The formation of bentonites via alteration of volcanic glassy rocks is associated with apparently contradictory mineralogical, geological and geochemical characteristics, which remain poorly understood. This contribution attempts to answer some of the questions based on the work carried out in our laboratory over the past 15 y. The first question concerns the formation of opal-CT-free bentonites from acidic precursors. It is suggested that the formation of opal-CT-free bentonites is closely related to the mechanism of eruption and emplacement of parent volcanic rocks if the temperature of the parent glass after emplacement is high and/or the establishment of hydraulic gradients coupled with high permeability after deposition of volcanic glass (i.e. diagenetic alteration) if the temperature of the parent glass is low. The second question pertains to the behaviour of Fe during bentonite formation. Based on mineralogical and geological characteristics of different bentonite deposits, it is suggested that Fe is present mainly as Fe2+ and hence may be mobilized during alteration. Oxidation of Fe may take place when bentonites are exposed to more oxidizing conditions close to the surface. Assignment of Fe as Fe2+ increases the layer charge of smectites and decreases significantly the range of layer charge. Thus the smectites may have considerably different properties when formed compared with their counterparts on the surface. The third question considers the possible influence of parent-rock chemistry on the crystal chemistry of smectites. It is proposed that the parent rock may affect the chemistry of the neoformed smectites to some degree; therefore the smectite chemistry may provide indications about the geochemical affinities of the parent rock.

Within the Variscan basement of Sardinia (Italy), two main Fe concentrations occur in the low-grade metamorphic tectonic units: (1) an uppermost-Ordovician oolitic ironstone of shallow anoxic water environment; and (2) a concentration of Fe oxyhydroxides lying on a palaeosurface. Two sets of samples were picked from the marine ironstone and from the continental Fe concentration following stratigraphic criteria. Chemical analysis, X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis and thin-section studies were performed on 34 samples.

Marine ironstones formed under a highly reducing, anoxic, non-sulphidic methanic environment, and their Fe phases are chamosite, siderite and magnetite. Detrital chlorite and illite, produced during physical weathering, were chamosite precursors. Using the V/Cr proxy, an emergence stage that caused a transition to an oxic environment is documented. In contrast, continental ironstones formed under oxic conditions and the dominant Fe phase is goethite, which can adsorb Zn2+ and U6+. Unexpected negative Ce anomalies occur in this set of samples, suggesting that the oxyhydroxides originated from Ce-depleted solutions. Although the ironstones of Sardinia formed in different environments (marine vs. continental) and under contrasting climatic conditions (sub-glacial vs. tropical) they share similar geochemical features. These dramatic palaeoenvironmental differences did not result in large differences between the geochemistry of the chemical sediments.

Eleven Oxisols with high clay contents, 2.6–59.7 wt.% Fe2O3, and containing hematite, goethite, magnetite and maghemite, from São Paulo, Minas Gerais and Goiás, Brazil, were studied for the purpose of microwave remote sensing applications in the 0.3 to 300 GHz range. Of special interest are: the pseudosand effect caused by Fe-oxide cementation of clusters of soil particles; the mineralogy; and whether the soil magnetic susceptibility affected by ferromagnetic magnetite and maghemite interferes with microwave propagation. Quantitative mineralogical analyses were conducted using X-ray diffraction with Rietveld refinement. Visible, near infrared and short wave infrared spectroscopic analyses were used to characterize the samples qualitatively for comparison with published spectral radiometry results. Quartz (3–88%), hematite (2–36%) and gibbsite (1–40%) occurred in all soils, whereas kaolinite (2–70%) and anatase (2–13%) occurred in nine samples. Ilmenite (1–8%) was found in eight soils and goethite (2–39%) in seven. Of the ferromagnetic minerals, maghemite occurred in seven soils (1–13%) and three contained magnetite (<2%). These results will be applied to the interpretation of the effect of Fe oxides, particularly the ferromagnetic oxides, on microwave interaction with high-Fe soils, with ultimate application to the monitoring of soil water content by microwave remote sensing.

The mafic rocks from boreholes studied here contain the assemblage typical for upper-subgreenschist to lower-greenschist facies (albite, chlorite, illite, titanite, quartz, Ti-oxide ± actinolite) as well as relict clinopyroxene.Metapelites underlying a metabasic rock sequence also record metamorphic alteration as demonstrated by the chlorite and illite crystallinities. Chlorite crystallinity suggests formation in conditions close to the subgreenschist/greenschist facies boundary, but this mineral is partly decomposed during metamorphic retrogression.

The Tertiary basin of Feres consists of sedimentary rocks, andesitic-rhyolitic volcanic rocks of K-rich calc-alkaline affinities, rocks with calc-alkaline and shoshonitic affinities and volcaniclastic fall and flow deposits. Volcanic and volcaniclastic rocks have variable concentrations of LIL elements (Ba, Sr, Rb, Th) and HFS elements (Zr, V) due to their mode of origin. The pyroclastic flows frequently show more or less intense devitrification, vapour-phase crystallization and, in some cases, evidence of fumarolic activity, as is indicated by the presence of scapolite. The volcanic and volcaniclastic rocks display various types of alteration including formation of zeolites (clinoptilolite, heulandite, mordenite, and laumontite) and smectite, as well as hydrothermal alteration (development of silicic, argillic, sericitic and propylitic zones) associated with polymetallic mineralization. The behaviour of chemical elements during alteration varies. Some are immobile and their distribution is controlled by the conditions prevailing during parent-rock formation and emplacement, but others, such as Ba and Sr, are mobile and selectively fractionate in zeolite extra-framework sites. The formation of zeolite from alteration of volcanic glass is accompanied by an increase in Mg and Al content, and a decrease in Si and Na content, whereas Ca is not affected by alteration. In certain pyroclastic flows, there is a significant difference in K-content between incipient glass and altered rock, due to K-feldspar formation during devitrification and vapour-phase crystallization.

Namacotche gem-bearing pegmatites of Alto Ligonha pegmatite district are heterogeneous, strongly fractionated, and have large Li and Ta and extremely large Cs contents. Clay samples were collected in fracture infillings and dilation cavities with gemstones and were studied using X-ray diffraction (XRD), polarized light microscope, scanning electron microscopy-energy dispersive spectroscopy, high-resolution transmission electron microscopy and chemical analyses. The <2 μm fraction contains cookeite, illite, illite-smectite and suggested irregular mixed-layer cookeite-smectite, beidellite, montmorillonite, kaolinite and goethite.The XRD patterns of chlorite and their d values suggest the presence of ‘di-trioctahedral chlorite’ similar to cookeite-Ia polytype. Cookeite chemical analyses show that Li contents range from 0.82 to 1.08 atoms per half unit cell.

A close relationship has been established between occurrences of gemstones and clay minerals. Some important textures and crystal chemistry are discussed.The main gemstones related to the Namacotche Pegmatite are: morganite (pink cesian beryl), kunzite (spodumene) and elbaite tourmaline. As the mechanisms responsible for the gemstone formation take place at low temperature, the clay minerals paragenesis cookeite ± cookeite-smectite interstratification ± beidellite + montmorillonite ± illite-smectite interstratification, represents a late-stage secondary paragenesis, generated by hydrothermal alteration.

Upper Cenomanian–Turonian clay mineral assemblages of sediments cropping out in the Western High Atlas basin are studied in four sections. Smectite and mixed-layer illite-smectite (I-S) have been identified as major constituents of the deposits. The composition of clay associations in black shales and associated sediments varies considerably according to age, but usually depends either on the general lithology, the abundance of organic matter, or the depth of burial. A distinct correlation is evident between clay mineral distribution and sea-level. Smectite and mixed-layer I-S with greater percentages of smectite layers increase in sediments deposited during transgressive periods, whereas they decrease progressively in the shallower facies deposited during regression in favour of illite and mixed-layer I-S with a greater percentage of illite. The vertical evolution and lateral distribution of clay assemblages and their relationships with sea-level as well as the palaeogeographic conditions prevailing during the Late Cenomanian–Turonian period (flattened topography and arid climate), indicate a detrital origin of the smectite minerals and a distribution pattern controlled by differential settling processes.

Kaolin deposits extending over an area of ~10 km2 in the western and southern parts of Limnos Island, northeast Aegean Sea, Greece, were studied. The kaolin deposits are alteration products of volcanic rocks, mainly trachytes, trachyandesites, andesites and dacites. Study of the collected samples was carried out using X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), energy-dispersive scanning electron microscopy (SEM-EDS), Fourier transform Raman spectroscopy (FT-Raman), Fourier transform infrared (FTIR) techniques and inductively-coupled plasma (ICP) bulk rock chemical analyses for major, trace and rare earth elements. The extensive alteration of the parent rocks resulted from the circulation of hydrothermal fluids through faults and fractures. The development of the various assemblages depends not only on the temperature and composition of the hydrothermal fluids but also on the distance of the rock from the fault or the channel of the ascending hydrothermal fluids.

Kaolinite, dickite, halloysite, illite, smectite and mixed-layer illite-smectite and jarosite were detected in the altered volcanic rocks forming locally various alteration zones. Smectite-rich zones and illite-rich zones are relatively infrequent. In the halloysite-rich zones, the kaolinization of feldspars is accomplished in four stages. The kaolinizaton of feldspars in the kaolinite-dickite-rich zones follows five discrete stages. In the less altered rocks, thin layers of kaolinite are formed on the surface of feldspars. With increasing kaolinization, kaolinite is developed on the surface of feldspars forming extended parallel booklets of newly formed kaolinite. In the third stage, feldspar crystals are partially altered to kaolinite booklets. As kaolinization advances, kaolinite becomes well formed and, in the most altered rocks, feldspars are partially or completely altered to dickite books, depending on the temperature of the hydrothermal fluids.

This work examines possible new applications for common, clay-based formulations. We processed spray-dried grains of a common clay (ZA-4) and collected fines/rejects from the production of lightweight aggregates (LWA), which were used as filtration beds for removal of Pb2+ and Ni2+ from aqueous solutions. Experiments were performed at different contaminant concentrations and by testing several operational conditions. The removal performance of both tested materials is strongly dependent on the particle-size distribution of the batch, being better for beds made of smaller grain size, since packing density and specific contact area for removal are greater. In general, the efficiency for removal of Pb2+ was greater than for Ni2+. When compared with granular activated carbon, clay-based beds performed well. In particular, finer fractions of LWA show stable behaviour, without saturation after consecutive runs. Saturated beds can potentially be reused, in appropriate amounts, for production of new lightweight aggregates or common ceramic items (e.g. bricks), since retained contaminants will be fixed by firing in the clay matrix. Calcination of pressed pellets made of contaminated filter grains created consolidated ceramic components and sorbed metallic species became inert.

Alkaline activation involves a reaction between aluminium silicates and compounds with alkalis or alkaline-earth elements in a caustic environment and under certain thermal and pressure conditions. It has been used in the production of non-structural building materials and in so-called ‘green’ concrete, as products display high mechanical strength, low chemical reactivity and high resistance to fire. Green concrete could be an eco-efficient alternative to concrete made using ordinary Portland cement (OPC), reducing CO2 emissions substantially. The main goal of this work is to test common clays, glass cullet and other industrial wastes as an alternative to the commonly used binders (metakaolin or fly ash) in geopolymers. Optimized formulations show interesting mechanical strength (approaching 20 MPa) after just 48 h of curing. The addition of F-sand as aggregate minimizes the shrinkage upon curing and the amount of activator required, and increases the density/degree of compaction of the bodies, thus improving their mechanical resistance within certain limits. Curing in ambient conditions also has a beneficial affect on the mechanical strength.