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X-Ray Powder Diffraction This Handout Provides Background on Theory and Methodology the Use and Theory of X-Ray Powder Diffraction

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X-Ray Powder Diffraction This Handout Provides Background on Theory and Methodology the Use and Theory of X-Ray Powder Diffraction X-Ray Powder Diffraction This handout provides background on Theory and Methodology the use and theory of X-ray powder diffraction. Examples of applications of The three-dimensional structure of this method to geologic studies are nonamorphous materials, such as provided. minerals, is defined by regular, repeating planes of atoms that form a crystal Introduction lattice. When a focused X-ray beam interacts with these planes of atoms, part Rocks, sediments, and precipitates are of the beam is transmitted, part is examples of geologic materials that are absorbed by the sample, part is refracted composed of minerals. Numerous and scattered, and part is diffracted. analytical techniques are used to Diffraction of an X-ray beam by a characterize these materials. One of crystalline solid is analogous to Figure 1. Simplified sketch of one these methods, X-ray powder diffraction diffraction of light by droplets of water, possible configuration of the X-ray source (X-ray tube), the X-ray detector, (XRD), is an instrumental technique that producing the familiar rainbow. X-rays is used to identify minerals, as well as and the sample during an X-ray scan. In are diffracted by each mineral this configuration, the X-ray tube and the other crystalline materials. In many differently, depending on what atoms detector both move through the angle geologic investigations, XRD make up the crystal lattice and how these theta (q), and the sample remains complements other mineralogical atoms are arranged. stationary. methods, including optical light In X-ray powder diffractometry, X-rays microscopy, electron microprobe are generated within a sealed tube that is microscopy, and scanning electron under vacuum. A current is applied that n l = 2 d sin q, where the integer n is the microscopy. XRD provides the heats a filament within the tube, the order of the diffracted beam, 1 is the researcher with a fast and reliable tool for higher the current the greater the number wavelength of the incident X-ray beam, d routine mineral identification. XRD is of electrons emitted from the filament. particularly useful for identifying fine- is the distance between adjacent planes This generation of electrons is analogous of atoms (the d-spacings), and q is the grained minerals and mixtures or to the production of electrons in a angle of incidence of the X-ray beam. intergrowths of minerals that may not television picture tube. A high voltage, Since we know l and we can measure q, lend themselves to analysis by other typically 15-60 kilovolts, is applied we can calculate the d-spacings. The techniques. XRD can provide additional within the tube. This high voltage geometry of an XRD unit is designed to information beyond basic identification. accelerates the electrons, which then hit a accommodate this measurement (fig. 1). If the sample is a mixture, XRD data can target, commonly made of copper. When The characteristic set of d-spacings be analyzed to determine the proportion these electrons hit the target, X-rays are generated in a typical X-ray scan of the different minerals present. Other produced. The wavelength of these X- provides a unique "fingerprint" of the information obtained can include the rays is characteristic of that target. These mineral or minerals present in the degree of crystallinity of the mineral(s) X-rays are collimated and directed onto sample. When properly interpreted, by present, possible deviations of the the sample, which has been ground to a comparison with standard reference minerals from their ideal compositions fine powder (typically to produce particle patterns and measurements, this (presence of element substitutions and sizes of less than 10 microns). A "fingerprint" allows for identification of solid solutions), the structural state of the detector detects the X-ray signal; the the material. minerals (which can be used to deduce signal is then processed either by a temperatures and (or) pressures of microprocessor or electronically, formation), and the degree of hydration converting the signal to a count rate. Applications for minerals that contain water in their Changing the angle between the X-ray XRD has a wide range of applications structure. Some mineralogical samples source, the sample, and the detector at a in geology, material science, analyzed by XRD are too fine grained to controlled rate between preset limits is an environmental science, chemistry, be identified by optical light microscopy. X-ray scan (figs. 1 and 2). forensic science, and the pharmaceutical XRD does not, however, provide the When an X-ray beam hits a sample and industry, among others. At the U.S. quantitative compositional data obtained is diffracted, we can measure the Geological Survey, researchers use XRD by the electron microprobe or me textural distances between the planes of the to characterize geologic materials from a and qualitative compositional data atoms mat constitute the sample by wide variety of settings; a few examples obtained by the scanning electron applying Bragg's Law. Bragg's Law is follow. microscope. Mineral-Environmental Studies Predictive Stratigraphic Analysis of airborne imaging In studies of areas affected by acid Analysis spectrometer data can directly map mine drainage, the identification of Mineralogical characteristics of mineral occurrences by detecting diagnostic spectral absorption bands, the secondary minerals and fine-grained paleosols(ancient buried soil horizons) shape and position of which are precipitates is a critical element. Acid is and underclays (the fine-grained detrital determined by individual mineral generated when iron sulfide minerals, material lying immediately beneath a structures. A detailed knowledge of such as pyrite, weather. Elements coal bed) have been instrumental in sample mineralogy, provided at least in derived from the alteration of the sulfide correlating coal zones from the minerals can form secondary minerals or Appalachian basin into the Western part by XRD, is required to understand go into solution. Elements that go into Interior basin. They have been the key to the observed spectral absorption features. solution may form mineral precipitates as quantifying the paleolatitudinal climate conditions (temperature, acidity, solution gradient in North America during the late Genesis of Coal Beds composition) change. Accurate Middle Pennsylvanian. XRD is one of the primary tools used mineralogical characterization of the to evaluate the lateral and vertical precipitates and secondary minerals, Remote-Sensing Studies variations in mineral matter and major, together with hydrogeochemical data, Mineralogical analysis by XRD is used minor, and trace elements in coal beds. helps us to better understand the in conjunction with remotely sensed data These data are used to help determine the solubility, transport, and storage of in several research investigations.XRD impact of geologic and geochemical metals. is used to identify the minerals processes on coal bed formation in order composing clay-rich, hydrothermally to understand and predict both inorganic Ore Genesis Studies altered rocks that occur on several and organic variability within and among Minerals form under specific ranges of Cascade volcanoes. Such rocks are mineable coal beds. temperature and pressure. Mineralogical believed to play an important role in the identification of ore minerals and generation of large landslides and Mineral-Resource associated minerals, including fine- mudflows. XRD is used to analyze Assessments grained hydrothermal alteration minerals, saline minerals, including borates. Mineralogical characterization provides evidence used to deduce the Many saline hydrate minerals produce provides part of the data required to conditions under which ore deposits diagnostic spectral bands, and spectral determine the particular kind of mineral formed and the conditions under which, data provide a basis for mineral deposits encountered in mineral-resource in many cases, they were subsequently exploration using remote-sensing data. assessment studies. XRD allows us to altered. identify fine-grained mixtures of minerals found in associated gangue and alteration assemblages, which cannot be resolved by other methods. -Prepared by Marta J.K. Flohr For more information, please contact: Frank T. Dulong U.S. Geological Survey, MS 956 Reston, VA 20192 Telephone: (703) 648-6416 E-mail: [email protected] John C. Jackson U.S. Geological Survey, MS 954 Reston, VA 20192 Figure 2.Example of an X-ray powder diffractogram produced during an X-ray scan. Telephone: (703) 648-6321 The peaks represent positions where the X-ray beam has been diffracted by the E-mail: [email protected] crystal lattice. The set of d-spacings (the distance between adjacent planes of atoms), which represent the unique “fingerprint” of the mineral, can easily be calculated from the 2-theta (2q) values shown. The use of degrees 2-theta in depicting X-ray powder diffraction scans is a matter of convention and can easily be related back to the geometry of the instrument, shown in figure 1. The angle and the d-spacings are related by Bragg’s Law, as described in the text. U.S. Department of the Interior U.S. Geological Survey May 1997.
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