X-Ray Diffraction in Soil Mineralogy.

Since 1930s, X-ray diffraction (XRD) was primarily used for mineral phase identification and quantification in soil science. Advances in electronics, materials, and mechanical designs have greatly simplified the operation of the instrument and improved data quality. With the development of linear detectors, position sensitive detectors, and energy dispersive detectors, an XRD pattern can be recorded within seconds. Simultaneous X-ray fluorescence can be analyzed. Robotic sample changers have been developed to load a large number of samples. Mineral distribution can be mapped using micro-diffraction. More reaction accessories have been developed to conduct in situ monitoring under controlled temperature, pressure, humidity, and gas composition. The XRD industry is also developing smaller XRD instruments. One of the mini 10-kg diffractometers will be installed in the Mars Science Laboratory rover to explore minerals on Mars. More and more diffraction analyses are conducted with synchrotron sources, taking the advantages of the high flux of the beam and additional analyses such as XANES and EXAFS. Data processing software, structure models, and theories have been developed to simulate XRD patterns based on physical parameters. Structure determination, crystallite size estimation, and mineral quantification can be performed using Rietveld refinement. Software packages have been developed to simulate the patterns of clay minerals and have been successfully used to study the stacking sequence of interstratification, rotational disorder, vacant sites distribution, cis/tran interstratification, water distribution in layer silicate minerals, the distribution of cations and vacant sites in birnessite. Nearly eighty years after its use in soil mineralogy, the X-ray diffraction still has its momentum in exploring areas that are farther-beyond the earth, and smaller-down to Angstrom scale to study the detailed structural defects. It requires soil mineralogists to better understand the principles, advantages, and limitations of this technology, and more importantly to have the creativity working on difficult and stimulating questions.