The crystallochemical features of Co in Co-substituted goethite solid-solutions prepared by two different procedures have been studied using infrared, X-ray photoelectron and electron energy loss spectroscopies. It was found that the path followed for the synthesis of Co-substituted goethite determines the oxidation state of Co in the goethite structure. Thus, in the solid-solution prepared by precipitation with Na2CO3 of an Fe(II) aqueous solution containing Co(II) cations, followed by the aerial oxidation of the precipitate, the Co cations were found to be divalent, whereas trivalent Co was incorporated into the goethite obtained by ageing a solution containing Fe(III) and Co(II) cations precipitated by the addition of KOH. This different behavior is explained by the higher pH of goethite formation in the latter case, which favors the oxidation of the Co(II) cations.

The properties of Si-associated goethite from sediments in the Atlantis II and Thetis Deeps in the Red Sea have been investigated in order to determine the effect of Si on the mineral. Two types of morphologies dominate in most samples: multi-domain crystallites, probably due to elevated Na concentration in the initial brine from which the mineral had crystallized, and mono-domain, acicular crystals. Goethite crystals with elevated Si/Fe elemental ratios are usually smaller and poorly crystalline, exhibiting numerous crystal defects, whereas larger crystals with higher crystallinity have lower Si/Fe elemental ratios. The higher Si/Fe ratios in Atlantis II Deep goethites and the lower ratio in Thetis Deep goethites probably reflect the levels of Si concentration in the hydrothermal fluids from which goethite precipitated. At relatively low Si/Fe ratios, the major effect of Si is to retard growth of the crystallites, but only a small number of defects are formed. At high Si/Fe ratios the defect concentration affects the properties of the crystals, as observed with Mössbauer spectroscopy. The Si association with goethite affects crystallinity and crystal size as indicated by X-ray diffraction, infrared spectroscopy and high-resolution transmission electron microscopy.

The ability of hydrotalcites to retain anionic wastes was studied. In particular, Cr(VI)-adsorbed hydrotalcites were heated to immobilize Cr(VI) in the solid sample. When the heating temperature increased up to 500°C, the lamellar structure of hydrotalcite was lost. At high temperatures (1200°C), the solids were recrystallized in the form of a spinel. Lixiviation with 1 N and 5 N NaCl solutions were utilized to simulate the effect of sea water and of concentrated NaCl solutions in salt mines on the immobilization of Cr. Radiation damage on the solid containing the immobilized Cr was studied by γ-irradiating with a 60Co source at 1000 and 6000 kGy. The Cr-containing samples heated at 1200°C, whether irradiated or not, safely immobilized Cr in the hydrotalcite mainly in the form of MgCr2O4 spinel. Irradiation of hydrotalcites revealed two different effects: (1) samples heated up to 1200°C evolved as a solid in which chromium was retained more firmly than in the non-irradiated material, irradiation enhancing the spinel formation; (2) the structure of samples heated at 1200°C developed a preferential crystallite orientation rather than a purely random one or new location of chromium ions, this effect did not affect Cr immobilization in the solid. Chromium lixiviation with 5 N NaCl solution was always less than the corresponding value with 1 N NaCl solution, probably due to the poor mobility of Cl− ions in the highly concentrated NaCl solution.

The order-disorder behavior of the isomorphous cation substitution of the octahedral sheet of phyllosilicates was investigated by Monte Carlo simulations based only on atomistic models in some three-species systems Al/Fe/Mg including a wide range of different octahedral compositions that can be relevant to clay compositions found in nature, especially for smectites and illites. In many cases, phase transitions do not occur, in that long-range order is not attained, but most systems exhibit short-range order at low temperature. The ordering of the octahedral cations is highly dependent on the cation composition. Variations in the tetrahedral charge (smectite vs. illite) produce slight differences in the cation distribution and the short-range and long-range order of octahedral cations do not change drastically. The average size of Fe clusters and the long-range order of Fe are not larger in illites than in smectites as previous reports concluded, but the proportion of Fe3+ cations non-clustered is higher in smectites than in illites. This behavior supports the experimental behavior of the Fe effect on the Al-NMR signal, which is lower in illites than in smectites.

Chinmen Island is located in the west of the Taiwan Strait, 15 km from the coast of mainland China. Mesozoic granitic gneiss forms the basement rocks of the island. High-defect kaolin deposits, both major sedimentary and minor residual types of clays, have been mined for ceramic uses for many years. The objectives of this study were to characterize the kaolin deposits and to discuss the genesis of kaolin minerals on the island. The kaolin samples were characterized by X-ray diffraction and transmission and scanning electron microscopy. In general, the particle-size distribution of the sedimentary kaolin was 0.5–5.0% sand, 15–55% silt and 30–85% clay. In the clay fraction, the ratio of kaolinite to illite ranged from 9:1 to 3:1. The sedimentary kaolin materials were originally transported by river from mainland China. Kaolinite occurred generally as pseudo-hexagonal platelets of ∼1 µm in diameter. The residual kaolin minerals resulted from the argillization of granitoid rocks by in situ weathering which possibly occurred during the Pleistocene. The residual kaolin contained more tubular halloysite.

A collection of green earths belonging to traditional artists’ pigments was examined in terms of mineralogy and provenance. The studied specimens included both mineralogical reference compounds and selected commercially available artists’ pigments, and contained green micas (glauconite or celadonite), chlorite, or smectite as pigmenting agents. The samples were examined by X-ray diffraction, Mössbauer spectroscopy, infrared (IR) spectroscopy, ultraviolet-visible (UV-Vis)-near-IR diffuse-reflectance spectroscopy and voltammetry of microparticles. Particular attention was paid to the Kadaň green earth, mined until the 20th century in the West Bohemia deposit. The Greene-Kelly charge-reduction test, detailed description of non-basal diffraction patterns and characteristic vibrations in the mid-IR spectra were used to classify the major pigmenting agent of the Kadaň green earth as ferruginous smectite with separately diffracting saponite-like clusters. The smectite contains ∼15% Fe, mainly in the trivalent form, a detectable fraction of Fe in tetrahedral sites, and it is accompanied by a significant amount of Ti-bearing relict minerals due to its volcanogenic origin. On the contrary, in green micas (glauconite and celadonite) the Ti content is much smaller. Diffuse reflectance spectroscopy was found suitable for distinguishing Fe as a constituent of free Fe oxides from Fe in the clay structure. It was also found to be useful for discriminating between green micas and smectites.

Cesium uptake by plants depends on adsorption/desorption reactions in the soil, as well as root uptake processes controlled by the plant. In this study, sorption and desorption of Cs+ on reference illite (IMt-1) was investigated in the presence of oxalate to gain understanding of mechanisms by which plant root exudates may influence Cs+ bioavailability in micaceous soils. Cesium sorption on illite decreased significantly as oxalate concentration increased from 0.4 to 2 mM. Cesium desorption from illite increased significantly with increasing oxalate concentration. Desorption of Cs+ by exchange with Na+, Ca2+ and Mg2+ was significantly enhanced in the presence of oxalate as selectivity for Cs+ decreased with respect to these ions in the presence of oxalate. On the other hand, oxalate had little effect on the Cs+/K+ selectivity coefficient. This suggests that oxalate treatments increase the relative proportion of exchange sites that are not highly selective for Cs+ and K+; e.g. ‘planar’ sites. The results indicate that oxalate plays an important role in Cs+ binding on illite and, therefore, plant rhizosphere chemistry is likely to alter Cs+ bioavailability in micaceous soils.

Fe(II)–Fe(III) green rust identified in soil as a natural mineral is responsible for the blue-green color of gley horizons, and exerts the main control on Fe dynamics. A previous EXAFS study of the structure of the mineral confirmed that the mineral belongs to the group of green rusts (GR), but showed that there is a partial substitution of Fe(II) by Mg(II), which leads to the general formula of the mineral: ${[{\text{Fe}}_{1-x}^{2+}{\text{Fe}}_x^{3+}{\text{Mg}}_y{(\text{OH})}_{2+2y}]}^{x+}{[x{\text{OH}}^{-}\cdot m{\text{H}}_2\text{O}]}^{x-}$${[{\rm{Fe}}_{1 - x}^{2 + }{\rm{Fe}}_x^{3 + }{\rm{M}}{{\rm{g}}_y}{({\rm{OH}})_{2 + 2y}}]^{x + }}{[x{\rm{O}}{{\rm{H}}^ - } \cdot m{{\rm{H}}_2}{\rm{O}}]^{x - }}$. The regular binary solid-solution model proposed previously must be extended to ternary, with provision for incorporation of Mg in the mineral. Assuming ideal substitution between Mg(II) and Fe(II), the chemical potential of any Fe(II)-Fe(III)-Mg(II) hydroxy-hydroxide is obtained as: $\text{μ}=X_1{\text{μ}}_{\text{1}}^{\text{o}}+X_2{\text{μ}}_{\text{2}}^{\text{o}}+X_3{\text{μ}}_{\text{3}}^{\text{o}}+\text{R}T[X_1\text{ln}{X}_1+X_2\text{ln}{X}_2+X_3\text{ln}{X}_3]+A_{12}{X}_2(1-X_2)$${\rm{\mu }} = {X_1}{\rm{\mu }}_{\rm{1}}^{\rm{o}} + {X_2}{\rm{\mu }}_{\rm{2}}^{\rm{o}} + {X_3}{\rm{\mu }}_{\rm{3}}^{\rm{o}} + {\rm{R}}T[{X_1}{\rm{ln}}{X_1} + {X_2}{\rm{ln}}{X_2} + {X_3}{\rm{ln}}{X_3}] + {A_{12}}{X_2}(1 - {X_2})$. All experimental data show that the mole ratio X2 = Fe(III)/[Fetotal + Mg] is constrained (1) structurally and (2) geochemically. Structurally, Fe(III) ions cannot neighbor each other, which leads to the inequality $X_2\alpha \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right..$${X_2}\leqslant {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 3$}}.$ Geochemically, Fe(III) cannot be too remote from each other for GR to form as Fe(OH)2 and Mg(OH)2 are very soluble, so $X_2\alpha \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.$${X_2}\geqslant {\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 4$}}$. A linear relationship is obtained between the Gibbs free energy of formation of GR, normalized to one Fe atom, and the electronegativity ϰ of the interlayer anion, as: μo/n = −76.887ϰ — 491.5206 (r2 = 0.9985, N = 4), from which the chemical potential of the mineral fougerite μ is obtained in the limiting case X3 = 0, and knowing ${\text{μ}}_{\text{1}}^{\text{o}}=-489.8$${\rm{\mu }}_{\rm{1}}^{\rm{o}} = - 489.8$ kJmol−1 for Fe(OH)2, and ${\text{μ}}_{\text{3}}^{\text{o}}=-832.16$${\rm{\mu }}_{\rm{3}}^{\rm{o}} = - 832.16$ kJmol−1 for Mg(OH)2, the two unknown thermodynamic parameters of the solid-solution model are determined as

${\text{μ}}_{\text{2}}^{\text{o}}=+119.18{\text{kJmol}}^{-1}$${\rm{\mu }}_{\rm{2}}^{\rm{o}} = + 119.18\;{\rm{kJmo}}{{\rm{l}}^{ - 1}}$ for Fe(OH)3 (virtual), and A12 = −1456.28 kJmol−1 (non-ideality parameter). From Mössbauer in situ measurements and our model, the chemical composition of the GR mineral is constrained into a narrow range and the soil solutions-mineral equilibria computed. Soil solutions appear to be largely overstaurated with respect to the two forms observed.

Bulk mineralogical and chemical properties of a weathering profile derived from serpentinite were studied using classical pedological methods (Munsell soil colors, particle-size distribution, density, cation exchange capacity, exchangeable bases, among others) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results. Bulk clay fractions were characterized using X-ray diffraction, thermal analysis, electron microprobe, Mössbauer and infrared spectroscopies. Bulk geochemical mass-balance calculated from ICP-AES results shows leaching of both Mg and Si which reflects the early weathering of serpentine minerals. As a consequence, newly formed clay minerals are enriched with the least mobile elements, i.e. Fe and Al, producing dioctahedral smectites. These dioctahedral smectites are complex, heterogeneous and consist mainly of two populations. One population is an Fe-rich montmorillonite with little or no tetrahedral charge and Fe3+ as the dominant octahedral cation whereas the second population exhibits tetrahedral charge. Both populations occur as interstratified layers in the lower horizon of the weathering profile but show increasing segregation into well-defined end-members towards the surface horizons. Considering total Al and Fe contents, these clays differentiate into two chemical composition domains, Fe-rich clays in the lower profile and Al-rich clays towards the surface horizons.