The octahedral composition of palygorskite in more than 300 samples from the Pefkaki deposit, W. Macedonia, Greece, has been studied by near-infrared (NIR) and X-ray diffraction (XRD), and evaluated according to the formula $y{\text{Mg}}_5{\text{Si}}_8{\text{O}}_{20}{\left(\text{OH}\right)}_2\cdot x{\text{Mg}}_2{\text{Fe}}_{\text{2}}^{\text{III}}{\text{Si}}_8{\text{O}}_{20}{\left(\text{OH}\right)}_2\cdot \left(1-x-y\right)$ Mg2Al2Si8O20(OH)2. Included in the study were PFl-1 and several commercial palygorskites. Our analysis of 2nd derivative NIR spectra shows that the dioctahedral composition is adequately described by three sharp overtone bands representing AlAlOH, AlFeIIIOH and FeIIIFeIIIOH in M2 dioctahedral sites, and that the summed intensity of these bands is proportional to the amount of dioctahedral component present (1−y). The samples show large variations in the degree of dioctahedral FeIII-for-Al substitution with FeIII occupying up to 70% of the dioctahedral M2 sites. Ternary analysis shows that the distribution of dioctahedral Al and FeIII is not random, but displays a tendency towards homoionic pairing. An overtone band at 7214 cm−1 and several combination bands are indicative of a trioctahedral Mg3OH component (y), and their appearance correlates with a distinct palygorskite signature in thermogravimetric analysis. Nevertheless, these bands cannot be used reliably for the quantification of a trioctahedral palygorskite component due to their close similarity to those of sepiolite. To circumvent this problem, we have evaluated y indirectly by calculating the difference between 1−y and the total concentration of palygorskite determined by the normalized intensity of the d110 XRD peak of palygorskite at 10.4 Å. Using this methodology, we have found that the samples conform to a trioctahedral limit of y ≈ 0.55, although within this limit they display large variations in octahedral character. Finally, we extend the above methodology to PLS chemometrics and show how NIR can be used to determine palygorskite content routinely in multimineralic geological samples.

The structure and dynamics of alkylammonium-intercalated smectites were simulated using molecular dynamics employing the clayff-CVFF force field, the reliability of which was firstly validated for these systems. The layering behaviors of alkyl chains confirm the scenarios of the monolayer, transition and bilayer configurations for short, medium-length and long carbon tails, respectively. In the systems without water, the alkylammonium groups are all anchored firmly above the surface six-member rings through H bonds between ammonium hydrogen and surface oxygen, and the alkyl tails are a little more mobile. With water involved, some ammoniums are dragged out of the potential barriers of the six-member rings by water molecules through the strong H bonds between water oxygen and ammonium hydrogen. The intercalated water scarcely affects the basal spacing, alkyl chain layering or alkylammonium dynamics. It is also found that the systems with alkyl chains of 11 to 14 exhibit the greatest density, resulting in the extremely limited mobility of the intercalated species.

The weathering stages of K-depleted biotite, K-depleted phlogopite, and natural biotite were investigated using hydrothermal treatment with A1C13 solution at 200°C for 12 to 72 h. Although there were some differences in the degree of weathering of the two K-depleted micas, both first transformed to hydroxy-Al interlayered vermiculite (HIV), which then altered to kaolinite. In case of the natural biotite, the biotite first transformed to a K-depleted mica-like phase, which then altered to kaolinite. The natural biotite had resisted weathering to kaolinite more than did the K-depleted biotite, as expected. The K-depleted phlogopite had less resistance in weathering to kaolinite than the K-depleted biotite. The transformation process changed the color of the micas. The K-depleted biotite changed from greenish-black through yellow to pale gray whereas the natural biotite changed from greenish-black through beige to yellow. However, in the case of K-depleted phlogopite, there was no significant color change during the transformation process. The presence of the interlayer K+ ions and the structural Fe2+ ions in mica appear to have contributed to the differences in the degree of weathering to kaolinite among the micas investigated.

Rapid dissolution of partly amorphized kaolinite in the systems kaolinite + NiCl2, kaolinite + Ni(OH)2, and kaolinite + NiCl2 + Ni(OH)2, at a temperature of 200°C and at pH between 5.3 and 7.4, leads to the precipitation of Ni-poor kaolinite, Ni-rich kaolinite and Al-Ni-serpentine. Identification of the phases was carried out using a combination of X-ray diffraction and transmission/analytical electron microscopy. Ni-bearing kaolinite shows variable morphologies in the systems studied: stacks of kaolinite with relatively small Ni contents and fine-grained curved particles of Ni-rich kaolinite dominate in the Cl-bearing system; spherical particles with a disordered structure and relatively uniform Ni contents (in the order of 0.15 atoms per formula unit (a.p.f.u.)) and platy particles of Al-Ni-serpentine characterize the products formed in the Ni(OH)2-richest systems. The presence of Ni(OH)2 in the systems (with and without Cl) favors the dissolution process as well as rapid precipitation of spherical particles, and the formation of serpentine. A difference from Mg systems studied previously is a well defined phase intermediate in composition between kaolinite and serpentine which originated in the Ni-bearing systems. Increasing Ni content is clearly reflected in the parallel increase in the b cell parameter of kaolinite. The average composition of the coexisting Al-Ni-serpentine is: (Al1.24Ti0.01Fe0.02Ni1.31) (Si1.58Al0.42)O5(OH,Cl)2.

A new occurrence of berthierine has been found in the Pulsifer granitic pegmatite, near Auburn, Maine. Berthierine was found in miarolitic cavities (‘pockets’) as aggregates of radial platy crystals with albite, as fracture-fillings in microcline and as fine-grained anhedral flakes with muscovite. Berthierine samples from all associations are depleted in Mg, but show variable enrichment in Mn. The textural relationships of the berthierine assemblage indicate that its formation is probably related to the dissolution of microcline in the presence of a low-temperature, neutral to alkaline, Fe-rich hydrothermal solution. This mode of occurrence is in direct contrast to the typical formation of berthierine in granitic pegmatites that is generally related to the alteration of cordierite-group minerals.

The formation of hydroxylated phases was investigated using K-depleted biotite (Na-biotite) and K-depleted muscovite (Na-muscovite) under hydrothermal treatment with alkali (Li+, K+, NH4+, Rb+ and Cs+), alkaline earth (Mg2+, Ca2+, Sr2+ and Ba2+), and aluminum (Al3+) cations at 200°C for 1 and/or 3 days. The K-depleted biotite treated with alkali cations produced anhydrous hydroxylated phases, while the K-depleted muscovite did not significantly exchange alkali cations but dehydrated to form Na-muscovite in all cases. The alkaline earth cations, however, produced hydrous hydroxylated phases with both K-depleted micas. The degree of hydration energy of cations and the charge density of micas were found to influence the formation of anhydrous and hydrous phases from the K-depleted micas. This type of topotactic cation exchange potentially could be used for fixation and immobilization of radioactive species such as Cs, Sr, Ra, etc. in the transformed micas. The K-depleted biotite and muscovite treated with Al3+ were transformed to hydroxy-Al interlayered vermiculites (HIV) because of hydrolysis and polymerization of Al3+. These HIV phases could also serve as useful adsorbents for soil and groundwater contaminants.

We have investigated the adsorption of adenine, adenosine, ribose, and adenosine-5′-phosphate (5′-AMP) by allophane at pH 4, 6 and 8. Adenine, adenosine and ribose gave similar isotherms, i.e. adsorption increased regularly with solution concentration and decreased in the order: pH 8 > pH 6 > pH 4. Allophane had a greater affinity for 5′-AMP than for adenine, adenosine or ribose. Further, the extent of adsorption for 5′-AMP increased in the order: pH 8 ≪ pH 6 ≈ pH 4. The adsorption of 5′-AMP at pH 4 and pH 6 was about 60 times greater than at pH 8. The strong adsorption of 5′-AMP accords with the well known high phosphate-retention capacity of allophane and allophane-rich soils. The experimental data may be rationalized in terms of the pH-dependent charge characteristics of the organic solutes and allophane. The large propensity of allophane to retain 5′-AMP is ascribed to ligand exchange between the phosphate of 5′-AMP and the hydroxyl of (HO)Al(OH2) groups, exposed at perforations on the wall of allophane spherules, giving rise to a surface (chelation) complex. The high affinity of nucleotides for allophane has implications for the possible role of allophane in the abiotic formation of RNA-type polynucleotides although nucleotide ‘immobilization’ by surface complexation might hinder RNA oligomerization.

The Allou Kagne (Senegal) deposit consists of different proportions of palygorskite and sepiolite, and these are associated with small quantities of quartz and X-ray amorphous silica as impurities. No pure palygorskite or sepiolite has been recognized by X-ray diffraction. Textural and microtextural features indicate that fibrous clay minerals of the Allou Kagne deposit were formed by direct precipitation from solution. Crystal-chemistry data obtained by analytical/transmission electron microscopy (AEM/TEM) analyses of isolated fibers show that the chemical composition of the particles varies over a wide range, from a composition corresponding to palygorskite to a composition intermediate between that of sepiolite and palygorskite, but particles with a composition corresponding to sepiolite have not been found. Taking into account the results from selected area electron diffraction and AEM-TEM, fibers of pure palygorskite and sepiolite have been found but it cannot be confirmed that all of the particles analyzed correspond to pure palygorskite or pure sepiolite because both minerals can occur together at the crystallite scale. In addition, the presence of Mg-rich palygorskite and very Al-rich sepiolite can be deduced.

It is infrequent in nature that palygorskite and sepiolite appear together because the conditions for simultaneous formation of the two minerals are very restricted. The chemical composition of the solution controls the formation of the Allou Kagne sepiolite and palygorskite. The wide compositional variation appears as a consequence of temporary variations of the chemical composition of the solution.

The processes of clay mineral formation were studied in seven podzol profiles developed on granitic regoliths in the Polish part of the Tatra Mountains. The selected profiles have similar parent material and macroscopically represent different stages of soil development (from initial to advanced). Bulk soil material (<2 mm fraction) and separated clay fractions (<2 µm) were studied using a petrographic microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy-dispersive spectrometry methods. The mineral compositions of the bulk soil samples are more or less the same (quartz, feldspars, mica and minor amounts of other phyllosilicates). The clay fractions are composed of mica and mixed-layer minerals which contain hydrated interlayers of vermiculitic and/or smectitic type, and kaolinite. Smaller amounts of chlorite, feldspars and quartz were also identified. Chlorite is present almost exclusively (except for one profile) in lower soil horizons (C, B/C, B). The amount of minerals with hydrated interlayers increases up the profiles. Kaolinite is present in all the samples except for the lowermost soil horizons (C) of two of the profiles. In some of the B horizons, the formation of hydroxy interlayers within hydrated interlayers is observed. The main processes of clay mineral formation recognized in the soils studied are: inheritance from the parent rocks; crystallization of kaolinite from soil solutions; the formation of dioctahedral vermiculite at the expense of inherited dioctahedral mica, and the formation of dioctahedral smectite at the expense of vermiculite. The recognized sequence of transformation is as follows: M → R0 M-V (12 Å or 14 Å) → R0 M-12 Å V → R1 M-12 Å V → 12 Å V → V-S → S. Observed formation of hydroxy interlayers seems to be pH dependent, starting when the pH ⩾ 4.4. The process of dissolution of primary silicates occurring simultaneously with the transformation is also documented.

Floating, mixed-valent Fe films have been observed worldwide in wetlands, ferrous iron-rich seeps, and in seasonally reduced soils, but are usually misidentified as oil or biofilms. There has been little characterization or explanation of their formation. Along the Oregon coast such films were found on ephemeral pools where Fe(II)-rich groundwater (∼100 µM Fe) has been discharged at the base of Pleistocene sand dunes. Fe(II) oxidized to Fe(III) at the air-water interface to form ∼100–300 nm thick films. Analyses indicated that the films contained both Fe(III) and Fe(II) in a ratio of 3:1; Si was the other main cation; OH was the main anion and some C was also identified. The film morphology was flat under optical and electron microscopy with some attached floccules having a string-like morphology. Energy-filtered electron diffraction patterns showed three diffraction rings at 4.5, 2.6 and 1.4 Å in some places and two rings (2.6 and 1.4 Å) in others. Upon further oxidation the films became 2-line ferrihydrite. We are proposing the name ‘schwimmeisen’ for the floating, mixed-valent Fe film.