X-Ray Diffraction (XRD)

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Exploration Technique: X-Ray Diffraction (XRD)

Exploration Technique Information
Exploration Group: Lab Analysis Techniques
Exploration Sub Group: Rock Lab Analysis
Parent Exploration Technique: Rock Lab Analysis
Information Provided by Technique
Lithology: Rapid and unambiguous identification of unknown minerals.[1]
Stratigraphic/Structural:
Hydrological:
Thermal:
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X-Ray Diffraction (XRD):
X-Ray Diffraction (XRD) is a laboratory-based technique commonly used for identification of crystalline materials and analysis of unit cell dimensions. One of two primary types of XRD analysis (X-ray powder diffraction and single-crystal XRD) is commonly applied to samples to obtain specific information about the crystalline material under investigation. X-ray powder diffraction is widely used in geology, environmental science, material science, and engineering to rapidly identify unknown crystalline substances (typically in less than 20 minutes). A pure, finely ground, and homogenized sample is required for determination of the bulk composition. 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.
Other definitions:Wikipedia Reegle


 
Introduction
X-Ray Diffraction (XRD) is a laboratory-based technique commonly used for identification of crystalline materials and analysis of unit cell dimensions. The technique was pioneered by Max von Laue in 1912, who discovered that crystalline substances act as a diffraction grating for X-ray wavelengths similar to the atomic-scale plane spacing in a crystal lattice.[1] One of two primary types of XRD analysis (X-ray powder diffraction and single-crystal XRD) is commonly applied to samples to obtain specific information about the crystalline material under investigation.

X-ray powder diffraction is widely used in geology, environmental science, material science, and engineering to rapidly identify unknown crystalline substances (typically in less than 20 minutes).[1] A pure, finely ground, and homogenized sample is required for determination of the bulk composition. 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.

Single-crystal X-ray diffraction is used most commonly for determination of unit cell dimensions and the position of atoms within a crystal lattice.[2] This can be applied to the identification of new minerals and to answering specific questions concerning the chemical makeup and atomic structure of crystalline substances

Photo of an X-Ray Diffraction machine. Photo from the Australian Microscopy & Microanalysis Research Facility Website, last updated February 22, 2013.

 
Use in Geothermal Exploration
The most obvious application of XRD analysis in geothermal exploration is the identification of fine-grained hydrothermal alteration minerals in powdered samples. 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. XRD analysis was utilized at Long Valley Caldera, CA for detailed characterization of alteration mineral assemblages identified in core samples in order to estimate the thermal maximum of the hydrothermal system.[3]

Single-crystal X-ray diffraction is most commonly used for identification of new minerals and for answering specific questions concerning the chemical makeup and atomic structure of crystalline substances.[2] As such, the application of this technique in modern geothermal exploration is fairly limited.

 
Related Techniques
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 in the field is relatively simple, and only requires a minimal amount (tenths of a gram) of the material of interest to be collected. The sample should be as pure as possible, and extra material should be collected if repeat analysis is desired.


 
Physical Properties
X-rays are generated in an X-ray tube, in which a target material (Cu, Fe, Mo, or Cr) 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, processed, and counted as the sample is scanned through a range of 2Θ angles. This method allows for all possible diffraction directions (for X-rays of a fixed wavelength) produced through beam interaction with a unique crystalline substance to be measured during analysis (i.e. the diffraction pattern). 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).

In X-ray diffraction, a device called a goniometer is used to rotate the sample in the path of the incident beam at an angle Θ, while an arm-mounted X-ray detector is maintained at an angle of 2Θ (typically from about 5-70° for a powdered sample). For X-ray powder diffraction, a single goniometer is used orient the sample and detector. In single-crystal X-ray diffraction, between 3- and 4-circle goniometers are used, each referring to one of four angles (2Θ, χ, φ, and Ω) that define the relationship between the crystal lattice, the incident ray, and the X-ray detector.[2]


 
Best Practices
Identification of an unknown requires a small amount of sample material, a device for grinding the sample, and a sample holder. A few tenths of a gram of pure sample is ground into a fine powder (approximately 10 micron grain size or 200-mesh), typically in a fluid to minimize the effects of induced strain during grinding and to help randomize the orientation of the grains. The powdered samples should then be placed into a sample holder, uniformly spread onto a glass slide such that the surface is flat, or sprinkled onto double-sided sticky tape.

Analysis of clays requires that the sample have a single orientation. This type of analysis requires specialized sample preparation techniques, which are outlined in detail on the USGS website.

 
Potential Pitfalls
Grinding can produce strain (excessive surface energy) on crystalline materials during sample preparation, which can offset peak positions and ultimately lead to a faulty analysis. A fluid lubricant is typically used during grinding to minimize these effects. In most cases, the sample must also be placed into/onto the sample holder such that a random distribution of lattice orientations is achieved, otherwise the analysis will not detect all peak positions that are characteristic of the material.

Additional limitations of XRD analysis include:

  • Unknown mineral identification is best accomplished using a homogeneous and single phase sample.[1]
  • Interpretation of the data requires access to a standard reference file of inorganic compounds.[1]
  • Requires a relatively small amount (tenths of a gram) of pure material that has been ground into a powder.[1]
  • The detection limit is about 2% of the sample for mixed materials.[1]
  • 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]
  • For single-crystal analysis, a single, stable, optically clear crystal sample is required, generally between 50 to 250 microns in size. Twinned samples can also be analyzed, but with greater difficulty.[2]
  • Data collection times for single-crystal analysis are significantly longer, usually between 24 and 72 hours.[2]


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

Campana, C.F., Bruker Analytical Application Note

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.

Putnis, A. (1992). Introduction to Mineral Sciences. Cambridge, UK: Cambridge University Press. Chapter 3 (pp. 41-80).





 
References
  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 X-ray Powder Diffraction (XRD)
  2. 2.0 2.1 2.2 2.3 2.4 Single-crystal X-ray Diffraction
  3. Michael L. Sorey,Gene A. Suemnicht,Neil C. Sturchio,Gregg A. Nordquist. 12/1991. New Evidence On The Hydrothermal System In Long Valley Caldera, California, From Wells, Fluid Sampling, Electrical Geophysics, And Age Determinations Of Hot-Spring Deposits. Journal of Volcanology and Geothermal Research. 48(3-4):229-263.
  4. Cite error: Invalid <ref> tag; no text was provided for refs named X-Ray_Fluorescence_.28XRF.29
  5. X-Ray Reflection in Accordance with Bragg’s Law




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