Kaolinite Crystals Research Articles

Microorganisms Isolated from Saharan Dust Intrusions in the Canary Islands and Processes of Mineral Atmospherogenesis

Global warming due to climate change has increased the frequency of sand and dust storms that affect air quality and ecosystems in general, contributing to air pollution. The Sahara Desert is the most potent emitter of atmospheric dust. The atmosphere is an extreme environment and microorganisms living in the troposphere are exposed to greater ultraviolet radiation, desiccation, low temperatures and nutrient deprivation than in other habitats. The Iberian Peninsula, and specifically the Canary Islands—due to its strategic location—is one of the regions that receive more Saharan dust particles annually, increasing year after year, although culturable microorganisms had previously never been described. In the present work, dust samples were collected from three calima events in the Canary Islands between 2021 and 2022. The sizes, mineralogical compositions and chemical compositions of dust particles were determined by laser diffraction, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Particle morphology and biological features were also studied by scanning electron microscopy (SEM-EDX) and confocal laser scanning microscopy (CLSM). The mineral–bacteria interactions were described from microscopic observations, which revealed the presence of iberulites and small neoformed kaolinite crystals in association with bacteria. This article defines the term “mineral atmospherogenesis” and its variant, “mineral bioatmospherogenesis”, through microbial interaction. This is the first described case of kaolinite produced through mineral bioatmospherogenesis. The bacterial growth in atmospheric dust was illustrated in SEM images, constituting a novel finding. Twenty-three culturable microorganisms were isolated and identified by 16S rRNA sequencing. Members of the phyla Pseudomonadota, Bacillota and Actinomycetota have been found. Some of these microorganisms, such as Peribacillus frigoritolerans, have Plant Growth-Promoting Rhizobacteria (PGPR) properties. Potential human pathogenic bacteria such as Acinetobacter lwoffii were also found. The presence of desert dust and iberulites in the Canary Islands, together with transported biological components such as bacteria, could have a significant impact on the ecosystem and human health.

The role of clay minerals in the concentration and distribution of critical metals in lateritic palaeosols from NE Iberia

The intense chemical weathering processes that lead to the formation of laterites and bauxites result in the enrichment of critical metals (Sc, V, Co, Ga, Ge, Nb, In, Hf, Ta, W, and REEs), which are of great economic interest and are directly involved in the manufacture of various devices essential for the technology transition. This work focuses on combined geochemical and scanning transmission electron microscopy (STEM) studies of lateritic palaeosols developed in continental profiles (Lower Cretaceous, NE Iberia) in order to evaluate the distribution and concentration of critical metals and their possible relationship with weathering processes. In this study, we show that during the process of laterization the highest weathering produces the highest critical metal concentrations (reaching concentrations of 1914.80 ppm in the lateritic palaeosols and 926.30 ppm in the ferruginous macropisoids). Concentrations normalized to the upper continental crust (Taylor and McLennan, 1985) indicate that both the lateritic palaeosols and the macropisoids are enriched in LREEs (256.81 and 223.01 ppm, respectively) and HREEs (107.11and 110.53, respectively). In the lateritic palaeosols, the (La/Sm)c values are higher than those of (Gd/Yb)c, indicating that the LREEs exhibit higher fractionation with weathering, whereas the opposite occurs for the macropisoids, in which the HREEs have higher fractionation. These data indicate that the HREEs are less mobile than the LREEs during laterization. The critical metals present a good positive correlation with Fe, Ti, and Al, but a good negative correlation with K. STEM images and EDS analyses allowed us to identify REE-phases (including monazite, xenotime, and La, Ce and Nd oxides) and nanoparticles of gold adsorbed on the surface of kaolinite crystals. This shows not only that during the laterization process Fe and Ti oxides act as scavengers for the critical metals, but also that kaolinite controls their distribution.

Analysis of the microstructure and morphology of disordered kaolinite based on the particle size distribution

The defected kaolinite is common in sedimentary kaolin, but its structure and disorder mechanism remains unclear. In this paper, three different size distributions of kaolinite particles: 0–300 nm, 200–1000 nm, and 500–2000 nm, were separated and classified from high-defect sedimentary kaolin, collected from Quaternary sedimentary kaolin in Guangxi Zhuang Autonomous Region of China. The three particle sizes of kaolinite have been monitored by particle size analyzer, high-resolution magic-angle spinning NMR (MAS NMR), small and wide-angle X-ray scattering (SAXS and WAXS), X-ray diffraction (XRD), BET surface analyzer, nitrogen gas-adsorption, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the smallest particle size of kaolinite exhibited high degrees of broken Al-O-Al and Si-O-Si bonds, more disordered structure, and elevated specific surface area. The degree of the structural disorder of kaolinite increases as the particle size decreases, which can be attributed to the more aluminum substituted for silicon in tetrahedron sheet in the finer kaolinite crystal, compared to the coarser kaolinite crystals.

Weathering events recorded in uppermost Hauterivian\u2013lower Barremian clay-dominated continental successions from the NW Iberian Range: climatic vs. tectonic controls

The facies and clay mineral study of clay/marl-rich levels from the Torrelapaja Formation (latest Hauterivian–early Barremian, NW Iberian Range, NE Spain) allowed to establish the palaeoclimatic and palaeoenvironmental conditions under they were generated. The muddy levels and pisoids contained therein of two logs were sampled and studied by X-ray diffraction and optical and electron microscopy. A similar mineralogical upwards trend is recorded in both logs, with a decrease in calcite coupled with an increase in quartz and orthoclase content and constant proportions in goethite, hematite, diaspore, anatase, rutile, ilmenite, and clay mineral content. The lower muddy levels have higher kaolinite content than the upper levels, where illitic phases are the dominant clay minerals. Smectite and intergrowths of illitic phases and kaolinite are also detected upwards. The kaolinite and smectite textures indicate an authigenic origin, whereas the illitic phases are former phases acting as a substrate for kaolinite crystallization. Pisoids mineralogy and texture show an in-situ origin, but some are fractured, indicating reworking processes. The mineral association found in the muddy levels is characteristic of oxisols formed under warm and humid conditions. The upward decrease in kaolinite content is coeval with an increase in the illitic phases and quartz content, related to siliciclastic input, but is also coeval with the presence of authigenic smectite. This indicates a decrease in chemical weathering, not fully registered due to the siliciclastic contribution, which was possibly associated with a change to colder, drier conditions during the latest Hauterivian–early Barremian in the studied area.

Provenance and Paleoenvironmental Studies of Cretaceous African and South American Kaolins: Similarities and Differences

The African and South American continents are of great interest in continental drift studies. Hence, this study assesses the possible correlations in the provenance and paleoenvironment of selected Cretaceous Nigerian and Cameroonian (in Africa), and Argentine and Brazilian (in South America) kaolins through an analysis of their mineralogical and geochemical characteristics. On the basis of their mineralogical composition, the Nigerian Lakiri and Brazilian soft Capim River kaolins are predominantly characterised as pure kaolins, whereas the kaolins from Cameroon (except for Yatchika) and Argentina are mainly considered as sandy kaolins. The present study revealed that the Brazilian soft Capim River kaolin had the highest kaolinite structural order, whilst the Argentine Santa Cruz kaolin had the least. The kaolins from Nigeria, Cameroon, and Argentina were dominated by subhedral to anhedral kaolinite crystals relative to the Brazilian kaolin, which possess more euhedral kaolinite crystals. The kaolins were formed by the intense weathering of intermediate to felsic rocks under anoxic conditions, which is consistent with the structural framework of the basins. The average paleotemperatures obtained for the kaolins (except for the one from Santa Cruz) indicates that the paleoweathering took place under tropical climates.

Particle size distribution and crystallinity as indicators of kaolinite genesis

The possibility of using the particle size distribution curve obtained by the laser diffraction method to determine the kaolinite genesis is considered in the paper. The full width at half maximum (FWHM) and the peak shape index (PSI) are proposed to characterize the distribution curve shape. The normal distribution of microcrystals with a perfect structure and clear faceting is associated with a single mechanism of the rock transformation into kaolinite. Most likely, such a mechanism is the solid-phase transformation of the parent rock minerals into kaolinite. The lognormal distribution, reflected by the deformation of the descending branch of the distribution curve peak, is associated with two mechanisms of kaolinite formation. In addition to solid-phase transformations, kaolinite crystallization from the degradation products of the parent rock occurs. Secondary kaolinites are characterized by a broad particle size distribution, a wide FWHM, and low crystal perfection in comparison with primary kaolinites. It is associated with a change in the kaolinite properties as a result of their transportation from the primary weathering crust to the sedimentation zone. It is proposed to use the data on the particle size distribution together with X-ray diffractometry (XRD) and scanning electron microscopy (SEM) for the first time to determine the kaolinite genesis.

Origin and characterization of clay deposits in the Dupi Tila Formation of the Bengal Basin, Bangladesh

Clay is one of the most important minerals having both geological and industrial significance. Clay lenses in the Dupi Tila Formation of the Bengal Basin have been investigated to characterize and determine their origin in terms of geological environment. Clay samples have been collected from the fresh cropped surface from the areas of Dupigaon (Jaintiapur Clay), Birisiri (Bijoypur Clay), and Bandarban (Bandarban Clay) in Bangladesh. Characterizations of the clay deposits have been carried out by field investigation, scanning electron microscope (SEM), X-ray diffraction (XRD), laser particle size analysis techniques, and X-ray fluorescence (XRF). Field investigation shows that clays are white, yellowish to variegated in color with a thickness of 15 cm to about 4 m, and moderately soft. The X-ray diffraction analysis reveals that kaolinite is the predominant clay mineral in all samples with minor amounts of quartz and feldspar. SEM analysis indicates that abundant tiny disc and irregular shape of kaolinite crystals are mixed with sparse gathers of K-feldspar and detrital quartz grains. The presence of silt in all the clay samples has been verified by the gran size ranges from laser particle size analysis. The grittiness is the result of the deposition of silt considering the highest percentage in Bandarban and Bijoypur and lowers in Jaintiapur. The XRF analysis shows that SiO2 (68–75%) is the most abundant among major oxides, Al2O3 and Fe2O3 come next, along with the lower percentage of MnO, MgO, CaO, Na2O, K2O, TiO2, P2O5, and SO3. The average concentration of different trace elements has been ranging from 3 to 430 ppm. Several geochemical indices suggest that the sediments might be derived from the Shillong plateau and the Himalayas in the north and the Naga-Lushai-China Hill-Arakan Yoma mobile belt in the northeast, which are highly weathered sedimentary terrains. Provenance studies and elemental ratio supports the evidence of the felsic origin of the studied sample. The Chemical Index of Alteration (CIA) value of 93% indicates the occurrence of significant weathering in different sediment recycling phases. From the results of the aforementioned analyses, it is clear that clay lenses in the Dupi Tila Formation are in-situ in origin and parent materials are detrital rather than authigenic nature confirmed by the presence of detrital quartz and reviewing the depositional environment, which gives us the insights of the geological condition of that time as well as the possible mode of sediment transportation.

Evaluating weathering of palaeosols in Cameroon (Central Africa) as a tool for paleoenvironmental reconstruction

Palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas (1400 mm mean annual rainfall, 22 °C mean annual temperature) of the Adamaoua region in north-central Cameroon by means of macro- and micromorphology, geochemical analyses and bulk mineralogy. The studied sequence at Mangoli is about 11 m deep and is divided into three sections, in terms of weathering products: (1) Precambrian granite, located in the base; (2) vesicular porphyritic basalt, in the middle section; (3) aphanitic basalt in the uppermost layer. The products of weathering from the Precambrian granite consist of the following horizons, from bottom to top: a saprolite with granite structure, a yellowish to whitish fine saprolite, and Btg and Bt horizons. The saprolite horizon is dominated by quartz, feldspars (plagioclases and microcline), and muscovite. Primary minerals, except quartz, show weathering, while clay fills the pores and fractures and kaolinite dominates the clay mineralogy. The granite weathering products are slightly acidic (pH ranging from 5.9 to 6.3) and display low values of Fe extracted with dithionite-soluble Fe (Fed) and high Chemical Index of Alteration (CIA) and Mineral Index of Alteration (MIA) values. The middle section of the sequence consists of different areas of the basalt’s spheroidal weathering zone, from the more advanced to the less weathered phase. The clay mineralogy is dominated by kaolinite. The zones with the most alteration are the fractures, where thick, reddish clay coatings are observable. In these areas, the minerals are well crystallized. The kaolinite crystals share areas with Fe oxides. The upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons. The lowest is a C horizon, overlaid by Bt horizons. The C horizon shows a high degree of weathering in some areas, but it is still possible to identify the primary minerals. Their products of weathering are slightly acidic (pH = 6) and have high Fed, CIA and MIA values, which are indicative of intense weathering. The results suggest warm and humid climates during the formation of all of the palaeosols, similar to modern environmental conditions. We conclude that Palaeosol 1, formed from the granite, represents a soil formed during the Mid-Miocene Climatic Optimum, when the temperatures are assumed to have been 3–6 °C warmer than today. Features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau. Meanwhile, the former top of the plateau is located on the slopes and lower altitudes; the burial of soils by volcanic materials at Mangoli increased the altitudes of the former plateau and resulted in landscape inversion (slopes). These palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide.

Effect of Mg(II) and Na(I) Doping on the Electronic Structure and Mechanical Properties of Kaolinite

Because kaolinite has multiple defects, it is very important to study the effect of different doped cations on the electronic structure and mechanical properties of kaolinite (Al4Si4O18H8) from the microscopic point of view with the first-principle calculation method. The results exhibited that the doping of Mg(II) and Na(I) makes the ion bond and layer spacing of kaolinite crystal change, and the bond length of the chemical bond between the doped and O atom is positively related to the atomic radius of the doped cations. Compared with undoped kaolinite crystal, the band gap width of the Mg-doped and Na-doped kaolinite crystal was larger, but the typical insulator characteristics were still maintained. Compared with undoped kaolinite crystal, Mg-doped and Na-doped kaolinite crystal had more electron transfer to O, while the Mg–O bond and Na–O bond had more ionic bond properties and less covalent bond composition than the Al–O bond. Finally, the elastic properties of undoped, Mg-doped, and Na-doped kaolinite crystal were further analyzed by calculating the elastic constant matrix. The influence of doping Mg(II) and Na(I) on C11 and C22 was greater than that on C33, indicating that doping had a greater influence on the stiffness in the direction of the parallel crystal plane. The doping of Mg(II) and Na(I) weakened the rigidity of kaolinite crystal materials and improved the plasticity and ductility of the materials. The atom-scale information provided a basis for explaining the mechanical behavior of kaolinite and is expected to provide guidance for solving the deformation problems in soft rock roadways.

Systematic exploration of the interactions between Fe-doped kaolinite and coal based on DFT calculations

In order to explore the interactions between Fe-doped kaolinite and coal, the lattice substitution of different valence Fe in kaolinite crystal was analyzed by Mössbauer spectrometer and density functional theory (DFT) calculations. The results show that there are three lattice substitution forms of Fe with different valence states in kaolinite, are mainly six-coordinate Fe2+, four-coordinate Fe3+ and six-coordinated Fe3+, and the difficult and easy level of lattice substitution from easy to difficult is six-coordinate Fe2+ > four-coordinate Fe3+ > six-coordinated Fe3+. According to the results of Mössbauer spectrum analysis and DFT calculation, the best calculation model of Fe-doped kaolinite was determined, and the interactions between Fe-doped kaolinite and coal were investigated in this study through DFT calculations. The results show that the interaction mechanism between Fe-doped kaolinite and coal is the result of hydrogen-bond interaction and the strong electrostatic attraction between benzene rings and Fe-doped kaolinite surfaces, and the interactions between Fe-doped kaolinite and coal are stronger than that between perfect kaolinite and coal. The lattice substitution of Fe mainly enhances the reactivity of kaolinite (0 0 1) surface, and the equilibrium cation Na+ has a certain influence on the reactivity of kaolinite (0 0 1) and (001¯) surface. The results can provide a theoretical basis for further study of the true adsorption state of kaolinite surface in coal slurry water.

Influence of texture on DC conductivity and dimensional changes of kaolin and illitic clay

The influence of the technological texture on the DC conductivity of an illitic clay and kaolin was studied. Textured samples were prepared by layering a wet plastic mass made from distilled water and clay. Anisotropy of the samples was assessed by studying the DC conductivity and dimensional changes as functions of temperature in parallel and perpendicular directions to the basal planes of illite and kaolinite crystals. It was found that the shrinkage of the samples after firing is more significant in the direction perpendicular to the basal planes. The DC conductivity both in the illitic clay and kaolin is higher parallel to the basal planes of illite or kaolinite crystals than perpendicular to them. In both materials, the dominant charge carriers are K+ and Na+ ions, complemented by H+ and OH– ions during the removal of the physically bound water and dehydroxylation. The influence of the texture remains observable in the kaolin samples even after a high-temperature treatment (1200 °C), contrary to the illitic clay where this influence is suppressed by the high amount of the glassy phase (~80 mass %) after the thermal treatment at temperatures above 1000 °C. Compared to the illitic clay, the influence of the technological texture on the DC conductivity of kaolin is more significant.

Synthesis and characterization of kaolinite/polyaniline nanocomposites and investigating their anticorrosive performance in chlorinated rubber/alkyd coatings

Kaolinite/polyaniline nanocomposites (KPAns) were prepared byin situchemical oxidative polymerization. The nanocomposites were synthesized with four different aniline to kaolinite weight percentage; 5, 10, 15, and 20% to produce KPAn5, KPAn10, KPAn15, and KPAn20, respectively. The structural, morphological and thermal properties of the nanocomposites were characterized using Fourier transform infrared, X‐ray diffraction, scanning electron microscope (SEM)–energy dispersive X‐ray analyzer, transmission electron microscope, and Thermogravimetric analysis. The results revealed a very slight increase in the d‐spacing of kaolinite XRD peaks. SEM images showed hexagonal platy sheets of kaolinite crystals that are indistinguishable in KPAn nanocomposites images, especially on using a high amount of polyaniline (PANI). The increase in the percentage of carbon and nitrogen elements in EDAX data proved the adsorption of PANI on the surface of kaolinite platelets. To investigate the anticorrosive performance of the synthesized nanocomposites, kaolinite and KPAn nanocomposites were loaded in various amounts (1, 5, and 9%) in a blend of chlorinated rubber/medium oil alkyd resin (CR/MA) to prepare different formulations that were applied and evaluated as protective coats on steel substrates. Visual corrosion test revealed that 5% KPAn‐loaded CR/MA coats exhibited the best protection against corrosion amongst other formulations as indicated by their lowest degree of rusting and blistering. Potentiodynamic polarization measurements confirmed the highest protection of 5% KPAn10‐loaded CR/MA coat as indicated by the quantitative corrosion rates obtained from Tafel extrapolation. POLYM. COMPOS., 40:2777–2789, 2019. © 2018 Society of Plastics Engineers

Effect of pretreatment on microstructure and photocatalytic activity of kaolinite/TiO2 composite

Kaolinite/TiO2 composites were prepared by using sol-gel method and raw kaolin, pretreated kaolinite and tetrabutyl titanate as the main raw materials. X-ray diffractometer, field-emission scanning electron microscope and infrared spectrometer analysis were carried out to characterize the phase composition and microstructure of the samples. The photocatalytic performance of the kaolinite/TiO2 composites were evaluated by degrading the methylene blue (MB) and phenol aqueous solution, respectively. The results show that intercalation and exfoliation reduced the size and thickness of kaolinite particles. Acid treatment improved the distribution and the loading quantity of TiO2 grains. When the kaolinite/TiO2 composites were calcined at 500 °C, the tetragonal structure of anatase particles of 30–100 nm in size were obtained, but the exfoliated kaolinite crystals were damaged. The degradation rate of MB increased gradually with the extension of photocatalytic reaction time and the enhancement of photocatalyst dosage. The adsorption performance of acid-treated kaolinite/TiO2 composite (AKT) was nearly the same as that of raw kaolin/TiO2 composite (RKT), but that of the exfoliated kaolinite/TiO2 composite (EKT) was the most excellent. The photocatalytic performance of AKT and EKT were better than that of RKT, and AKT exhibited the optimum property. Under a certain photocatalyst dosage and photocatalysis time, the absorption rate and the degradation rate decreased gradually with the enhancement of initial concentration of MB. Similar result was also acquired for the degradation of phenol. Both the acid treating and the exfoliating to kaolinite enhanced the photocatalytic performance of the kaolinite/TiO2 composite photocatalysts, but acid treatment may be more helpful to the preparation of high performance kaolinite/TiO2 composite photocatalyst.

Weathering and clay formation in semi-arid calcareous soils from Northeastern Brazil

Despite the great number of studies exploring the inter-profile genetic relationships in a catena, the microrelief and microtopographic features have been overlooked. This study investigated the role of microrelief surfaces (convex, flat and concave) on the mineral formation and pedogenesis in soils derived from limestones in NE Brazil. Soil profiles were studied in different micro-landscape surfaces, and soil samples were taken by horizon for chemical (weathering indexes) and mineralogical analyses. Mineralogy was determined in samples from diagnostic subsurface horizons, which were subjected to the proper chemical treatment for identification (X-ray diffraction - XRD) and quantification (Newmod II and Thermogravimetry - TG) of the phyllosilicates. The XRD patterns showed similar composition among the parental material of the studied profiles. The chemical also showed similar dynamics along the surfaces; however, the concave surface had soils with a higher degree of weathering, when compared to others. The clay minerals identified on the convex and flat surfaces were vermiculite, illite, smectite and kaolinite; while on the concave surface, only illite and kaolinite were found. Based on the information generated by the Newmod II, there was an increase in the amount of kaolinite towards the concave surface, from 21%, on the convex surface, to >50% of pure kaolinite crystals on the concave surface. These data were supported by the TG curves. Our results evidence that microrelief features, marked by low topographic amplitudes, are determinant for the formation and distribution of phyllosilicates in calcareous soils; and, thus, may be of key importance for predicting the occurrence of contrasting mineralogy in soils derived from calcareous rock.

Characterizing fluids associated with the McArthur River U deposit, Canada, based on tourmaline trace element and stable (B, H) isotope compositions

The origin and nature of fluids responsible for U deposits of the Athabasca Basin have been in debate. This study presents the trace element abundances and stable isotope ratios (D/H and 11B/10B) of well characterized tourmaline samples determined by in situ analytical techniques, in order to evaluate the nature of fluids during the mineralization of the McArthur River U deposit, the world's largest high-grade U deposit. Magnesio-foitite, an alkali-deficient Mg-rich tourmaline, is abundant along the 13-km long P2 fault, a reverse structure that controls the location of the McArthur River deposit. Magnesio-foitite contains variable concentrations of rare earth elements (REEs, up to 46ppm in total) and Y (up to 35ppm). Chondrite normalized patterns indicate low light REEs (LREEs) relative to heavy REEs (HREEs) (CeN<YN, Y as a proxy for HREEs) and slight negative Eu anomalies, similar to those of uraninite from the deposit. The data combined with textural evidence suggest that magnesio-foitite and uraninite co-crystallized with LREE-rich aluminum phosphate sulfate minerals. Relatively high REE contents of magnesio-foitite in the ore zone indicates the incursion of a REE-rich basement fluid. Low values of δD (−98 to −41‰) for magnesio-foitite suggests that the mineralizing fluid originated from groundwater. Variably high values of δ11B (+13.1 to +23.2‰) are explained by the dissolution of B from carbonate or evaporitic rocks and preferential removal of 10B by the crystallization of illite and kaolinite. The results of this study support the ascent of a REE-rich basement fluid during mineralization and extensive modification of basinal fluids through the crystallization of clay minerals.

SEM and TEM evidence of mixed-layer illite-smectite formed by dissolutioncrystallization processes in continental Paleogene sequences in northwestern Argentina

AbstractIn the northernmost Calchaquí Valley (Salta, Argentina), the Paleogene continental sediments show a transition from smectite, at the top, to R3 I-S (&gt;90% illite) through R1 I-S (65–80% illite), in contrast to the remaining sectors, containing smectite up to the bottom. Samples at the base of the succession were characterized by high-quality step-scan X-ray diffraction (XRD), scanning electron microscopy (SEM) and analytical high-resolution transmission electron microscopy (HRTEM). Analysis by SEM demonstrated dissolution of primary phases (feldspars, micas and quartz) and crystallization of illite, I-S and kaolinite. As this alteration is not pervasive, an intermediate fluid/rock ratio could be inferred. The lattice-fringe images of the samples from upper parts of the sequence show abundant I1-rich areas, whereas in the lower parts of the sequence, illite packets and I3 I-S coexist and compositions evolve towards muscovite (tetrahedral-charge increase, principally compensated by Mgby- Al substitution in octahedral sites and by a slight decrease in Ca in interlayer sites). As burial temperatures were probably similar in all the samples, depth was not responsible for the illite formation at the bottom. The TEM textures suggest that illitization proceeded mainly by dissolution-crystallization. The active faults close to the northern Calchaquí Valley probably promoted the circulation of hot, deep fluids, favouring illitization.

The effect of citric acid on the kaolin activation and mullite formation

The mullite phase in porcelain is important to the porcelain strength; however, its formation from clay minerals usually requires high temperature. Here we found that the activation of kaolin by citric acid would lower the formation temperature of mullite phase to 935.1°C, which is 67.5°C lower than that for the raw kaolin. Needle-like mullite crystal, a desired crystal shape in kaolin, could be observed in activated kaolin at 1050°C. The reduction of mullite formation temperature is due mainly to the surface change in kaolin. Upon citric acid activation, the pseudohexagonal platelets of kaolinite crystal are disintegrated to non-uniform flakes, and the specific surface area increases from 8.18m2/g to 31.21m2/g, both leading to improved activity of kaolin and increased sintering driving force. The results are of interest in producing high-strength porcelain at reduced sintering temperatures and consequent energy saving for sintering process.

Kinetic studies of the dehydroxylation and crystallization of raw kaolinite and fluorides-modified kaolinite

The effect of some fluoro compounds on the dehydroxylation and crystallization of kaolinite was investigated using thermal analysis. Samples were prepared starting from the raw kaolin (K-0) by adding 1 mass% fluorine, introduced as LiF (K-Li), NaF (K-Na) and CaF2 (K-Ca), respectively. The results show that the peak temperatures of the dehydroxylation process are less influenced by the fluorides presence, whilst the peak temperatures of the crystallization process decrease in the presence of fluorides, which confirm the mineralizer effect of fluorides on the crystallization process. The activation energies of the dehydroxylation and crystallization processes were calculated using the isoconversional methods described by Kissinger–Akahira–Sunose and Vyazovkin. For the dehydroxylation process, the values of the activation energy calculated as a function of the conversion extent are not significantly influenced by the fluorides presence. For the crystallization process, all the samples with fluorides show values of the activation energy higher than those of the sample without fluorides on the entire range of the conversion extent.

The Pico de Navas slump (Burgos, Spain): a large rocky landslide caused by underlying clayey sand fluidification

The Pico de Navas landslide was a large-magnitude rotational movement, affecting 50x10 6 m 3 of hard to soft rocks. The objectives of this study were: (1) to characterize the landslide in terms of geology, geomorphological features and geotechnical parameters; and (2) to obtain an adequate geomechanical model to comprehensively explain its rupture, considering topographic, hydro-geological and geomechanical conditions. The rupture surface crossed, from top to bottom: (a) more than 200 m of limestone and clay units of the Upper Cretaceous, affected by faults; and (b) the Albian unit of Utrillas facies composed of silty sand with clay (Kaolinite) of the Lower Cretaceous. This sand played an important role in the basal failure of the slide due to the influence of fine particles (silt and clay), which comprised on average more than 70% of the sand, and the high content presence of kaolinite (>40%) in some beds. Its geotechnical parameters are: unit weight (δ) = 19-23 KN/m 3 ; friction angle (φ) = 13o-38o and cohesion (c) = 10-48 KN/m 2 . Its microstructure consists of accumulations of kaolinite crystals stuck to terrigenous grains, making clayey peds. We hypothesize that the presence of these aggregates was the internal cause of fluidification of this layer once wet. Besides the faulted structure of the massif, other conditioning factors of the movement were: the large load of the upper limestone layers; high water table levels; high water pore pressure; and the loss of strength due to wet conditions. The 3D simulation of the stability conditions concurs with our hypothesis. The landslide occurred in the Recent or Middle Holocene, certainly before at least 500 BC and possibly during a wet climate period. Today, it appears to be inactive. This study helps to understand the frequent slope instabilities all along the Iberian Range when facies Utrillas is present.

Mineralogy of volcanically derived alluvial soils at Moshi, Tanzania

Despite intensive commercial agriculture in the rift zone of Tanzania, mineralogical studies on the soils influenced by volcanic parent materials are scarce. A mineralogical investigation of the soils and two buried ash layers from an irrigated sugar estate was undertaken using X-ray diffraction, transmission electron microscopy and measurements of extractable iron, aluminium and silicon and the specific surface area (SSA) of the clay fraction. The dominant mineral in the sand and silt fractions was sanidine. The clay fractions contained mainly high-defect kaolin, illite and K-feldspar, with small amounts of calcite, talc and gibbsite in some samples. Electron microscopy revealed the presence of tubular and spheroidal halloysite and &lt;0.5µm kaolinite crystals, as well as nanocrystalline material, probably allophane, and volcanic glass. The amounts of ferrihydrite (0.34–1.84%) and allophane (0.52–6.84%) were low in the soils but higher in the buried ash layers. The surface areas of the clay fractions were high (up to 145m2g–1) and it was calculated that 5% allophane constituted 22% of the total SSA. Although all soils were dominated by halloysite and small kaolinite particles, it is likely that the small amounts of allophane with high SSA has a strong effect on their physical and chemical properties.