Analytical Technique [DEPARTMENT OF GEOLOGY, GZRSC]
X-RAY DIFFRACTION
X-ray diffraction (XRD) is the primary, non-destructive tool for identifying and quantifying the
mineralogy of crystalline compounds in rocks, soils and particulates. Every mineral or compound
has a characteristic X-ray diffraction pattern whose 'fingerprint' can be matched against a
database of over 250 000 recorded phases. Modern computer-controlled diffraction systems can
interpret the diffraction traces produced by individual constituents and highly complex mixtures.
XRD is an essential technique for identifying and characterising the nature of clay minerals,
providing information which cannot be determined by any other method.
Working Principles of XRD
• when monochromatic X-rays are projected onto a crystalline material at an angle (θ),
diffraction occurs when the distance traveled by the rays reflected from successive planes
differs by an integer(n) of wavelengths (λ)
• By varying the angle θ, the Bragg's Law conditions [nθ = 2d sinθ] are satisfied by
different d-spacings
• Plotting the angular positions and intensities of the resultant diffracted peaks produces a
characteristic pattern
• where different phases are present, the diffraction trace represents the sum of the
individual patterns
Analytical Technique [DEPARTMENT OF GEOLOGY, GZRSC]
Powder X-Ray Diffraction :
In powder X-ray diffraction, the diffraction pattern is obtained from a powder of the
material, rather than an individual crystal. Powder diffraction is often easier and more convenient
than single crystal diffraction as it does not require individual crystals.
X-ray diffraction is based on constructive interference of monochromatic X-rays and a
crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce
monochromatic radiation, collimated to concentrate, and directed toward the sample. The
interaction of the incident rays with the sample produces constructive interference (and a
diffracted ray) when conditions satisfy Bragg’s law:
nλ = 2d sin θ
where n is an integer, λ is the wavelength of the X-rays, d is the interplanar spacing generating
the diffraction, and θ is the diffraction angle.
The diffracted X-rays are then detected, processed and counted. By scanning the sample
through a range of 2θ angles, all possible diffraction directions of lattice should be attained due
to the random orientation of the powdered material. Conversion of the diffractions peaks to d-
spacings allows identification of the compound because each compound has a set of unique d-
spacings. Typically these is achieved by comparision of d-spacings with standard reference
pattern.
X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder and
an X-rays detectors. X-rays are generated in a cathode ray tube by heating a filament to produce
electrons, accelerating the electrons toward a target by applying a voltage, and bombarding the
target material with electrons. When electrons have sufficient energy to dislodge inner shell
electrons of the target material, characteristic X-ray spectra are produced. The spectra consists of
several components, the most common being Kα and Kβ
As the sample and detector are rotated, the intensity of the reflected X-rays imparting the
sample satisfies Bragg’s law, constructive interference occur and a peak in intensity appears. A
detector records and processes this X-ray signal and converts the signal to a count rate , which is
then output to a device such as a printer or a computer monitor.
The geometry of an X-ray diffractometer is such that the sample rotates in the path of the
collimated X-ray beam at an angle θ while the X-ray detector is mounted on an arm to collect the
diffracted X-rays and rotates at an angle of 2u. The instrument used to maintain the angle and
rotate the sample is termed a goniometer. For typical powder patterns, data are collected at 2 θ
from 50 to 700, angles that are preset in the X-ray scan.
Analytical Technique [DEPARTMENT OF GEOLOGY, GZRSC]
Common Applications of XRD in Geology:
- It can identify clay rich minerals which can prevent big landslides and mudflow
- Determining extractability of metals from ore-bearing deposits
- Evaluating purity of pharmaceutical-grade limestone, used in antacid products
- Analyzing core samples taken for reservoir characterization
- Speciation and concentration of clay minerals within drilling mud for indirect monitoring of
physical properties
- Identifying garnet type in kimberlites as an indicator for diamond concentration and quality
- Quantifying dolomitization for iron and zinc ore potential
- Determining metamorphic grade of rock.
