Synchrotron X-Ray Methods in Clay Science

By W. Bassett, P. Bertsch, D. Chateigner, S. Fendorf, C. Jacobsen, R. Lu, A. Goncharov, R. Hemley, H. Mao, A. Manceau, M. Schlegel, B. Lanson, C. Bartoli, W. Gates, U. Neuhäusler, J. Niemeyer, J. Parise, D. Schulze, S. Sutton, M. Rivers, J. Thieme and T. Wu
Edited by Darrell G. Schulze, Joseph W. Stucki and Paul M. Bertsch


The discovery of X-rays over 100 years ago and the subsequent discovery of X-ray diffraction 17 years later had a profound impact on almost all areas of the physical sciences. Clay Science is no exception. Modern concepts of clays are shaped to a great extent by information obtained from X-ray based techniques. The X-ray intensity obtainable from the sealed-tube laboratory X-ray sources used for most clay research, however, has not increased substantially from that available during Röntgen's time. The increasing availability of synchrotron X-ray sources that produce X-ray beams up to a billion times more intense than laboratory sources are opening up whole new areas of research in many different fields of science. The Clay Minerals Society workshop entitled 'Synchrotron X-ray Methods in Clay Science' was held prior to the 11th International Clay Conference on Saturday, June 14, 1997, at Carleton University, Ottawa, Ontario, Canada. The purpose of the workshop was to introduce clay scientists to the wide range of synchrotron-based techniques now available. This volume of the CMS Workshop Lectures includes papers by all but one of the workshop speakers. The authors were asked to provide tutorial introductions to the various techniques now available at synchrotron facilities. The first chapter gives an introduction to the synchrotrons, the properties of synchrotrons X-rays, and the jargon of this complex technology. The next eight chapters describe most of the techniques likely to be used by clay scientists. Topics include: X-ray absorption spectroscopy, X-ray diffraction, X-ray microprobe, X-ray microscopy, infrared spectroscopy, and diamond anvil cell work, all with examples of applications to clays or closely related minerals. The final chapter explains how to obtain access to synchrotron facilities. The workshop also included a presentation on synchrotron-based Mössbauer spectroscopy, but unfortunately a chapter on this topic is not included.

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    The discovery of the crystalline nature of colloidal clay particles in the 1930’s was a major breakthrough made possible by the then-new technique of x-ray diffraction. X-ray powder diffraction remains an essential tool for clay mineralogy research today, while other x-ray based techniques such as x-ray fluorescence spectroscopy, radiography, and computed tomography are important to individual researchers based on availability of equipment and the needs of particular research projects.

    Commercially available x-ray instrumentation relies on specialized vacuum tubes as the x-ray source. The capability of sealed-tube x-ray sources has not increased significantly since Wilhelm Conrad Rontgen’s discovery of x-rays a century ago. The introduction of rotating anode x-ray tubes in the 1960’s brought about a 10-fold increase in x-ray intensity, but the basic constraints of a vacuum tube x-ray source, namely significant intensity over only a few narrow energy ranges and a highly divergent source, remain. Beginning in the 1950’s the high energy physics community began to build particle accelerators to study the fundamental properties of matter. One type of particle accelerator, the synchrotron, was designed to accelerate charged particles around a nearly circular trajectory so the particles could be made to strike a target at high energy.

    Synchrotrons produced large quantities of electromagnetic radiation, including x-rays, as a by-product of steering the particles around the ring. This radiated energy was originally considered a nuisance because it had to continually be replaced, but it soon became apparent that the synchrotron radiation had many useful properties for x-ray-based techniques. New generations

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    present address: Geological and Environmental Sciences Department, Stanford University, Palo Alto, CA 94305

    A detailed characterization of elements within clay minerals and soils usually requires the use of a number of different techniques. Spectroscopic methods are often (and should be) incorporated in the characterization procedure as they provide a wealth of information on the chemical and structural nature of an element within solids. A spectroscopy that has recently proven to be a powerful means for obtaining the speciation and local structure of elements present in clay minerals and soils is x-ray absorption fine structure (XAFS) spectroscopy-the subject of this chapter. XAFS has a number of advantageous qualities for studying clays and soils which include: element specificity, the local chemical and structural state of an element, and the ability to analyze materials in situ. It probes the local chemistry and structure of a single element throughout a sample. What is captured by this technique can be thought of as a ‘view’ of the x-ray absorber’s electronic structure and the atoms that coordinate it; Figure 1 illustrates the structural ‘view’ obtained with this method. The oxidation state, type of nearest neighbors, coordination number, bond distances, and orbital symmetries of the x-ray absorbing element can be accurately determined in an array of media (Eisenberger and Lengeler 1980). Because the information obtained with XAFS differs from that of other spectroscopies and microscopies, when used in conjunction with them XAFS offers a complementary means for detailing the properties of clay minerals or soils.

    Basic, general steps for performing XAFS and a brief background on its physical basis are

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    Present address: CSIRO Land and Water, Glen Osmond, SA 5064, Australia

    This chapter describes the new possibilities offered by application of polarized EXAFS (P-EXAFS) spectroscopy to structural studies of fine-grained layered minerals. X-rays delivered by synchrotron sources are more than 95% polarized in the central part of the plane of rotation of the synchrotron electrons, and the polarization rate is further increased because of the polarization that occurs during monochromation of the incident beam. Because of the highly polarized nature of synchrotron radiation, one can obtain angularly resolved structural information through analysis of the angular dependence of X-ray absorption spectra for anisotropic samples. Originally, this technique was applied to single phyllosilicate crystals (Manceau et al. 1988; Manceau et al. 1990), and we show here that it can be extended to self-supporting films of hne-grained layered minerals. The angular variation of P-EXAFS spectra sensitively depends on the orientation distribution of individual mineral platelets in the prepared film. In the case of smectites, highly oriented films can be prepared, which allows us to precisely probe their 3-dimensional local structure without loss of spatial resolution as compared to single crystals.

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    Since the discovery of X-ray diffraction over eight decades ago, crystallography has matured into a very precise, highly tested, widely applicable and definitive tool. The success of the conventional method is due to the fact that the same theory of scattering can be applied to materials as diverse as simple close packed solids, macromolecules with thousands of atoms per unit cell and mineral surfaces. The method, although so pervasive that it is now taken for granted, is as important as ever. Without models for the atomic arrangement within a crystalline or aperiodic solid, functionality, chemical reactivity and physical properties are difficult to interpret. Discussions outside the framework of a testable crystallographic model are, rightly, viewed less credibly.

    In the past two decades a major new source of X-rays has become available which has made possible unprecedented sensitivity, accuracy and precision in the determination of crystal structures and the study of the dynamics of chemical reactions (Bourgeois et al. 1996). Compared to conventional sealed tube sourcess, synchrotron radiations is 104 102 times brighter and rather than having the sharply peaked spectrum, generally has a broad spectral range caused by emission of photons

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    Analytical techniques with high sensitivity and high spatial resolution are crucial for understanding the chemical properties of complex materials such as clay minerals. Several techniques are capable of trace element microanalysis, notably electron microprobe analysis (EMPA), proton-induced x-ray emission (PIXE), and secondary ion mass spectrometry (SIMS). These techniques are complementary, but none of them is suitable for all analyses and each has unique capabilities. EMPA is currently capable of mm-sized spots but minimum detection limits are no better than 50 ppm. PIXE is well-suited for analyses of relatively light elements with 10 ppm sensitivity and μm-sized spots. The x-ray fluorescence (XRF) microprobe exceeds both of these techniques in sensitivity, especially for heavy elements, and currently has comparable spatial resolution (Smith and Rivers, 1995). All three of these techniques are fluorescence-based so sensitivities are smoothly varying functions of atomic number. Elemental sensitivities for SIMS are highly variable depending on ion yield, and quantification is difficult because of matrix effects. SIMS has higher sensitivities than the other techniques for some elements and lower sensitivity for others. Quantification is comparatively straightforward for XRF because the physics of photon interactions with matter is well understood. Trace element microdistributions with the synchrotron x-ray microprobe can be determined with <10 μm resolution and <1 ppm sensitivity. Oxidation state maps can be produced with <100 μm resolution and <100 ppm sensitivity. Oxidation state maps can be produced with μ100 urn resolution and μ100 ppm sensitivity. Microtomography can provide three-dimensional images of microstructure with micrometer resolution. The purpose

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    Infrared (IR) spectroscopy continues to be an invaluable tool to study the physical and chemical properties of minerals, including bonding characteristics, local structural symmetry, thermoelasticity and electronic properties (e.g., Kieffer 1979a, 1979b, 1979c, 1980; Rossman 1988a; Keppler 1996; McMillan et al. 1996). In particular, IR spectroscopy is ideal for studying hydrogen bonding and for quantifying trace amounts of hydroxyl and hydrogen-bearing (e.g., aqueous) inclusions in minerals (Nakamoto et al. 1955; Novak 1974; Farmer 1974; Paterson 1982; Aines and Rossman 1984; Rossman 1988b; Rossman 1996). Study of the hydrous component in key minerals provides important insight on global budget and evolution of volatiles in the Earth (Thompson 1992; Bell and Rossman 1992). In addition, microscopic inclusions containing H2O in minerals provide information on the geological environment and chemical conditions in which these inclusions were formed (Navon et al. 1988; Schrauder and Navon 1993). The presence of H2O or structurally bound hydrogen, which can be quantitatively measured by IR spectroscopy, can have a large effect on physical properties (e.g., elasticity, rheological and transport properties) as well as on the phase relations of host minerals (Griggs and Blacic 1965; Mackwell et al. 1985; Liu 1985; Kronenberg et al. 1986). IR reflectivity spectra in the lattice vibration region can be used to ascertain minor structural and symmetry variations and as a finger-printing technique for identifying mineral phases (Farmer 1974; McMillan and Hofmeister 1988; McMillan et al. 1996).

    Spectroscopic studies of minerals often pose special technical challenges (Farmer 1974; Hofmeister 1995). Hydrous inclusions in minerals

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    The use of soft x-ray microscopy to study problems in soil science is relatively new (Thieme et al., 1992). The potential, however, is large: soft x-rays are ideally suited to < 0.1 μm resolution imaging of micrometer-thick biological specimens, and offer unique capabilities for microchemical characterization. We briefly describe here some of the characteristics of soft x-ray microscopes and how they might be used for soil science studies. Further details are provided in recent review papers (Kirz et al, 1995), monographs (Michette, 1986; Spiller, 1994), and conference proceedings (Aristov and Erko, 1994; Thieme et al., 1998).

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    X-rays have been used for many decades to obtain information on the structure of clays. X-ray diffraction analysis is especially important in this respect, because it allows one to obtain structural information in the picometer and nanometer size range. The resolution achievable depends on the wavelength of X-rays, 0.154 nm for Cu-Kα, for example. In the following we will describe another method, X-ray microscopy, for obtaining structural information which results in direct images of the samples under investigation rather than the indirect information provided by X-ray diffraction, unique feature of X-ray microscopy is that it is capable of imaging particles with colloidal dimensions directly in aqueous media (Thieme et al., 1992; Jacobson and Neuhäusler, Chapter 7, this volume).

    X-ray microscopy of aqueous samples is possible because water shows very little X-ray absorption at wavelengths slightly above 2.35 nm, the K-absorption edge of oxygen. Substances like clays or organic matter absorb X-radiation of this wavelength range more strongly. Figure 1 shows the linear absorption coefficient of three substances as a function of wavelength, i.e. water, a typical montmorillonite, and phenol as a model for an organic compound. The phase shift of X-rays penetrating water and inorganic or organic substances shows similar trends. Therefore, X-ray microscopy yields images either in amplitude contrast or in phase contrast (Schmahl et al., 1995). Thus, there is a natural contrast in X-ray microscopic images and preparation techniques like staining or drying are not necessary (Wolter, 1952).

    The resolution of a microscope is directly related to the

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    The hydration states of montmorillonite and other smectites have been the subject of numerous studies. X-ray diffraction has been one of the most valuable means of studying these hydration states because the effect that hydration has on the c-axis lattice parameter makes them so easy to observe by this method. The three commonest configurations of cations and H2O molecules found in montmorillonites lead to three discrete d001-spacings of ∼19Å, ∼15Å, and ∼12.5Å. These hydration states are called 3-layer, 2-layer, and 1-layer, respectively, even though the distribution of H2O molecules is more complex than simple layers (Chang etal., 1995; McBride, 1994). For this reason we prefer to reference the hydration states by their d-spacings rather than by the number of layers of water.

    The retention and release of water by clays is of interest to geologists. For example, because it affects the origin and mobility of fluids in deeply buried sediments and possibly even in subducted sediments. In addition, the degree of hydration can have very profound effects on the rheologic properties of sediments and sedimentary rocks.

    The hydrothermal diamond anvil cell (HDAC) and synchrotron radiation (Wu et al., 1997) has given us the means to make more detailed observations at a significantly greater range of temperatures and pressures than in earlier studies of clays (e.g., Stone and Rowland, 1955; Koster van Groos and Guggenheim, 1984, 1986, 1987).

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    Synchrotron radiation laboratories are large complicated facilities and obtaining initial access may appear to be a formidable task. Although there is considerable effort associated with gaining initial access, it is useful to keep in mind that most synchrotron X-ray laboratories are funded as user facilities, charged with providing access to the scientific community at-large. Thus, the metric of success and usefulness of a synchrotron radiation laboratory is the scope, amount, and quality of science conducted at the facility. The commissioning of third generation synchrotron X-ray sources over the past several years coupled with decreased federal spending has resulted in a much greater emphasis on user satisfaction, as these facilities try to expand both capabilities and user community to solidify their funding and maintain full operation potential. It has been our experience that the staff of synchrotron research facilities are generally very helpful to new users, and support services available to users at these facilities have increased significantly over the past five or so years. Synchrotron facilities that are funded by the U.S. Department of Energy are user facilities that have beam time available to individual researchers. The time is allocated based on a peer review, general user proposal system.

    There are generally two groups of users at synchrotron facilities. One consists of scientists who build the instrumentation on particular beamlines and who maintain and operate the beamlines on a day-to-day basis. At the National Synchrotron Light Source (NSLS) these groups are called the participating research teams (PRT), while at the

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