moderate/low swelling index and viscosity and they are not suitable for the drilling and foundry industries. However, preliminary results show that after processing, most of the deposits might be utilised in high added-value applications such as bleaching earths or in the synthesis of clay-based polymer nanocomposites., The Samos bentonites of Upper Miocene age are medium-low grade (40-70% smectite), formed at the expense of acidic pyroclastic flows mainly in a subaerial-lacustrine environment, and consist mainly of Ca-rich medium-high-charge montmorillonite, opal-CT and sanidine with minor quartz and locally mordenite. The Chios bentonites of Lower-Middle Miocene age were formed at the expense of trachyandesitic tuffs in a lacustrine environment and consist mainly of Ca,Mg-rich high-charge montmorillonite (33-75%), similar to the SAz-1, and opal-CT, minor plagioclase, chlorite, plagioclase, carbonates and traces of talc and serpentine. In Thrace, Lower Oligocene pyroclastic flows, tuffs and lavas of acidic-intermediate composition were altered to Ca-rich and in places, Na-rich, dioctahedral montmorillonite or Fe-rich beidellite (Sappes area) and with minor opal-CT, mordenite, quartz, plagioclase and, in places, illite. The bentonites are medium-high grade (50-58% smectite in Feres-Pefka, 74-86% in Skaloma and 29-40% in Sappes). The bentonites have moderate cation exchange capacity and moderate/low swelling index and viscosity and they are not suitable for the drilling and foundry industries. However, preliminary results show that after processing, most of the deposits might be utilised in high added-value applications such as bleaching earths or in the synthesis of clay-based polymer nanocomposites.