Benchtop X-ray Diffraction Spectroscopy Contact: World Agroforestry Centre (ICRAF), P.O. Box Nairobi, Kenya. Tel: X-Ray Diffraction (XRD) is a high-tech, non-destructive technique for qualitative and quantitative analysis of crystalline compounds. About 95% of all solid materials are crystalline. When X-rays interact with a crystalline substance or powder, a diffraction pattern is produced. In a mixture of substances each crystalline substance produces its pattern independently of the others and can be quantified. Information obtained includes types and nature of crystalline phases present, structural make-up of phases, degree of crystallinity, amount of amorphous content, microstrain & size and orientation of crystallites. Soil mineralogy is a key determinant of basic soil functional properties. New benchtop instrumentation is enabling routine application of XRD in soil diagnostics. Soil mineralogy largely dictates function: nutrient quantity (stock) and intensity (strength of retention by soil) pH and buffering, variable charge anion and cation exchange capacity carbon saturation; protection aggregate stability, dispersion/flocculation resistance to erosion These properties in turn determine soil agricultural, environmental and engineering qualities. Yet soil mineralogy is currently not used to predict soil functional properties. High throughput, benchtop quantitative XRD could change this. XRD information on mineralogy can be combined with information from infrared spectroscopy, which characterizes soil organic properties, to provide powerful diagnostic capabilities. Introduction Quantitative analysis of actual minerals in topsoils and subsoils. Classification of soils in terms of weatherable minerals: soil fertility potential. Use in pedotransfer functions to directly predict soil functional properties. Applications XRD has become an indispensable method for materials investigation, characterization and quality control. Soil Mineralogy and Function When a sample is irradiated with a beam of monochromatic X-rays, the sample atomic lattice acts as a 3-dimensional diffraction grating causing the X-ray beam to be diffracted to specific angles. The diffraction pattern, angle and intensity of diffracted beam, provide information about a sample. The angles are used to calculate the interplanar atomic spacings (d-spacings). The position (d) and intensity (I) information is used to identify the type of material, by comparing patterns for data entries in standard databases. Identification of any crystalline compounds, even in a complex sample, can be made by this method. The position (d) of diffracted peaks provides information about atoms arrangement within the crystalline compound. The intensity (I) information used to assess the type and nature of atoms. Width of the diffracted peaks is used to determine crystallite size and micro-strain in the sample. The ‘d’ and ‘I’ from a phase also used to quantitatively estimate the amount of that phase in a multi-component mixture. Non-destructive analysis No sample preparation No chemicals Qualitative and quantitative mineral profiles High throughput Ability to distinguish between elements and their oxides. Possibility to identify chemical compounds, polymorphic forms, and mixed crystals. XRD spectrometer with slide-up front cover for sample loading and integrated computer Good instrument resolution resolves overlapping diffraction peaks in complex patterns. Working principles Analysis and Quantification Key Advantages of XRD