W.J. McHardy

Podzolisation mechanisms and the synthesis of imogolite in northern Scandinavia

Abstract The nature of the short-range ordered Al and Fe minerals of the spodic B horizons of northern Scandinavia was studied by selective dissolution, transmission electron microscopy, infrared absorption and sulphate adsorption experiments. Imogolite-type materials (proto-imogolite allophane and well-developed imogolite) were the predominant oxalate-extractable Al minerals. Minor amounts of gibbsite and kaolinite were also found. The Fe oxides were less abundant and relatively well crystallized. Imogolite-type materials were the predominant source of positive charge, as evidenced by the amount of sulphate adsorbed. The Fe:AI ratio of the pyrophosphate extracts was positively correlated with the ratio of pyrophosphate-C to oxalate-extractable Al—this observation provides field evidence that imogolite-type materials were particularly unstable in the presence of organic substances. It is hypothesized that most accumulated Al and Fe originated from the congruent dissolution of primary minerals followed by the downward migration and degradation of metal-organic complexes, but incongruent weathering within the B horizon might have made additional contributions to the precipitation of the short-range ordered minerals. Historical climatic change might, to some extent, explain why metal-organic complexes were almost absent in many B horizons.

Surface reactivity of poorly-ordered minerals in podzol B horizons

The surface reactivity of mineral soil horizons from three podzolised forest soils in Scandinavia was examined. The amount of accumulated C was low, between 1.8 and 2.3% in the top of the B horizon ...

Mineralogy and chemistry of ochre sediments from an acid mine drainage near a disused mine in Cornwall, UK

A B S T R AC T: Ochre sediments from acid mine drainage in Cornwall have been investigated using X-ray diffraction, thermal methods, infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and chemical methods in order to determine their mineralogical and chemical composition. Fresh sediments consist of ferrihydrite and goethite. Large fractions of these minerals are dissolved by the ammonium oxalate treatment reflecting their poorly crystalline structure. Fresh sediments contain large amounts of surface-adsorbed SO4 (up to 9.3%) which is readily desorbed by the PO4 treatment. Goethite is the only mineral present in relatively older sediments and the mineral is well crystallized with rod-shaped morphology. Environmental conditions, such as pH and SO4 content, are not favourable for the presence of schwertmannite at the site. Iron minerals appear to be precipitating around filamentous algae and the shape of algae is preserved in the Fe oxide matrix. The ubiquitous presence of algae in close association with Fe minerals indicate their possible role in the crystallization of Fe oxides.

hk-ordering in aluminous nontronite and saponite synthesized near 90 degrees C; effects of synthesis conditions on nontronite composition and ordering

In studies on the fate of aluminium in the environment, nontronite and saponite have been obtained by synthesis in reducing alkaline conditions close to those prevailing in poorly drained soils developed from limestones. The two minerals obtained have different structures and organizations corresponding to two different growth and/or maturation mechanisms. High-resolution transmission electron microscopy of ultrathin sections of a synthetic aluminous nontronite embedded in resin showed the presence of crystallites consisting of two to ten co-terminating parallel layers, indicating synchronous growth. Electron diffraction showed that the individual crystallites had hk-ordering, i.e., orientation of layers with respect to the six-fold pseudosymmetry of the unit cell. Deposits of a synthetic saponite included hk-ordered crystallites and crinkled films with turbostratic stacking. The two saponite phases had slightly different b dimensions. Lattice fringe images of sections of saponite embedded in resin showed a high angular disorientation of the layers in the stacking direction, suggesting multiple nucleation and growth of individual layers, subsequently aggregated with imperfect parallelism.Exploration of the synthetic conditions of the aluminous nontronite indicated that calcium was essential for an hk-ordered product. Syntheses using potassium or sodium hydroxides and carbonates for pH control gave poorly organized nontronites. Hydrazine was not essential for nontronite formation, but better crystallized products—judging by their IR spectra—were obtained in its presence by maintaining reducing conditions in the early stages of synthesis. Attempts to prepare ferruginous beidellites under similar conditions to those in which aluminous nontronites formed were unsuccessful.

PARTICLE SHAPE VERSUS COHERENT SCATTERING DOMAIN OF ILLITE/SMECTITE: EVIDENCE FROM HRTEM OF DOLNA VES CLAYS

Fundamental particle thickness measurements of Dolná Ves hydrothermal illite/smectite (I/S) samples confirmed earlier findings regarding the content of fixed cations in illite interlayers (ca. 0.9/O10(OH)2). The distributions of fundamental particles and mixed-layer crystals of a given sample are internally consistent. In samples dominated by bilayer fundamental particles, mixed-layer crystals most often contain even numbers of layers. The expandabilities measured by XRD are much higher than so-called minimum expandabilities obtained from HRTEM measurements. This discrepancy is explained by assuming that the coherent scattering domains of Dolná Ves clays do not correspond to natural mixedlayer crystals but are thicker, probably due to parallel association of crystals on the oriented XRD slide. This tendency to produce intercrystal contacts is probably related to the unusually large ab dimensions of crystals of Dolná Ves clays.

Methods for the study of interrelationships between micro-organisms and soil structure

Abstract Common methods of assessing soil structure that could be used by microbiologists are reviewed and involve size fractionation of soil aggregates, water release characteristics, intrusion porosimetry, gas adsorption and microscopic methods with image analysis. These methods could also be used for studying aspects of the interactions between soil structure and larger fauna or plant roots. Some recent refinements of these methods are described and the use of artificial soil aggregates in microbial studies is discussed.

Effects of Lichens on Mineral Surfaces

The paper examines the effects and significance of lichens, in particular the crustose species, on natural rock and on building and monumental stones. Weathering and secondary products, as well as the effects of lichen acids, are documented and reference to control of lichen growth on mineral substrates is reviewed. A particular aspect of the diagnostic techniques referred to in the degradation effects of lichen activity is scanning electron microscopy with the associated electron probe micro-analysis facility.

SEM observations in the back-scattered mode of the soil–root zone of Brassica napus (cv. Rafal) plants grown at a range of soil pH values

Abstract In the study of the root–soil interface, scanning electron microscopy (SEM) in secondary emission mode and transmission electron microscopy are commonly used techniques. As these methods have some limitations, there is a need to explore other techniques which may be suitable for the observation of the root–soil zone. One of the most promising methods is the application of SEM in the back-scattered electron (BSE) emission mode. The root zone of plants of winter oilseed rape ( Brassica napus cv. Rafal) grown at a range of soil pH and soil phosphorus (P) concentrations was examined by SEM in BSE mode. The method for the preparation of transverse sections of the root–soil contact zone is presented. The BSE scanning images of the root–soil interface obtained allowed investigation of the flow paths of plant nutrients from the soil towards roots and their distribution in the roots. Concentration of aluminium in the cell walls of immature roots and at the soil–root interface was observed in the most acid soils. Potassium, calcium and phosphorus were found in differentiated roots associated with the external tissues of the central cylinder. The intimate contact of root surface with soil mineral particles could also be observed.

An Optical, Scanning Electron Microscopic and Microanalytical Study of Cementation in Some Podzols

Abstract McHardy, W.J. and Robertson, L., 1983. An optical, scanning electron microscopic and microanalytical study of cementation in some podzols. Geoderma, 30: 161–170. The nature of a strongly cemented horizon occurring in a soil profile at Lochend Farm on the shores of Loch Leven has been investigated by optical and scanning electron microscopy and electron probe microanalysis, and comparisons have been made with more weakly cemented Bh horizons of two other podzol profiles developed on sands and gravels. Under the optical microscope the monomorphic organic coatings present on skeleton grains in the cemented horizons of all three profiles appear similar. Of the elements detectable by microanalysis, Al is dominant in all the coatings whereas a low Fe and relatively high Ca content is characteristic only of the Lochend material.

Scanning electron microscopy of calcium oxalate on mantle hyphae of hybrid larch roots from a farm forestry experimental site

Abstract An examination of roots of hybrid larch from a farm forestry site by scanning electron microscopy has revealed crystalline deposits encrusting mantle hyphae of the associated ectomycorrhizal fungus. Electron probe micro-analysis identified calcium in the crystals and X-ray diffraction showed them to be whewellite, the monohydrate form of calcium oxalate. The significance of the finding is discussed.