This review focuses on the clay mineralogy of the most important Brazilian soils: the Latosols, which cover >60% of the country by area, and occur in association with other soils. They are typically deep, highly-weathered soils, dominated by low-activity 1:1 clay minerals and Fe and Al oxyhydroxides, with varying proportions of these minerals, depending on parent material and weathering intensity. They are usually of low fertility, although eutric types also occur. Latosols are generally correlated with Oxisols (American soil taxonomy) and Ferralsols (WRB system). Clay mineralogy is typically monotonous: kaolinite, gibbsite, hematite, goethite, maghemite and Ti minerals (mainly ilmenite and anatase) are the prominent mineral phases in the clay fraction. Some Latosols developing on basalt from southern Brazil contain significant amounts of hydroxyl-interlayed vermiculite. Among the pedogenic oxides the most frequent are goethite (α-FeOOH), indicated by yellowish colours (2.5Y–10YR; in the absence of hematite), and hematite (α-Fe2O3), which imbues reddish colors (2.5YR–5R), even when present in very minor amounts. Maghemite (γ-Fe2O3) is less frequent; it imparts a reddish-brown colour (5YR–2.5YR) and magnetic properties. Both goethite and hematite show Al-substitution, with a greater relative proportion in soil goethites. Hence, in similar drainage conditions, goethite is less prone to dissolution than hematite. Most reddish Latosols also contain maghemite, due to partial or complete oxidation of magnetite, which generally occurs naturally or is fire-induced. Magnetite and/or maghemite are associated with trace elements which are important in plant nutrition, such as Cu, Zn and Co. The contents of gibbsite in Latosols are extremely variable, from a complete absence in brown Latosols, to 54% in red Latosols from mafic rocks. Relatively large amounts of gibbsite are found in the clay fraction of these soils and this mineral is important in P sorption in deeply weathered Latosols in association with goethite and hematite. Even though most Latosols are dystrophic, some are eutrophic, revealing an unusually large base saturation in areas under ustic regimes where the parent material is particularly rich in bases, such as basalts. This eutrophic nature is attributed to the protecting role of micro-aggregates in ferric red Latosols, which retard baseleaching from the inner aggregate. At the other extreme, some Brazilian Latosols are acric and positively-charged in sub-surface horizons, as revealed by the relationship pH KCl > pH H2O. These acric Latosols are the result of long-term weathering and intensive leaching, during which pH tends to increase to values close to the zero point charge of Fe and Al oxides (between 6 and 7), greatly increasing P adsorption, which is mainly attributed to gibbsite, goethite and hematite. Soil kaolinites in Brazilian Latosols are mostly of low crystallinity, with Hughes and Brown indexes of between 6 and 15. In this review we have discussed the role of these clay-fraction minerals in soil genesis and fertility, highlighting the marked role of inheritance from deeply-weathered parent material. Latosols typically retain large amounts of Fe oxides, some of which are magnetic, with spontaneous magnetization >1 J T–1 kg–1. In this regard, reddish Latosols developed from mafic rocks are the most representative magnetic soils, and cover as much as 3.9% of Brazil. An overview of magnetic soils on four representative examples of mafic lithologies is presented, together with some aspects of their Fe-oxide mineralogy and related field and laboratory technqiues.

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.

Respirable forms of crystalline silica including the mineral cristobalite are known carcinogens and the subject of increasing regulation and codes of good practice. Some bentonites may contain cristobalite, but more commonly they contain opal-CT, or opal-C, which can be confused with cristobalite. In the light of increasing regulation there is a clear need for simple and reliable methods that can distinguish cristobalite from the opals in bentonites and other clay materials. Here we propose the use of a simple NaOH method that is easily applied to bulk samples of bentonites. The method consists of boiling the clay in 0.5 M NaOH for 10 min and measuring the weight of undissolved solid. The method is shown to be accurate for both the identification and quantification of opal in bentonites since these phases are soluble when thus treated. It is also a straightforward method to distinguish the opals from cristobalite as the latter is essentially insoluble. Adoption by industry of a simple operationally-defined method such as this would be preferable to ad hoc reliance on direct instrumental methods alone such as X-ray powder diffraction, as the latter almost always require a large element of judgement and experience on the part of the analyst, although direct methods do need to be used in a supporting role.

Physicochemical properties of representative samples from talc deposits discovered at Pout-Kelle and Memel in Cameroon have been investigated using a variety of techniques. The data allow the study of the mineral crystal-chemistry, chemical and modal compositions, grain size distribution, textures and surface heterogeneity, and morphometric characteristics. As a result, we found that talc (platy or round, Fe-rich) prevails (65–90%) over Cr-chlorite (7–26%), halloysite, chromite, rutile, brucite and magnesite. Chemically, talc ores are made up of SiO2, MgO, Fe2O3 and minor Al2O3. Mode values range from 50 to 55 μm at Memel and from 30 to 90 μm at Pout-Kelle. Specific surface areas measured by BET and t-plot methods range from 1.3 to 2.5 m2/g. The large values of morphometric characteristics are indicative of high crystallinity and platiness. Potential uses of these talc ores in the rubber, plastics and paper industries require beneficiation processes. Interestingly, the Memel deposit and most zones of the Pout-Kelle deposit are free of needle-shaped crystals, an advantage for environmental safety.

The Fe3+ uptake from aqueous and methanol solutions of FeCl3·6H2O in two vermiculites from Santa Olalla and Ojén (Spain) and in a montmorillonite from Wyoming has been investigated by Mössbauer spectroscopy at room temperature and 4.2 K. The samples were further characterized by X-ray diffraction. The low temperature Mössbauer spectra reveal the deposition of akaganeite and perhaps some ferrihydrite on the external surface of the clay minerals. The room temperature spectra exhibit two Fe3+ quadrupole doublets in addition to the Fe2+ components of the structural iron in the sheet silicates. On treatment in ⩾0.04 M Fe3+ solutions the Fe3+ quadrupole splittings of 0.90 mm/s and 0.55 mm/s can be assigned to the mineral akaganeite; at smaller Fe3+ concentrations one of these doublets has a quadrupole splitting similar to that of the Fe3+ in the original vermiculites, the other one a quadrupole splitting of 0.6–0.7 mm/s, which can be considered as a hint for an uptake of Fe dimers in the interlayer space irrespective of the solvent used for iron deposition. Preliminary results show that the adsorption of pyrrole in all of our Fe3+-containing samples, regardless of the preparation in aqueous or methanol solution, results in the formation of polypyrrole and a concomitant reduction of some of the Fe3+. An uptake of pyrrole into the interlayer space has been observed only in the Fe3+-containing Wyoming montmorillonite and in the Ojén vermiculite treated with the methanol solution of iron chloride.

Two natural clays (kaolinite and montmorillonite) were deposited onto a platinum electrode surface using two deposition techniques and under different experimental variables. For both clays, the percentages of surface coverage (%θ) were optimized in the 75–96% range. A greater surface coverage was observed at higher temperatures for both clays, which confirms the endothermic nature of the deposition process. The maximum surface coverage (96%) was achieved for kaolinite. The surface coverage of kaolinite on a platinum electrode was constant for deposition times between 6 and 20 h. A surface coverage of 91% with montmorillonite particles was achieved. There was a very small increase in surface coverage by increasing the concentration of clay in the modified solution. The maximum surface coverage was observed under acidic conditions and smaller coverage values were reported for neutral and basic solutions. For both clays, a complete surface coverage for the electrode surface was achieved using the spin-coating technique. The experimental variables that affect the deposition of the clay, such as the stoichiometric ratio of clay/PVC and centrifugation speed and time, were studied and optimized to obtain full surface coverage. The spin-coating method achieved the required durability and stability for the modified-electrode. The characterization showed that the metallic surface chemistry of the platinum electrode was totally suppressed. Both modified electrodes were found to be useful for determination of Ag(I) ion with a detection limit as small as 1.00×10–10 M. The analytical precision was also satisfactory (RSD <5.0%).