GeoScienceWorld
Volume

Advanced Applications of Synchrotron Radiation in Clay Science

Abstract

This volume presents the majority of topics in synchrotron science that are of use to the clay science community. The chapters presented in this volume serve not only as significant statements on the state of these applications, but also as useful primers to potential new users of synchrotron facilities.

  1. Page 1
    Abstract

    A continuous spectrum of photon radiation is produced when accelerated charged particles are deflected by electric or magnetic fields. For electron storage rings this radiation is produced by path bending magnets or insertion devices, and is called synchrotron radiation. It commonly spans energies from the far infrared to hard X-ray regions, has extremely small beam divergence and is orders of magnitude brighter than photons from X-ray tubes. The radiation is also strongly polarized in the plane of the storage ring, and has a pulsed structure in time, allowing for experiments that can resolve kinetic phenomena on the order of the pulse separation. Specialized insertion devices augment the radiation available from bending magnets. Wigglers essentially act as the sum of many bending magnets, producing high photon fluxes. Undulators produce interfering radiation fields that result in non-continuous spectral output, but extremely high brightness and high coherency. Other important X-ray optical elements include grazing-incidence mirrors for beam focusing, crystal monochromators for energy selection, zone-plate optical elements for nanoscale beam focusing and phase contrast microscopy, and detector systems for measuring high count rates with highenergy resolution. Present synchrotron facilities allow a number of powerful spectroscopic, diffraction, and imaging techniques to be widely available to a broad user base. Access to such facilities is also described.

  2. Page 33
    Abstract

    A new generation of hybrid powder diffraction instruments will provide increased access to high-energy (>30 keV) synchrotron X-rays and facilitate a variety of diffraction experiments that are either not possible or practical using conventional diffractometers in the home laboratory. The coupling of a beam with increased brightness, tunability, low divergence, and low emittance with the availability of fast area detectors, opens up the possibilities of new classes of experiments including: (1) PDF studies of nano-minerals and the testing of structure models; (2) micro-powder diffraction from heterogeneous mineral composites; (3) the development of whole nanoparticle models that include core structure, defects, surface reconstruction, and surface-sorbed species to provide more realistic descriptions of nanominerals; (4) in situ and time-resolved studies following the evolution of atomic arrangements in supersaturated solutions and the nano-particulates precipitating from them.

  3. Page 69
    Abstract

    Good knowledge of the atomic-scale structure is a prerequisite to understanding and exploiting minerals. When minerals are ordinary single crystals or powders, the structure is routinely obtained by traditional Bragg X-ray diffraction. Traditional X-ray diffraction is difficult to apply to minerals that are substantially disordered at the atomic scale, however, because the diffraction patterns of such minerals show a small number of Bragg-like peaks, if any, and a pronounced diffuse component. A nontraditional approach based on high-energy X-ray diffraction coupled to atomic pair distribution function data analysis may be used instead. The essentials of this approach are described here and its great potential demonstrated with examples from recent studies. The purpose is to encourage the mineralogical community go beyond traditional X-ray diffraction when the atomic-scale structural features of substantially disordered minerals are to be determined in detail.

  4. Page 89
    Abstract

    Total scattering experiments using synchrotron X-rays and spallation neutrons are providing new insights into the structural characteristics of synthetic and natural ferrihydrite, a poorly crystalline nanomineral that has industrial applications and is pervasive in the environment. Modern synchrotron and neutron facilities with dedicated total scattering beamlines now enable the collection of scattering data for ferrihydrite to large scattering angles and with superior signal/noise ratio. The pair distribution function, or PDF, derived from these scattering data is a real-space depiction of atomic structure with exceptional resolution. A number of recent high-energy X-ray total scattering studies have focused on as-precipitated, pure synthetic ferrihydrite of different particle sizes, as well as ferrihydrite co-precipitated with environmentally relevant impurities, such as aluminum, arsenic, chromium, and silicon. The findings from these investigations have contributed to our understanding of the extended framework of iron and oxygen in ferrihydrite, and the specific associations of these various impurities over a range of impurity content. In situ and ex situ synchrotron total scattering methods are being applied in studies of the structural, physical, chemical, and magnetic changes taking place during ferrihydrite transformations. Neutron total scattering is shedding new light on the association of water with the surfaces and in the bulk structure of ferrihydrite. In several of these studies, real-space fitting of PDFs derived from both neutrons and X-rays are being used to test both new and previously proposed structural models for synthetic ferrihydrite. Total scattering and PDF analysis is also revealing new insights into the structural characteristics of different samples of natural ferrihydrite that were formed in a diverse range of geochemical settings. The results of these studies, combined with new PDF data for a suite of natural samples presented in this review, show that the structure of natural ferrihydrite can vary almost continuously between high- and low-crystallinity end members. The natural samples with the greatest degree of structural order are virtually identical to pure laboratory ‘six-line’ ferrihydrite. Interestingly, total scattering shows that the majority of the natural ferrihydrite samples in the suite presented here are less crystalline than ‘two-line’ ferrihydrite, the variety of synthetic ferrihydrite that is often assumed in laboratory studies to be equivalent to natural ferrihydrite. The implications of this decreased crystallinity are not yet fully understood in terms of evaluating and predicting the chemical behavior of natural ferrihydrite in the environment.

  5. Page 137
    Abstract

    Computed X-ray tomography is a technique that produces cross sections of an object from a series of projections at different angles. The technique has found widespread use in medical CAT scanners, which typically have resolutions of ~1 mm. Microtomography is the extension of this technique to smaller spatial resolution down to <1 μm. In the last 15 years the development of high-brightness synchrotron X-ray sources, high resolution CCD detectors, and high-performance computing have allowed the field of microtomography to progress rapidly. It is now being applied widely in Earth and soil science, where it is used to image the 3-D distribution of minerals, fluids, and pores. By exploiting X-ray absorption edges, 3-D images of the distribution of specific chemical elements can be produced. This is used to image the distribution of aqueous and organic fluids that have been doped with contrast agents such as iodine and cesium. The method is also being used to locate trace-mineral phases containing high-atomic-number elements such as zirconium and cerium. With fluorescence tomography 3-D images of trace element abundances and even oxidation states can be produced. This is being applied to understand the chemical contamination and remediation by plants in the environment. Diffraction tomography images the 3-D distribution of crystalline phases based on their powder diffraction peaks, and is very useful for imaging materials with similar X-ray absorption and composition but different crystalline structures.

  6. Page 167
    Abstract
    A version of this chapter is also available at www.xafs.org and a longer version has recently been published in the Mineralogical Society of America/Geochemical Society Reviews in Mineralogy series, volume 78
    : Spectroscopic Methods in Mineralogy and Materials Sciences (G.S Henderson, D.R. Neuville, and R.T. Downes, editors).

    X-ray Absorption Fine-Structure (XAFS) is an element-specific spectroscopy in which measurements are made by tuning the X-ray energy at and above a selected core-level binding energy of a specified element. Although XAFS is a well established technique providing reliable and useful information about the chemical and physical environment of the probe atom, its requirement for an energy-tunable X-ray source means it is primarily done with synchrotron radiation sources and so is somewhat less common than other spectroscopic analytical methods. XAFS spectra are sensitive to the oxidation state and coordination chemistry of the selected element, and the extended oscillations in the spectra are sensitive to the distances, coordination number, and species of the atoms immediately surrounding the selected element. Because it is element-specific, XAFS places few restrictions on the form of the sample, and can be used in a variety of systems and bulk physical environments, including crystals, glasses, liquids, and heterogeneous mixtures. Additionally, XAFS can often be done on low-concentration elements (typically down to a few ppm), and so has applications in a wide range of scientific fields, including chemistry, biology, catalysis research, material science, environmental science, and geology.

  7. Page 212
    Abstract

    Modern synchrotron-based hard X-ray microprobes allow clay and soil scientists to characterize clay mineralogy and the abundance, distribution and speciation of elements that are both essential structural constituents in clays and adsorbed or incorporated within the clay structure. The instruments can be used to evaluate minute samples that are heterogeneous at the sub-micrometer scale, often in an as-collected state. Synchrotron radiation sources are ideal for developing high-intensity, highly focused X-ray probes and these instruments offer distinct advantages over other analytical techniques by allowing analyses to be done in situ with little or no chemical pretreatment and with low detection limits. The current generation of hard X-ray microprobes provides attogram detection levels for transition and heavy metals, allows for coupled molecular speciation analysis using X-ray absorption fine structure (XAFS) analysis at parts per million concentration, and can be used to evaluate mineralogy at spatial resolutions of <1 μm using X-ray microdiffraction. Microfocused XAFS provides researchers with the ability to evaluate micrometer-scale heterogeneities in elemental valence state and molecular speciation. Synchrotron X-ray microdiffraction now allows mineralogists to obtain diffraction patterns from sample masses on the order of 1 μg or less, with data quality (signal to noise ratio) considerably better than the best diffraction patterns obtainable from commercial laboratory-based diffractometers. Advances in X-ray optics and the construction of new synchrotron sources with low emittance have led to the recent availability of hard X-ray microprobe beamlines with spatial resolutions as small as 30 nm. Examples of the application of these techniques to the study of the mineralogy and geochemistry of clays include the mobility and adsorption of potentially toxic elements in natural clays and geosynthetic clay liners, evaluation of how clays impact the valence state of actinides in soils and in the subsurface, and in visualizing clay gels in a wet state.

  8. Page 240
    Abstract

    Scanning transmission X-ray microscopy (STXM), or X-ray microscopy in general, is an ideal molecular probe for investigating the chemistry of clay minerals, clay-sized mineral particles, and mineral-organic aggregates in heterogeneous aqueous matrices. This technique allows mapping of different elements, elements in different valence states and bonding environments, and mineral phases at a spatial resolution of a few nanometers. In addition, high-resolution XANES spectra can be obtained at the same spatial resolution, and the characteristic XANES features of individual phases can also be used in mapping. While STXM studies can be conducted using X-ray transmission, X-ray fluorescence, or electron-yield, the type of detection used can limit the sample thickness, bulk vs. surface sensitivity, and sensitivity to detecting dilute samples. Using STXM and associated Al K-edge XANES spectra, researchers have examined successfully mixtures of clay suspensions. These studies indicate that STXM can be used to identify specific clay-mineral assemblages in natural samples. STXM has also been used successfully in studying Fe- and Mn-biominerals, their characteristics, and rates of production under different biogeochemical conditions. More recently this technique has been applied successfully to sstudy aquatic colloids from both marine and terrestrial systems. A detailed discussion of the method and its applications are summarized here.

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