V.C. Farmer

The infra-red spectra of layer silicates

The infra-red absorption bands arising from the structural hydroxyl groups of a number of dioctahedral and trioctahedral layer silicates are reported with grating resolution in the 3750–3500 cm−1 region. The frequency differences and orientation behaviour of these bands are discussed in terms of the known structures of the minerals. The effect of the deviation from hexagonal symmetry shown by the tetrahedral layers of dioctahedral minerals on their vibrations is theoretically analyzed, and shown to account for some of the principal differences between the spectra of dioctahedral and trioctahedral layer silicates in the 1300 to 400 cm−1 region. Comparison of the spectra of random and oriented specimens of dioctahedral layer silicates in this region permits an assignment of some of the principal absorption bands to particular vibrational modes of the crystal lattice, and reveals some surprising differences between corresponding vibrational frequencies in related structures. Effects on the spectra of Al-for-Si substitution in the lattice of both dioctahedral and trioctahedral minerals are reported. A striking effect of particle size on the appearance and position of some of the stronger absorption bands of the kaolin minerals is related to the direction of the dipole moment change associated with these vibrations. Minerals examined include pyrophyllite, beidellite, rectorite, muscovite, margarite, montmorillonite, nontronite, celadonite, lepidolite and kaolins in the dioctahedral series, and talc, hectorite, saponite, phlogopite and biotite in the trioctahedral series.

Characterization of the chemical structures of natural and synthetic aluminosilicate gels and sols by infrared spectroscopy

Infrared spectra in the 300–1400 cm−1 region indicate that the non-crystalline products of interaction between hydroxyaluminium species and orthosilicic acid in dilute aqueous solutions of pH 6. The compound formed in acid solution has an infrared spectrum similar to imogolite, and is termed proto-imogolite; like imogolite, it contains orthosilicate groups and 6-coordinated aluminium, and has an ideal Si:Al ratio of around 0.5, but it can incorporate some excess alumina or silica and it does not have the regular tubular structure of imogolite. Compounds formed in alkaline solutions that are not too dilute have infrared spectra resembling the feldspathoid group of minerals and are termed hydrous feldspathoids. They incorporate a condensed tetrahedral framework with an Si:Al ratio greater than one, but can also contain 6-coordinated aluminium. Natural allophanes of the proto-imogolite and hydrous feldspathoid types exist. The allophane of weathered pumice, however, contains a condensed silicate anion that incorporates little tetrahedral aluminium. Proto-imogolite forms stable sols at pH < 5 and must play an important role in the transport of aluminium in acidic natural waters containing dissolved silica.

Transverse and longitudinal crystal modes associated with OH stretching vibrations in single crystals of kaolinite and dickite

Raman spectra of randomly-oriented kaolinite, dickite and nacrite show, for coarsely crystalline material, an extra band in the OH stretching region which is absent from the IR spectra of clay-size samples. Oriented single-crystal Raman spectra of these minerals provide confirmation for the assignment of the extra bands to transverse optical modes involving in-phase coupled vibrations of the layer-surface hydroxyl groups. The corresponding IR bands have transition moments nearly perpendicular to the layer surface, and appear at the higher frequencies of the longitudinal optical modes of macroscopic crystals.

An assessment of complex formation between aluminium and silicic acid in acidic solutions

Abstract The extent of the reaction A1 3+ + Si(OH) 4 → AlOSi(OH) 3 2+ + H + has been assessed by measuring the shift in log[H + ] between solutions containing A1(ClO 4 ) 3 , HClO 4 , and NaClO 4 with and without added silicic acid. The observed shifts of 0.018–0.027 units are about one tenth of those predicted from the formation constant ( pK 110 = 1.07 ± 0.06) proposed for the monomeric aluminosilicate species by Browne and Driscoli (1992). An estimate of this formation constant on the basis of the shifts in log[H + ] ( pK 110 = 2.50 ± 0.05) indicates that the concentration of aluminosilicate species can be neglected in many natural waters.

Effects of particle size and structure on the vibrational frequencies of layer silicates

Abstract The dependence on particle size of the position and shape of certain absorption bands in the spectra of kaolin minerals is ascribed to an effect of the oscillating electric fields induced by the vibrating dipoles. These dipole-induced fields can also account for some discrepancies between calculated and observed frequencies in layer silicate spectra. The effects of dipole coupling between layers and within layers are distinguished. The OH vibration of the mica, phlogopite, is shown to be affected by the electrostatic field due to interlayer potassium ions.

Lignin in sphagnum and phragmites and in peats derived from these plants

Abstract The occurrence of phenolic substances in the cell wall of plants is reviewed with particular reference to sphagnum moss. Chemical, ultraviolet and infrared absorption studies of the phenolic component of Sphagnum sp. show that it is distinguished from the lignin of higher plants by its ready liberation in water-soluble form by acid or alkaline hydrolysis, by its low content of guaiacyl and syringyl groups, and by its high content of p-hydroxyphenyl and carbonyl groups. Its acidity does not, however, distinguish it from the lignin of gramineous plants. Infrared spectra and the results of nitrobenzene oxidation show the presence of lignin-derived aromatic rings in the humic acid of phragmites peat. Little-altered lignin can account for 25–40 per cent of the alcohol/benzene-soluble fraction of this humic acid. The same aromatic rings are present in lower concentration in humic acids from sphagnum peats and are derived from the remains of higher plants in these peats, and not from the sphagnum itself.

Utilization by Soil Fungi of p -Hydroxybenzaldehyde, Ferulic Acid, Syringaldehyde and Vanillin

SUMMARY: A number of fungi isolated from soils under a variety of vegetational types was found to attack p-hydroxybenzaldehyde, ferulic acid, syringaldehyde and vanillin. These compounds were used as sole source of carbon by the organisms tested. By means of spectrochemical methods and paper chromatography it was shown that vanillin and ferulic acid were converted to vanillic acid before the breaking of the benzene ring, and syringaldehyde was converted to syringic acid. The bearing of these results on the breakdown of lignin in soil is discussed.

Synthesis of imogolite: a tubular aluminium silicate polymer

Heating weakly acidic dilute solutions containing hydroxyaluminium orthosilicate complexes gives a fibrous polymer, each fibre consisting of tubes about 2·2 nm external diameter and 1·0 nm internal diameter, essentially identical with those of the natural mineral, imogolite.

Effects of grinding during the preparation of alkali-halide disks on the infra-red spectra of hydroxylic compounds

Abstract Changes in the spectra of phenols and organic acids induced by grinding with alkali halides are ascribed to the adsorption of molecules on the surface of the alkali halides through their hydroxyl groups. Carboxylic acids are shown to be adsorbed principally as monomers, linked to halide ions by hydrogen bonding, so that the spectral changes which occur serve to identify vibrations to which the carboxyl group contributes. In addition, a rearrangement of the crystalline structures of cinnamic acid and hydroquinone on grinding with alkali halides is reported. Glucose gives a new crystalline spectrum when ground with potassium iodide, but other sugars are either unchanged or converted to an amorphous state; similar effects occur with some samples of potassium bromide. The pressed-disk technique is discussed in the light of these results.

The spectrographic analysis of plant ash in the carbon arc

A spectrographic method using the cathode layer arc technique for the determination of Cu, Fe, Mn, Sr, Ba, Na, Mg and Ca in plant ashes without chemical pretreatment is reported. Potassium sulphate is used as a spectrographic buffer, while traces of Ag and Cr incorporated in the carbon powder provide internal standards for the elements determined. A method for preparing CaCO3 free from Sr and Ba is reported.