Olduvai Gorge, Tanzania (East African Rift) exposes a 100 m thick Plio-Pleistocene sequence of dominantly volcaniclastic sediments deposited in a 50 km wide closed basin containing a playa lake. A scanning transmission electron and analytical electron microscopy (STEM/AEM) study of authigenic clay minerals in sediments from representative depositional environments in the basin (pyroclastic fan, fluvial plain, wetland, lake margin and lake basin) was undertaken to determine whether clay compositions and textures could provide unique geochemical fingerprints characteristic of source area (Plio-Pleistocene trachytic volcanics vs. Precambrian quartzose-feldspathic basement) or paleoenvironmental conditions.

Our study shows that compositional signatures obtained by clay minerals during early pedogenesis are inherited from their parent source rocks. Sediments sourced from volcanics contain highly disordered, dioctahedral smectite. Those sourced from Precambrian basement are similar, but are more Al-rich. Subsequent neoformation in the pedogenic (soil) or diagenetic (lake-margin, lake) environments results in the modification of original clay mineralogy, compositions, and textures, and unique paleoenvironmental fingerprints are acquired. Soils developed on the distal pyroclastic fan contain smectite with more Fe(III) and Mg than smectite from the proximal pyroclastic fan sediments. A trend of decreasing Al and increasing Mg content occurs in smectite compositions in samples from the fluvial to lake-margin and lake environments as a result of partial replacement of original dioctahedral Al-rich smectite by neoformed trioctahedral Mg-rich smectite (stevensite). Neoformed celadonite replaces smectite in the most saline lake sediments.

The STEM/AEM data collectively indicate that diagenesis in the saline-alkaline lake results in the replacement of Al-rich dioctahedral smectite by Mg-rich trioctahedral smectite (stevensite) and Mg- and Al-rich celadonite. Thus, determination of clay mineral compositions at a basin-wide scale provides a useful tool for interpreting the spatial distribution of depositional and diagenetic environments.

Soils developed on Pleistocene andesitic lava flows and fluvial detritus in the Atlantic coastal plain of Costa Rica display a clay mineral assemblage that includes 10 Å and 7 Å halloysite and lesser amounts of kaolinite and dioctahedral vermiculite. Other secondary minerals include gibbsite, goethite, hematite, maghemite, allophane and amorphous Al hydroxides. Active floodplain soils are dominated by 10 Å halloysite and contain less allophane, while soil clays from Pleistocene terraces consist of a mixture of 10 Å and 7 Å halloysite as well as less dioctahedral vermiculite, kaolinite, and amorphous Al hydroxides. Residual soils formed on Pleistocene lava flows are dominated by 7 Å halloysite with less abundant kaolinite, dioctahedral vermiculite, 10 Å halloysite and amorphous Al hydroxides. This sequence suggests transformations of 10 Å halloysite to 7 Å halloysite and allophane to amorphous Al hydroxides with time. The presence of 10 Å halloysite in Pleistocene terrace soils implies slow reaction rates or metastability.

Quantitative X-ray diffraction (QXRD) analysis indicates a decrease in the amount of plagioclase feldspar from 34 wt.% in the 1–2 year floodplain to 0–1.6% in terrace and residual soils. Plagioclase weathering is paralleled by the formation of dioctahedral clay, allophane and Al hydroxides. Analysis by QXRD also indicates that crystalline minerals comprise 70–95% of the soil fraction, implying 5–30% X-ray-amorphous material. These data are verified by selective extraction using ammonium oxalate, which indicates 8–30% amorphous material. Chemical analysis of the extractant by inductively coupled plasmaatomic emission spectrometry indicates that allophane (Al:Si ratios of 0.92–3.82) occurs in floodplain and some terrace soils while amorphous Al hydroxides appear to coexist with allophane in Pleistocene terrace and residual soils with Al:Si ratios of 6.53–8.53. Retention of Mg to a greater extent than Na, Ca and K suggests Mg incorporation into hydroxide sheets in dioctahedral vermiculite as well as substitution into hydroxides.

Clays of a soil sequence with five profiles in the Val Genova (northern Italy) along an elevation gradient with climate ranging from moderate to Alpine were investigated with XRD using several diagnostic treatments. Smectites developed in the surface horizons of podzolic soils either from chlorite through the removal of hydroxy interlayers or from mica, which weathers in a first step to regularly or irregularly interstratified clay minerals. Citrate treatment allowed the detection of low-charge expandable minerals in the Bhs or Bs horizons. Therefore, the reduction of the charge of 2:1 clay minerals occurred before the removal of hydroxy polymers by fulvic acids and low-weight organic acids. Due to the more intense podzolization process near the tree line, the d060 region showed a temporal evolution of trioctahedral to dioctahedral mineral structures in the well developed Podzols. The pedogenic smectites of the E or Bhs horizons generally included one or several populations with various charges. In most cases, smectite was a heterogeneous mixture of montmorillonite and interstratified beidellite-montmorillonite. A pure beidellite phase could not be detected. The soils near the tree line, where weathering processes were most intense, had two main components: one with a charge >0.75, representing vermiculite-like minerals, and the other with a charge near 0.25, representing smectite. The charges of the beidellitic component and montmorillonite were almost equal. The higher the weathering state of the investigated soils, the lower was the layer charge of smectites.

The Serinhisar-Acıpayam basin of western Anatolia hosts a Neogene alkaline lake which formed in some graben and semi-graben depression zones as a result of N–S tension. The basin is filled with fluvial and lacustrine sediments dominated by clayey materials. The filling of the basin with fine sediments and associated water level changes caused the development of swampy and/or semi-swampy, alkaline-lake environments where sepiolite, palygorskite, saponite and dolomitic sepiolite or palygorskite precipitated periodically in the basin. Sepiolite is predominant in the Kuyucak section and is intercalated with saponite-dominated levels, whereas saponite accompanied palygorskite at Kocapınar where basaltic volcanism occurred. The contacts between sepiolite-palygorskite and saponite levels are more or less sharp, reflecting rapid changes in the physicochemical conditions of the depositional environment. Micromorphological images reveal that both sepiolite and palygorskite grew as interwoven fibers or fiber bundles and masses where dolomite was absent, indicating direct precipitation from solution, whereas fibrous networks grew authigenically on and out of dolomite in dolomitic sepiolite and dolomitic palygorskite. Saponite is either green or reddish brown due to its organic material-rich content and derivation from products of basaltic volcanism. Synsedimentary basaltic volcanism was the main source of Fe and Al, whereas Si and Mg were derived from surrounding ultrabasic and detrital units and partly from the volcanism. It can be concluded that sepiolite, palygorskite and saponite formed either by direct precipitation from alkaline lake water or authigenically from interstitial pore-water between dolomite rhombs as controlled by concentration of Si, Mg, Al and Fe, rather than by mutual transformation.

It has been suggested that interstratified illite-smectite (I-S) minerals are composed of aggregates of fundamental particles. Many attempts have been made to measure the thickness of such fundamental particles, but each of the methods used suffers from its own limitations and uncertainties. Small-angle X-ray scattering (SAXS) can be used to measure the thickness of particles that scatter X-rays coherently. We used SAXS to study suspensions of Na-rectorite and other illites with varying proportions of smectite. The scattering intensity (I) was recorded as a function of the scattering vector, q = (4 π/λ) sin(θ/2), where λ is the X-ray wavelength and θ is the scattering angle. The experimental data were treated with a direct Fourier transform to obtain the pair distance distribution function (PDDF) that was then used to determine the thickness of illite particles. The Guinier and Porod extrapolations were used to obtain the scattering intensity beyond the experimental q, and the effects of such extrapolations on the PDDF were examined. The thickness of independent rectorite particles (used as a reference mineral) is 18.3 Å. The SAXS results are compared with those obtained by X-ray diffraction peak broadening methods. It was found that the power-law exponent (α) obtained by fitting the data in the region of q = 0.1–0.6 nm−1 to the power law (I = I0q−α) is a linear function of illite particle thickness. Therefore, illite particle thickness could be predicted by the linear relationship as long as the thickness is within the limit where α <4.0.

Single-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) Å, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, <T−O> = 1.641 Å, is close to the mean <T−O> value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance <M2−O> = 2.020 Å which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Hematite (α-Fe2O3) possesses distinct spectral properties which facilitate its identification in mineral mixtures. This paper reports the relationships between the visible diffuse reflectance (DR) spectrum and the color and crystal properties of a group of 81 natural and synthetic hematites. The visible DR spectra for powdered hematite samples diluted to 4 wt.% with BaSO4 white standard were recorded and used to calculate the corresponding Munsell colors. The second derivative of the Kubelka-Munk function of the DR was used to estimate the position and intensity of the main absorption bands, which occurred at ∼435, ∼485 and ∼545 nm. The Munsell hue ranged from 9.5P to 5.3YR and was negatively correlated with the position of the ∼545 nm band, so it became yellower as the band shifted to shorter wavelengths (higher energies). The Munsell value, which ranged from 4.9 to 8.6, and chroma, which ranged from 1.4 to 8.3, were negatively and positively correlated, respectively, with the intensity of the ∼545 nm band, the position of which exhibited a weak negative correlation with the degree of Al substitution (x). The position and intensity of this band also exhibited a weak negative correlation with the specific surface area (SSA); both, however, were uncorrelated with domain shape as measured by the ratio between the X-ray mean coherence lengths (MCLs) in the [110] and [104] directions. The properties of the ∼435 and ∼485 nm bands were unrelated to x, SSA or MCL110/MCL104. The negative relationship between the position of the ∼545 nm band and x does not support the assignment of this band to the 2(6A1) → 2(4T1g(4G)) electron pair transition (EPT), which relates to the to Fe3+−Fe3+ magnetic coupling between face-sharing octahedra. The ∼485 nm band might reflect a ‘goethitic’ structural component in Fe-defective hematites as it appears at the same wavelength as the hypothetical EPT related to Fe3+−Fe3+ magnetic coupling between edge-sharing octahedra in goethite (α-FeOOH).

Palygorskite-indigo and sepiolite-indigo adducts (2 wt.% indigo) were prepared by crushing the two compounds together in a mortar and heating the resulting mixtures at 150 and 120°C, respectively, for 20 h. The samples were tested chemically to ensure that they displayed the characteristic properties of Maya Blue. Textural analysis revealed that no apparent changes in microporosity occurred in sepiolite or palygorskite after thermal treatment at 120°C (sepiolite) and 150°C (palygorskite) for 20 h. Micropore measurements showed a loss of microporosity in both sepiolite and palygorskite after reaction with indigo. The TGA-DTG curves of the sepiolite-indigo and palygorskite-indigo adducts were similar to their pure clay mineral counterparts except for an additional weight loss at ∼360°C due to indigo.

The 29Si CP/MAS-NMR spectrum of the heated sepiolite-indigo adduct is very reminiscent of the spectrum of dehydrated sepiolite. Crushing indigo and sepiolite together initiates a complexation, clearly seen in the 13C CP/MAS-NMR spectrum, which can be driven to completion by heat application. In contrast to the broad peaks of the pure indigo 13C CP/MAS-NMR spectrum, the sepiolite-indigo adduct spectrum consists of a well-defined series of six narrow peaks in the 120.0–125.0 ppm range. In addition, the sepiolite-indigo spectrum has two narrow, shifted peaks corresponding to the carbonyl group and the C-7 (C-16) of indigo. A model is proposed in which indigo molecules are rigidly fixed to the clay mineral surface through hydrogen bonds with edge silanol groups, and these molecules act to block the nano-tunnel entrances.

The mechanism of decolorization of crude maize and sunflower oils was studied by means of adsorption of β-carotene by a low-grade bentonite, containing mixed-layered illite-smectite. Decolorization depends on temperature and the time required for equilibrium decreases with increasing temperature. The study of the kinetics of adsorption showed that decolorization of maize oil is a first-order process which occurs in two steps: a first fast step with higher activation energy (25.6 kJ mol−1), indicating the influence of a chemical interaction between the pigment and the clay surface, followed by a second slow step with low activation energy (12.3 kJ mol−1), characteristic of physical adsorption on the previously adsorbed molecules. Decolorization of sunflower oil is also a first-order process, described by a single mechanism with intermediate activation energy (19.0 kJ mol−1). Adsorption isotherms of decolorization of maize oil follow the Freundlich equation, indicating the existence of heterogeneous adsorption sites on the solid's surface. Heterogeneity is attributed both to different active centers on the smectite surface (Brönsted and Lewis centers) and to the different phases present in bentonite, such as illitic layers and clinoptilolite, which also have active centers on their surfaces.

The mechanism by which the anionic polymers interact with platelets of 2:1 clay minerals is not fully understood. The objective of the present study was to evaluate the effect of the electric field associated with the basal and edge surfaces of the clay minerals on the adsorption of anionic polymers. Two negatively charged polymers, PAM90 and ACC86, of the same molecular weig ht (2 × 105 g mol−1) but with different degrees of hydrolysis (90 and 20%, respectively), were used. The effect of pH (6, 10), NaCl concentration (0, 10 mmolC L−1) and clay particle size on PAM90 adsorption by pyrophyllite and the effect of NaCl concentration on the adsorption of these polymers by Na-montmorillonite in aqueous suspensions were studied. Adsorption of PAM90 on pyrophyllite was greater at pH 6 than at pH 10 and greater in 10 mmol L−1 NaCl than in distilled water. Adsorption of PAM90 on pyrophyllite increased with decreasing particle size. On pyrophyllite the high-charge-density PAM90 was adsorbed to a greater extent than the low-charge-density ACC86; by contrast, ACC86 adsorption was greater than that of PAM90 on Na-montmorillonite. These results were attributed to the repulsive forces which develop between the negatively charged extended-chain polymer and the extended negative electric field associated with the basal surfaces, around the Na-montmorillonite platelets. The results may suggest that the edge surfaces play a major role in PAM adsorption.

The surface acidic properties of pillared clays (PILCs) with Al or Zr oxide pillars (prepared from a natural Portuguese smectite and a synthetic Laponite), and of a protonic NaHY zeolite, were studied by two methods: pyridine adsorption followed by infrared (IR) spectroscopy, and the catalytic transformation of 1-butene. The results of the pyridine adsorption revealed that all the pillared clays studied have mainly Lewis-type acidity, the exception being the clay pillared with Zr oxide, obtained from natural smectite, which also presents a significant number of Brönsted acid sites. The results of 1-butene transformation showed that the pillared clays exhibit catalytic properties similar to those of the protonic Y zeolite. The acid character of the solids was confirmed by the values of the cis/trans 2-butene isomers ratio. At short reaction times, product distribution showed that the main reaction is oligomerization, followed by cracking. After 15 min the products are mainly the linear isomers, cis and trans-2-butene, showing that the majority of the catalytic active sites, are already poisoned after 15 min of reaction. The particular behaviour of Laponite intercalated with Al oxide pillars is discussed. The IR spectra of the pyridine adsorbed on the fresh samples and after the catalytic assays, revealed that Lewis acid sites behave as active sites for 1-butene catalytic transformation. The consistency between the results of the two complementary techniques used for the characterization of the acidity of the solids was proved.

The nature and morphology of Al(OH)3 polymorphs obtained by precipitating Al and tartrate at pH 12.0 and at a tartrate/Al molar ratio (R) of 0.1 were studied by X-ray diffraction, transmission electron microscopy and electron diffraction. Co-crystallization of unusually long (up to 18 µm) ‘stick-shaped’ gibbsite with platy particles of both nordstrandite, mainly, and bayerite was observed. Face-to-face associations of plates nucleated perpendicularly on the gibbsite surfaces parallel to the c axis produced peculiar elongated ‘brush’ -shaped particles. In a strongly alkaline environment the inhibiting effect of tartrate on Al(OH)3 polymorph formation is reduced and production of co-crystallized phases with structural crystallographic similarities can be observed.