Rocks and many other materials show complex polycrystalline arrangements with chemical and structural heterogeneity over a variety of lengthscales. The inhomogeneities may have been caused during genesis processes or later in response to environmental physicochemical changes. Microprobe techniques provide satisfactory knowledge of such materials but their use is limited by their restricted field of view. For example, X-ray absorption near-edge spectroscopy (XANES) allows elemental oxidation states to be probed with high spatial resolution. However, scanning a large area is often not feasible with realistic acquisition times. To overcome this limitation, we show the potential of coupling XANES and full-field absorption radiography with a large hard X-ray beam. We developed a new full-field experimental setup on the ID21 beamline. It has been optimised to acquire stacks of mega-pixel XANES images with submicrometre resolution, using a CCD array detector. Acquisition times are short considering the millions of spectra obtained, and the precision of a single spectrum is sufficient for advanced XANES studies. Another originality of this approach is the association of XANES with polarisation contrast imaging (PCI). As well as providing mineralogical orientation maps, PCI was used here for the first time to correct the elemental oxidation state estimates from polarisation effects induced by polarised synchrotron radiation.

The full-field XANES setup consists of a fixed exit double crystal monochromator, a special sample holder, and a 2D-detector to detect transmitted X-rays. The detector comprises a scintillator coupled to a CCD camera through magnifying light optics. First trials showed that measurements of fine spectral features at the limit of the monochromator resolution require a quasi-perfect flat-field correction. For this purpose, the sample stage has been equipped with a linear stage able to remove and reposition the sample accurately in the beam in less than a second, thus permitting flat field acquisition with high frequency. The experimental setup is also switchable into PCI mode in a few minutes. A series of radiographs are acquired by rotating the sample in the azimuthal plane over π. Radiographs are then re-aligned by rotation with computing scripts to create a PCI image stack. PCI resulting from the interaction between X-ray linear dichroism and preferential absorption in relation to the minerals’ refractive indices (and thus their orientation) have been successfully recorded on Fe-bearing phyllosilicates.

The potential of the full-field XANES and PCI association is demonstrated on a metasediment that underwent two distinct metamorphic events. These events led to the formation of a rock with micrometric scale heterogeneities, deformation structures associated with mineralogical phases, and with expected Fe redox variations. A crystallographic orientations map derived from PCI shows two populations of phyllosilicates (Figure 145a). Fe3+/Fetotal estimates change drastically after PE corrections (Figure 145b). The amplitude of variation stays around 0.4 but the shape of the histogram becomes comparable to a Gaussian. The Fe3+ map highlights subtle redox variations recorded during the rock exhumation and can be used to retrieve the pressure-temperature conditions undergone by the rock and so, to build a geodynamic model.

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Fig. 145: a) C-axis orientation map of chlorite derived from PCI. b) Quantitative iron redox map of chlorite corrected for polarisation effects. c) Selection of 1 and 3x3 chlorite XANES spectra, amongst the 1M spectra constituting the image.

The second example concerns an analogous material to nuclear waste storage systems. This material is a bentonite (smectite and iron oxides) mixed with micrometric large metallic iron spherules. It was submitted to a forced fluid percolation for 4 years at 25°C. Bentonite is made of finely intercalated mineralogical phases to the micrometre or pixel scale. Mapping of these phases have been computed by fitting XANES spectra composing the hyperspectral data by linear combination of reference spectra selected from a database (Figure 146a). Results show that starting material is preserved from chemical reactions in smectite and goethite rich nodules, and highlight reactive areas with apparition of reaction products like hematite and cronstedtite, while proportions of the initial bentonite compounds decrease. Sensitivity and resolution of the technique make it possible to detect small amounts of phase with low crystallinity. The spatial information reveals local chemical evolutions in clays responsible for changes in the bentonite mechanical properties.

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Fig. 146: a) Spectra retrieved from a database of bentonite end-members mapping from a Fe K-edge XANES stack (pixel width: 645 nm) with a least square fitting method. b) Example of line fitting corresponding to a single spectrum selected in the clayey matrix.

In conclusion, this new method is unique in providing mega-pixel elemental speciation maps, with a large field of view (≈ 1 mm2), while conserving a submicrometre resolution. The combined use of PCI and XANES allows a reliable redox estimate, eliminating artifacts resulting from polarisation effects that can occur when analysing monocrystals. These 2D-hyperspectral data are also very effective for the local identification of phases in finely divided materials.

 

Principal publication and authors

V. De Andrade (a,b), J. Susini (b), M. Salomé (b), O. Beraldin (b), C. Rigault (c), T. Heymes (d), E. Lewin (d) and O. Vidal (d), Anal. Chem. 83, 4220-4227 (2011).

(a) NSLS II, BNL, Upton (USA)

(b) ESRF

(c) HYDRASA, CNRS, Poitiers University (France)

(d) ISTerre, Maison des Géosciences, Grenoble (France)