Main Clay Minerals Research Articles

Characteristics of clay minerals in soil particles from an argillic horizon of Alfisol in central China

The soil particles (<2000, 450–2000, 100–450 and 25–100nm) in an Alfisol were studied using inductively coupled plasma-optical emission spectroscopy (ICP-OES), conventional and synchrotron X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG) to investigate the mineral composition of clays and changes in crystalline structure in the particles of an argillic horizon. The results showed that the clay minerals in the particle fractions (<2000, 450–2000 and 100–450nm) were illite, kaolinite, vermiculite and a trace amount of hydroxy interlayered vermiculite and chlorite. In the nanoparticles (25–100nm), the main clay minerals were illite and kaolinite. The molar ratios of SiO2 to Al2O3 and SiO2 to R2O3 (Al2O3 and Fe2O3) were higher in the nanoparticles compared to other particle fractions. With decreasing particles size, kaolinite and vermiculite decreased gradually and illite increased. The Al-Mg-OH and Si–O–Si (Al) stretching modes of vermiculite were broadened in the 100–450nm particles and disappeared in nanoparticles while the hydroxyl group (OH) of clay minerals in the samples was reduced with decreasing particle fractions. The broadening of the band characteristics of clay minerals, the dehydration and dehydroxylation were less pronounced in particle fractions from the argillic horizon comparing to the topsoil. Clay minerals from 450 to 2000 and 100–450nm particle fractions were well ordered whereas illite and kaolinite were poorly ordered in nanoparticles of the argillic horizon. The “crystallinity” of clay minerals was weakly affected in the particle fractions of argillic horizon compared to topsoil.

Pozzolanic activity assessment of calcined clays with complex minerals content

The present work deals with the possibilities for using calcined clays of different compositions and mineral content as pozzolanic admixtures to cement mortars. The optimal burning conditions for each type of selected clay are determined according to the losses on ignition. The contribution of the main clay minerals to formation of the total pozzolanic activity by the burning process is evaluated by way of various physical, chemical (Frattini, Chapelle and the method of active admixtures) and mechanical (strength activity index) methods. The pozzolanic properties of the calcined clays are determined by 30% cement replacement. A calculation procedure for the prediction of the pozzolanic activity of burned clays, depending on their raw mineral content, is proposed.

Adsorption of Lead by Bentonite Clay

Removal of heavy metals from industrial waste water is a worldwide challenge. Inorganic pollutants are difficult to treat as they are non-biodegradable, hence they remain in nature. Such pollutant when enters the food cycle, affect human, plant, animal lives and aquatic systems. Therefore, their removal becomes critical from the environmental point of view. In this study lead is being used as a pollutant in water and unmodified bentonite clay is used as adsorbent material. The bentonite clay was characterized using technique XRD. The results of XRD show that montmorillonite is the main clay mineral. The applicability of the bentonite clay from Bhawnagar area for removal of lead from aqueous solution was tested using the batch process. Experimental parameters affecting the adsorption process like pH value, initial metal concentration, dose of adsorbent (clay), agitation time, and speed were analyzed to get optimum conditions for maximum removal of lead. Experimental data were fitted well to the Langmuir adsorption isotherm with maximum adsorption capacity of 26.3 mg/gram at correlation coefficient of 0.955. It was found that Langmuir adsorption model is fit well with experimental data. Successful application of bentonite clay from Bhawnagar area for removal of lead from aqueous solution greatly supports its field application.

Deep Rock Fabric Characteristics and Optimization of Drilling Fluid Composition for Deeper Zones of the Longmenshan Fault

The main objective of this study was to improve borehole wall stability in deep drilling into the Lonmenshan fault zone by developing a drilling fluid with ultra-fine inert materials. The mineral composition and microstructure of rock samples from deeper zones of the fault were analyzed by X-ray diffraction and scanning electron microscopy. It was found that the rock has a loose fabric and is microporous and highly permeable. The main clay minerals in the rock are illite and chlorite, which are prone to hydration and swelling. Under high pressures, their hydration and swelling capacity increase, which may cause instability of boreholes. Based on the obtained mineral composition and surface microstructure data and analysis of the mechanism of additive action, the following was found to be the optimum composition of the drilling fluid that helped rapid drilling into the fault zone: water + 5% sodium bentonite + 0.3% Na2CO3 + 5% sodium bentonite in the form of hollow glass microspheres (HGM) + 2% polysaccharide polymer + 3% carboxymethyl cellulose.

Behavior of ammonium adsorption by clay mineral halloysite

Ammonium pollution becomes severe during mining of ionic rare earth-ores in southern China. As one of the main clay minerals in soils of ionic rare earth mines, halloysite plays an important role in ammonium adsorption. In this study, the saturated adsorption capacity, factors affecting adsorption and adsorption kinetics of halloysite for ammonium were investigated. The results indicated that the ammonium adsorption of halloysite was saturated with 1.66 mg/g at 303 K, pH of 5.6 and initial ammonium concentration of 600 mg/L (about half of the actual initial in-situ leaching concentration). When the initial concentration of –N, pH values and temperatures (288 K to 313 K) increased, the ammonium adsorption capacity of halloysite increased. The ammonium isothermal adsorption of halloysite matched the Langmuir and Freundlich isotherms. The adsorption process of ionic rare earth mining soils for ammonium was favorable. And the adsorption process followed closely the pseudo-second kinetic equation.

Removal of Leishman stain from aqueous solutions using natural clay of Qulapalk area of Kurdistan region of Iraq

Clay sample of Qulapalk – Sulaimani of Kurdistan region of Iraq was analyzed for its chemical composition and mineralogy. Calcite (a non-clay mineral) was a major constituent of the sample (34%). The main clay mineral was smectite as depicted by X-ray diffraction patterns of the oriented slides. Adsorption of Leishman stain from aqueous solution onto the natural clay sample was studied in batch system. The effect of operational conditions like initial pH, adsorbent dose, contact time and temperature were studied. The pseudo second order kinetic model fitted well in correlation to the experimental results.The rate of the adsorption process was found to follow the pseudo second-order kinetics, which was found to be increased with the increase of temperature. The activation parameters Ea, ΔH∗ and ΔS∗ were found to be 37.94 kJ mol−1, 35.34 kJ mol−1 and 148.02 J mol−1 K−1 respectively. The sorption isotherm fitted to both Freundlich and Langmuir isotherm models. High adsorption capacity was recorded for the current natural clay (455.37 mg/g) toward methylene blue. Equilibrium thermodynamic parameters ΔH°, ΔS° and ΔG° indicated that the adsorption processes was spontaneous and endothermic in nature.

The use of Mössbauer spectroscopy in environmental research

The impact that mining has on the environment is becoming an ever increasing problem all over the world. South Africa, Brazil and India are main producers of various valuable resources such as for example iron ore, platinum, gold and coal, of which coal and platinum mining will be discussed in this paper. Dumping of ash, waste and discards, result in the formation of acid mine drainage (AMD) due to the high sulphur content of the coal and the waste products. The main Fe-S-bearing minerals in the coals investigated were pyrite, jarosite and ferrous sulphate, a weathering product of pyrite. In the ash produced due to combustion or gasification of the coal, the main Fe-constituents are Fe2+,3+ glass (≈ 30%) and hematite (70%). The amorphous phase of the sample was composed mainly of SiO2 and Al2O3 with trace element inclusions of Hg, Ti, Cd and As. The soil, sediment and overburden in the coal mining areas contain pyrite as Fe-S-mineral and also ferrous sulphate as weathering product, with illite the main clay mineral. From laboratory leaching products of coal and ash, sulphur in the form of SO\(_{4}^{2-}\), was found to be one of the most leached ions with a concentration ranging between 100–1000 ppm. The amount of Fe leached out from the ash samples was between 2–5 ppm, but the Fe-leachability depends on the pH, with higher amounts leached out at pH ≤ 1.5. Magnetite losses, to the amount of about 1kg per tonne of magnetite used, occur during the dense medium separation process (DMS) used in cleaning the coal, which also reports in the waste product. South Africa is the largest producer of platinum and smelting of the ore can lead to various forms of pollution. Magnetite formation in the 2-stage furnace process is used as an indicator of the effectiveness of the reduction and the Fe2+ and Fe3+ ratio is used to monitor the process. In the flash furnace the ratio is 2–6, whilst in the electric furnace it is ≤0.02. If not monitored closely a large amount of nickel loss will occur if sent to the waste dump. Mossbauer spectroscopy was used to identify the Fe-species and the results were augmented by High Resolution-Transmission Electron microscopy (HR-TEM), Field Emission-Scanning Electron Microscopy (FE-SEM)/EDS, X-Ray Diffraction (XRD) and ICP-EOS results.

Chemical weathering east and west of the emerging Caledonides in the Silurian – Early Devonian, with implications for climate

The late Llandovery (Silurian) – early Lochkovian (Devonian) climate of the tropical zone is tracked considering orogenesis, global glaciation events, and cratonic drift. Mineral and chemical compositions of clay fractions of Canadian (the Franklinian Basin) and Estonian (the Baltoscandian Basin) sedimentary rocks from different sides of the emerging Caledonides were studied, using clay as an index of climatic conditions of the provenance area. Illite is the main clay mineral, with addition of orthoclase, muscovite, and chlorite in both regions. Authigenic chert is present in some Canadian sections. The similarity of clay minerals and elemental composition in the two regions suggest common chemical weathering conditions. Nevertheless, the Al/Ti ratio of the &lt;1 μm clay fraction, the proxy for climate’s aridity–humidity and weathering intensity, reveals differences. The Franklinian Basin in Laurentia shows a shift of climate from humid to semi-humid in pace with the uplift of the Caledonides. The sub-meridional mountain chain in the east reduced the amount of rainfall and, therefore, caused drier climate than could have been expected from the low latitudinal position of the area. The material from the Baltoscandian Basin reveals semi-arid and arid climate for most of the Silurian. Although the southern part of the Fennoscandian Shield was situated in desert latitudes, the evidence from the provenance of clay suggests that the climate was not fully arid. In Baltoscandia, covariation between low Al/Ti of clay and negative shifts of δ13C of marine carbonates is recorded at some stratigraphical levels, suggesting a link between humid climatic episodes in mainland and waning of glaciers at the South Pole.

Structural, dielectric and surface morphological properties of ball clay with wet grinding for ceramic electrical insulators

In this work, a study on structural, dielectric and surface morphological properties of ball clay with wet grinding is undertaken. The wet grinding treatment was performed using ball and vibro mills for different time spells of 2, 4, 8 and 16 h. The raw material and ground ball clay samples were subjected to the XRD, LCR meter and SEM to investigate the structural, dielectric and morphological properties respectively. The compositional analysis of the raw material was also carried out using energy dispersive spectroscopy (EDS). The XRD patterns reveal that the kaolinite is the main clay mineral along with quartz and the samples become amorphous in nature with grinding treatment. The dielectric constant and dissipation factors are also calculated with frequency and temperature. The ground sample for 8 h shows maximum dielectric constant and dissipation factor at 100 kHz and 1 MHz frequencies respectively while ground sample for 16 h shows maximum dielectric constant at 1 MHz. An increase in dielectric constant at the high frequency range (∼MHz) is observed with grinding treatment. The SEM studies reveal that the grains are densely packed, well defined and having continuous solid phase. The grain size is observed to be reduced with wet grinding due to increase in surface area.

Alteration of compacted GMZ bentonite by infiltration of alkaline solution

AbstractConcepts for geological disposal of high-level radioactive waste usually include bentonite buffer materials. Numerous studies have been performed with most usingWyoming bentonite. Gaomiaozi (GMZ) bentonite has been selected as a potential buffer/backfill material for the deep geological repository of high-level radioactive waste in China. In this context, the highly alkaline environment induced by cementitious materials in the repository is likely to alter montmorillonite, the main clay mineral in GMZ bentonite. This alteration may result in deterioration of the physical and/or chemical properties of the buffer material. To acquire quantitative data which would allow us to assess the dissolution of montmorillonite and changes in the diffusivity of hydroxide ions as well as their effects on the swelling pressure and permeability of the compacted GMZ bentonite, an experimental study was conducted under highly alkaline (NaOH solutions with various pH values were used), simulated groundwater conditions. The GMZ bentonite also contains cristobalite which may also have been dissolved. The microstructure of the compacted bentonite samples after the experiments was determined by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDX) was carried out to identify mineralogical changes. At pH &gt;13, the permeability of specimens increased significantly; the swelling potential decreased with increasing pH. Furthermore, the pore volume and pore size of GMZ bentonite changed when exposed to alkaline solution, resulting in an increase in porosity and permeability. The main alteration mechanisms of compacted GMZ bentonite undergoing infiltration by highly alkaline solution are likely to be dissolution and modifications in terms of the microstructure and mineralogy.

Identification and Simultaneous Optimisation of Tunisian Clay Properties: Preliminary Study for the Ceramic Products

The relationship between composition and physical parameters such as specific surface area, cationic exchange capacity and plasticity is studied with the aim of developing regression models that would permit the prediction of clay properties. These models could be useful for mineralogists and industrial applications. Nineteen representative clay samples were collected from Jebel Ressas in north-eastern Tunisia. Mineralogical data show that clay samples cover a very large variety of minerals. The main clay mineral is illite (50–60 wt%), secondary minerals including quartz, calcite and minor amounts of Na-feldspar. This study reveals that the average amount of silica (SiO2) and alumina (Al2O3) are 51.9 and 19.6 wt%, respectively. The contents of lime (CaO) and iron (Fe2O3) vary between 4 and 8 wt% whereas the amount of alkalis (Na2O + K2O) is on average 4.1 wt%. The grain size data indicates a significant amount of silt fraction, and the fraction <2 µm varies between 23 and 35 wt%. Values for plasticity index range from 16 to 28 wt%. The cation exchange capacity and the specific surface values are 34.1–45.7 meq/100 g of air-dried clay and 302–374 m2/g, respectively. Lastly, regression models are used to correlate the properties with the mineralogical and chemical compositions. The significance and the validity of models were confirmed by statistical analysis and verification experiments. The regression models can be used to select the clay properties (plasticity index, cation exchange capacity and specific surface) in relation with clay minerals proportions and the finer fraction amounts.

Nubian Sandstone reservoir characteristics: sedimentological and petrophysical evaluation in the Southern Gulf of Suez, Egypt

The Nubian Sandstone is a thick sequence of clastic and thin carbonate sediments in the Jebel El Zeit section of eastern Egypt. The sequence is composed mainly of sandstone with shale and minor carbonate interbeds and was deposited in continental and fluviomarine to marine settings. Petrographically, quartz arenite and quartz wacke are the main facies of Nubian Sandstone. They have different secondary components, cement and matrix types, reflecting their different depositional environments and diagenetic histories. The Nubian Sandstone is one of the most prolific hydrocarbon reservoirs in the Gulf of Suez oil province.The Nubian Sandstone has good reservoir quality with porosity varying from 10% to 29%, and permeability of up to 5000 millidarcies. The quality of the reservoir depends on its proportions of shale, diagenetic history and the depth of burial (compaction). X-ray diffraction and scanning electron microscope analyses indicate that kaolinite is the main clay mineral and is derived from the alteration of potash feldspar. Enhancement of oil recovery from the reservoirs of the Nubian Sandstone can be achieved by alkaline injection to dissolve the kaolinite.

Microstructural, Chemical and Mineralogical Analyses for Understanding the Geotechnical Properties of Clayey Soils

This paper presents a study research in the laboratory of three clayey soils from the depression of the Lama, a tropical climate area where disorders appear on construction and roads. Samples were tested to establish the relationship between their mineralogy and their geotechnical properties. On the three clayey soils, analyses were performed to characterize the macroscopic behavior (physical tests, free swelling test and compressibility tests) and the microstructural, chemical and mineralogical characterizations by thermal analysis, chemical analysis, X-Ray diffraction, Scanning Electron Microscopy observations complementary X-ray EDS analyses. At first glance, the results of physical and compressibility tests appear not to be consistent with the free swelling test results. Indeed, these results highlighted differences in the behaviors of the samples because the swelling potential revealed by the results of physical and compressibility test does not follow the same trend as the one given by the free swelling test result. The main clay minerals in the studied clayey soils varied from fibrous clays (palygorskite) to best-known clays such as smectite and kaolinite. Microstructural, chemical and mineralogical analyses allowed to understand and explain the different trends obtained by the different types of geotechnical characterization tests of studied clayey soils.

Estimate Erosion in Ladiz Dam in Sistan and Baluchistan with Use of Trace Minerals—South East Iran

A survey uses this new information: the statistical calculation and interpretation to determine the characteristics of sediments in the reservoir’s “Dodar Ladiz” dam. It is a useful and efficient way to use clay minerals as mineral tracer for identifying the sources and assessing erosion rate in the Dodar Ladiz dam watershed in a statistical method. The existence of four main clay minerals includes Kaolinite, Montmorillonite, Illite and Chlorite. The estimated watershed erosion rate is 2220.64 tons per square kilometer per year. Data ANOVA table in the inter/intra group communication of the clay minerals in the Dodar Ladiz dam watershed samples is shown by F-test statistics; there is a significant difference between the frequency of clay minerals in the samples with 95% confidence rate (P value < 0.05). Therefore, they can be divided into six groups. The average rates of the clay minerals are not equal in any of homogeneous hydrological reservoir units of the dam. Kruskal-Wallis test shows that the rate of frequency of Iolite mineral is identical among all samples of the hydrologic units and the dam reservoir; this mineral cannot be used as a tracer to identify the source. Reviewing the obtained data from statistical tests and studying clay minerals as a trace in determining the rate of degradation of upstream catchment “Dodar Ladiz” dam, it shows that the maximum annual erosion rate is 994.85 tons per sq/km, in the basin A within a 105.13 square kilometers area. Among the most important factors involving erosion and production of basin sediments, we can refer to lithology as the most critical factor and tectonic features, topography and climate as minor factors.

Mineralogical and geochemical characteristics of Miocene pelitic sedimentary rocks from the south-western part of the Pannonian Basin System (Croatia): Implications for provenance studies

Fifty-two samples of Miocene pelitic sedimentary rock from outcrops on Medvednica, Moslavačka Gora and Psunj Mts., and boreholes in the Sava Depression and the Požega Sub-depression were investigated. These sediments formed in different marine (with normal and reduced salinity), brackish, and freshwater environments, depending on the development stage of the Pannonian Basin System. Carbonate minerals, clay minerals and quartz are the main constituents of all pelitic sedimentary rocks, except in those from Moslavačka Gora Mt in which carbonate minerals are not present. Feldspars, pyrite, opal-CT, and hematite are present as minor constituents in some rocks. Besides calcite, dependent on the sedimentary environment and diagenetic changes, high-magnesium calcite, aragonite, dolomite and ankerite/Ca-dolomite are also present. Smectite or illite-smectite is the main clay minerals in the samples. Minor constituents, present in almost all samples, are detrital illite and kaolinite. In some samples chlorite is also present in a low amount. Major elements, trace elements and rare earth elements patterns used in provenance analysis show that all analysed samples have a composition similar to the values of the upper continental crust (UCC). The contents of major and trace elements as well as SiO2/Al2O3, K2O/Al2O3, Na2O/K2O, Eu/Eu*, La/Sc, Th/Sc, La/Co Th/Co, Th/Cr, Ce/Ce* and LREE/HREE ratios, show that the analysed pelitic sedimentary rocks were formed by weathering of different types of mostly acidic (silicic), i.e. felsic rocks.

Characteristics of clay minerals in soil particles of two Alfisols in China

The composition, structure and transformation characteristics of clay minerals in various size particles (<2000, 450–2000, 100–450 and 25–100nm) of two Alfisols (Haplustalf and Hapludalf) in China were investigated by elemental analysis, X-ray diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FT-IR) and 27Al and 29Si Magic-angle Spinning Nuclear Magnetic Resonance (MAS NMR). The clay minerals in <2000, 450–2000 and 100–450nm size particles of the two soils were illite, vermiculite, smectite, kaolinite and kaolinite interstratified minerals. In Haplustalf, vermiculite was dominant clay mineral in 450–2000nm particles and illite was dominant clay mineral in 100–450nm particles. In Hapludalf, kaolinite (and kaolinite interstratified minerals) predominated in all size particles. With decreasing particle sizes, the two clays showed a decrease in K2O, Na2O, MgO and SiO2 contents and an increase in Al2O3 content, with the minimum molar ratio of SiO2 to Al2O3 observed in nanoparticles (25–100nm). Meanwhile, the smectite and vermiculite contents decreased or disappeared, leaving illite and kaolinite (and kaolinite interstratified minerals) as the main clay minerals in the two soil nanoparticles. Additionally, with decreasing particle sizes, the clay mineral structures were transformed from well crystalline to poor crystalline, with illite being the poor crystalline in nanoparticles of the soils. The hydroxyl groups between the tetrahedral and octahedral structures of the 2:1 clay minerals and the surface of kaolinite gradually broke down, and IVAl of the tetrahedral structure gradually reduced. In nanoparticles, Al mainly appeared in the octahedral structure as VIAl, and Si in the structure of Q3 (Si linked to three other Si atoms via oxygen). The composition and structure of clay minerals in different size particles showed that the clay minerals weathered from larger particles (>100nm) were an important mineral source of nanoparticles, and the 1:1 clay minerals were mainly inherited from the minerals in larger particles and transformed from 2:1 clay minerals into nanoparticles.

Occurrences of kaolin in Koutaba (west Cameroon): Mineralogical and physicochemical characterization for use in ceramic products

AbstractThirty clay samples collected from three hills in Koutaba (west Cameroon) were characterized in order to evaluate their potential use as raw materials for ceramics. After preliminary mineralogical identification by X-ray diffraction, three representative samples from the three different hills, referred to hereafter as K1M, K2M and K3M, were selected for further investigation by X-ray fluorescence, plasticity, granularity and thermogravimetric analysis. The main clay minerals are kaolinite (32–51%) and illite (up to 12%). Additional major phases are quartz (32–52%), goethite (6–7%) and feldspars (0–4%). The chemical composition showed variable amounts of SiO2(60–72%), Al2O3(15–20%) and Fe2O3(1–9%), in accordance with the quartz abundance in all of the samples studied. The particle-size distribution showed a large proportion of silty fraction (64–88%) with moderate sandy (9–19%) and clayey fractions ( &lt; 5% for K2M, 12% for K1M and 20% for K3M). All of the clays showed moderate plasticity-index values (8–11%). Because of these characteristics, K1M and K3M may be suitable for use in common bricks and hollow ceramic products. Sieving or the addition of ball clays is recommended to increase the plasticity of sample K2M for use in common bricks.

Palaeontology, sedimentology, and biostratigraphy of a fossiliferous outcrop of the Early Miocene Querales Formation, Falcón Basin, Venezuela

The top of the Querales Formation is well exposed at the Quebrada Corralito section, 17 m thick, in northern Venezuela. The section, dominated by siliciclastic accumulations of fine-grained sediments, preserves one cycle of transgressive–regressive phases of the deltaic environments facing the marine platform. An X-ray analysis revealed kaolinite as the main clay mineral. The age ranges from late Early Miocene to early Middle Miocene—zones N8 to N9, Late Burdigalian to Langhian. The section shows low biodiversity, including eleven macroinvertebrate taxa and two ichnotaxa; at its base, it contains mostly plant debris and amber from the continent, thus revealing a continental source near the littoral zone. Its middle portion preserves macroinvertebrates from the littoral zone, but also including the cephalopod Aturia sp., transported by a hydraulic flow to an area of greater depth; these macroinvertebrates are absent from overlying sections, at a time in which the basin was flooded most. The X-ray diffraction on six rock samples revealed mainly the presence of quartz with calcite, subordinated plagioclase and traces of potassium feldspar. Other minerals that compose this stratigraphic section are pyrite, dolomite and siderite, but they are disseminated in a siltstone. The section is dominated by mudstone sediments. The succession is kaolinite in the most pure fraction, and in lesser proportion smectite and illite; the stratified illite–smectite, chlorite and chlorite–smectite are present in low quantity. The mineralogical components of a molluscan valve were also examined, showing abundant pyrite, and moderate amounts of siderite and dolomite. In a part of the section, we recorded a rich assemblage of calcareous microfossils with foraminifers, nannofossils and marine ostracods. In the foraminiferal assemblage, 99 % correspond to planktonic forms. Most macroinvertebrates from benthonic assemblages suggest that they may have inhabited a near-shore marine environment, and sporadically experienced storm transport to deeper, shelf waters.

Synchrotron X-ray diffraction of bole layers from Portuguese gilded baroque retables

Studies based on a scientific approach to materials and techniques used in Portuguese gilded retables from the Baroque are very scarce and focus particularly on works with erudite features and on the characterization of the superficial gold leaf. The conservation and appearance of gilded surfaces, however, depended on the qualities of the clayey ground layer underneath, which is the bole. Colour and texture are closely related to its mineralogical composition.Boles were healing clays. Red to orange varieties could also be used for gilding, usually agglutinated with animal glue when the gold surface was meant to be burnished. Armenian was the name used to identify the best quality material.Microsamples collected from erudite and popular gilded retables, respectively belonging to the city of Oporto and its rural surroundings, were selected for elemental and mineralogical characterization. It was intended to shed light on the characteristics of boles used in Portuguese retables and to understand if there are any differences between materials used in works of distinct artistic quality.Elemental analysis was performed through SEM–EDS. SR-XRD was used for phase identification, performed with a six-circle diffractometer at the DIFFABS beamline of SOLEIL Synchrotron. Portuguese clay standards of identifiable composition and provenance were also analysed.The results suggest that boles are mainly kaolinitic, with variable amounts of illite and smectite. Gypsum was used as an extender. Although the proportions of the main clay minerals are similar in erudite and popular works, in Oporto homogeneity is clearly higher.

The geochemistry and mineralogy of high-S coals, with examples mainly from the East Pennines Coalfield in Yorkshire and Nottinghamshire, UK: an overview

Over a number of years, coals have been analysed in the geochemical laboratories of the University of Sheffield. Much of this work has been published and this paper presents an overview and, in some cases, a reinterpretation of the data. Most of the coal samples were obtained from seam sections in the Yorkshire and Nottinghamshire coalfields (now included in the East Pennines Coalfield), and from the Parkgate Coal in particular. Sulphur variations are essentially due to pyrite. In individual plies of the seam, lateral variation in total sulphur is thought to be due to the availability of sulphate at the time of deposition. Access of sulphate into the coal swamp environment can be directly related to river channels, many of which are known to have been long-lived features. The pyrite is of concern because of S emissions, but in addition it is the location in the coal for Hg, Tl, Pb, As and Se, all of which are of major environmental concern. Concentrations were determined by direct and indirect methods. The most common of the latter methods is statistical, and some of the potential problems are reviewed. Also contained within pyrite, but not exclusively so, are Cd, Ni and Sb. Trace element concentrations from different pyrite samples are similar, strongly suggesting a common source. Comparable trace element enrichment patterns for coals and marine shales provide evidence of a seawater origin. The chloride concentration in the coals is high and can also be linked to seawater. The two elements, S and Cl, differ in that Cl is conservative, is not involved in diagenetic reactions and is retained in the coal in residual pore fluids. Illite and kaolinite are the main clay minerals present, with subsidiary chlorite and mixed-layer illite-smectite. In most coals, the clay minerals are dominantly detrital, but in low ash coals, kaolinite of diagenetic origin can be detected. In some samples from the Barnsley Coal, the source of this kaolinite, based on the trace element geochemistry, is dispersed volcanic ash (basaltic). It is also highly likely that organic matter contributes small amounts of Si and Al in the coal, and phytoliths are one potential source. However, the detrital sediment is a major contributor to the major and trace element geochemistry of the coals. Trace elements with a dominantly detrital origin are Rb, Cs, Cr, Ga, Th, Nb and Li. Trace elements are also associated with the organic matter to a greater or lesser extent. Germanium is mainly associated with organic matter, but V is also present in the clays. These elements are mainly contained in vitrinite, although Ge concentrations are more variable, suggesting some mobility during diagenesis. This is borne out by Ge, V and Be showing enrichment at the margins of the the Parkgate Coal, following ‘Zilbermints Law’. Mobility during diagenesis is also demonstrated by the mineral infill of the cleat (joints). Stages of cleat mineralization have been recognized and related to diagenetic stages in siliciclastic sequences. Although there is evidence of element mobility after deposition in the coals, this is not thought to be related to the mineralizing fluids in the Pennine Orefields.