Portable X-Ray Diffraction (XRD)

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Exploration Technique: Portable X-Ray Diffraction (XRD)

Exploration Technique Information
Exploration Group: Field Techniques
Exploration Sub Group: Data Collection and Mapping
Parent Exploration Technique: Data Collection and Mapping
Information Provided by Technique
Lithology: Rapid and unambiguous identification of unknown minerals.[1]
Portable X-Ray Diffraction (XRD):
Portable X-Ray Diffraction (XRD) is a field-based technique that can be used for identification of crystalline materials and analysis of unit cell dimensions. Portable XRD analysis is similar to X-ray powder diffraction, which has traditionally been used in geology, environmental science, material science, and engineering to rapidly identify unknown crystalline substances. Portable XRD analysis allows for simpler sample preparation, faster analytical times than traditional methods (less than 2 minutes), and can be performed at the sampling site in the field. A pure, finely ground sample is required for determination of the bulk composition and identification of fine-grained minerals.
Other definitions:Wikipedia Reegle

Portable X-Ray Diffraction (XRD) is a field-based technique made possible by recent miniaturization of components of the traditional laboratory-based XRD instrument. Devices that are currently available weigh less than 15 kg, and are similar in size to a small briefcase. The technique can be used to analyze the bulk composition of a sample and to identify unknown minerals in the field.[1] The first commercially available portable XRD device was the Terra Mobile XRD system, originally manufactured by InXitu, Inc. in early 2008. The device is capable of X-ray powder diffraction analysis, which can be applied to samples to obtain specific information about the crystalline material under investigation. Current portable XRD devices utilize high sensitivity CCD X-ray detectors to perform both XRD and XRF analyses with a single device.[2]

X-ray powder diffraction has been widely used in geology, environmental science, material science, and engineering for rapid identification of unknown crystalline substances since the 1940s, however these analyses were historically restricted to a laboratory setting.[1] Additional uses include detailed characterization of crystalline samples, determination of unit cell dimensions, and quantitative determination of modal amounts of minerals in a sample. X-ray powder diffraction can also be applied to the identification of fine-grained minerals. More specifically, the technique can be used to distinguish between different clays and mixed layer clays that are optically similar, but form from distinctly different weathering and hydrothermal alteration processes.

Recent innovations have led to the development of a vibrating sample holder for use in XRD analysis.[3] Such sample holders cause convection of a powdered sample during bombardment, allowing for analysis of samples with grain sizes between 5-150 microns. This capability drastically simplifies sample preparation and greatly reduces the number of moving parts required for the device to function, allowing for miniaturization and portability.

Photo showing the Terra Mobile XRD system (and operator), the first commercially available portable XRD device originally manufactured by InXitu, Inc. Photo from the Olympus Corporation merchant website.

Use in Geothermal Exploration
Reports of portable XRD analysis in geothermal exploration are scant. Potential uses include differentiation of optically similar clays associated with hydrothermal alteration in the field.
Related Techniques
Portable XRD analysis depends on the fundamental principles of electron beam and x-ray interactions with solid materials, similar to other analytical techniques.[4] Other techniques that operate on these principles include X-Ray Spectroscopy (through Energy-Dispersive X-ray Spectroscopy (EDX)) and Wavelength Dispersive Spectroscopy (WDS) typically performed using a SEM or EPMA, and X-Ray Fluorescence (XRF) analyses.

Field Procedures
Sample collection and preparation in the field are relatively simple, and only requires a minimal amount (about 15 mg) of the material of interest to be prepared.[3] The sample is ground into a powder using a mortar and pestle, and is then passed through a series of small sieves to concentrate a powder within the required 5-150 micron grain size range (100 mesh screen). This powdered sample is placed in a sample vial, which is inserted into the vibrating sample holder for analysis.

Physical Properties
X-rays are generated in an X-ray tube, in which a target material (Cu or Co) is excited using an electron beam, causing inner shell electrons to be ejected and replaced by electrons from higher energy outer orbitals. This interaction produces X-rays that are characteristic of the target material, which are then filtered and concentrated into a monochromatic incident beam of X-rays that is focused on the sample.[1] Interactions between the incident X-ray beam and the sample produce intense reflected X-rays by constructive interference when conditions satisfy Bragg’s Law (n λ = 2d sinΘ). This law describes the general relationship between the wavelength of the incident X-rays, the incident angle of the beam, and the spacing between the crystal lattice planes of atoms.[5] Constructive interference occurs when the differences in the travel path of the incident X-rays is equal to an integer multiple of the wavelength. When this occurs, a diffracted X-ray beam leaves the crystal at the same angle as the incident angle (Θ). Diffracted X-rays are detected and counted as the sample is scanned in the vibrating sample holder. This method allows for imaging of the diffraction pattern in the range of 5-55° 2Θ (for X-rays of a fixed wavelength) produced through beam interaction with a unique crystalline substance. Identification of the specific mineral phase is achieved by converting the diffraction peaks to d-spacings and comparing the d-spacings to those known from analysis of standard reference materials.[1]

Diagram illustrating Bragg's Law angle of deviation 2Θ. Interference can either be constructive (left) or destructive (right). Figure from Wikipedia, contributed by Christophe Dang Ngoc Chan (2011).

Data Access and Acquisition
Portable XRD analysis is rapid compared to its lab-based counterparts, usually taking no more than a minute or two for a single analysis. Samples can also be quickly scanned for their major and trace element constituents (Ca through U) using the device’s XRF capability.
Best Practices
Identification of an unknown requires a small amount of sample material, a device for grinding the sample, and a sample holder. Approximately 15 mg of pure sample is ground into a powder (<150 micron grain size or 100-mesh) in the field prior to analysis. Additional sample preparation details are included in the Field Procedures section of this page (above).
Potential Pitfalls
Limitations of Portable XRD analysis include:

  • Interpretation of the data requires access to the AMSCD mineral database (included in the XPowder software that ships with the Terra device) or to a standard reference file of inorganic compounds.[1]
  • Requires a relatively small amount of pure material that has been ground into a powder.[2]
  • Indexing of patterns for non-isometric crystal systems can be complex for unit cell determinations.[1]
  • Peak overlay may occur for some samples and becomes worse for high angle “reflections.”[1]

Additional References
Bish, DL and Post, JE, editors. 1989. Modern Powder Diffraction. Reviews in Mienralogy, v. 20. Mineralogical Society of America.

Cullity, B. D. 1978. Elements of X-ray diffraction. 2nd ed. Addison-Wesley, Reading, Mass.

Eby, G.N., 2004, Principles of Environmental Geochemistry. Brooks/Cole-Thomson Learning, p. 212-214.

Klug, H. P., and L. E. Alexander. 1974. X-ray diffraction procedures for polycrystalline and amorphous materials. 2nd ed. Wiley, New York.

Moore, D. M. and R. C. Reynolds, Jr. 1997. X-Ray diffraction and the identification and analysis of clay minerals. 2nd Ed. Oxford University Press, New York.

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