X-ray diffraction (XRD)
Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi
Minerals and most technical products may be swiftly and reliably identified quantitatively and qualitatively with the X-ray diffraction method (Fig. 1). The method is economical and even small amounts of sample, less than 1 mg in weight, may be studied. However, the method is best suited for crystalline samples; amorphous materials, such as glass, cannot be identified with this method. X-ray diffraction studies are typically complemented with scanning electron microscope studies, and occasionally also with other methods like thermal analysis or infrared spectrophotometry.
Figure 1. Bruker D8 Discover A25 X-ray diffraction instrument in GTK’s mineralogical laboratory. Photo: GTK
Description of the method
XRD measurements are made by causing a beam of X-radiation to fall onto a suitably prepared (powder or polished bulk surface) specimen of sample material and measuring the angles at which a specific characteristic X-ray wavelength “λ” is diffracted. The diffraction angle “θ”, can be related to the interplanar spacing “d”, by the Bragg’s law:
nλ = 2d sinθ
By measuring the d-spacings and obtaining integrated intensities a diffraction pattern can be produced and the mineralogical properties determined by comparing the diffraction data or peaks to a standard database. An example of a diffraction pattern of goethite and the peaks in data is presented in the figure 2. (Bish & Post 1989)
Figure 2. XRD powder pattern of goethite. Image: Wikimedia Commons.
The XRD-method is widely used for the identification and characterisation of crystalline materials e.g. minerals and inorganic components. The XRD method is a useful tool in analyzing clay minerals. Clay minerals have a number of unique properties, e.g., some clay minerals may show cation exchange capacity, interaction with water or interaction with organic compounds. As a result of these properties the minerals may swell to several times their original size and this gives them an important role within the environmental field. Additionally, clays are versatile in industrial products such as paper and paints. The growing use of these minerals has created an increasing demand for their study. (Räisänen 1996)
Besides mineral composition different XRD-methods can be used to determine crystalline orientation and structural properties (e.g. Lattice parameters (10-4Å), strain, grain size, expitaxy, phase composition, preferred orientation (Laue) order-disorder transformation, thermal). In mine waste characterisation, XRD can be used as a fast method to analyze the overall mineralogical composition.
- Non-destructive technique
- Easy sample preparation
- Relatively straight forward data interpretation
- Old, widely used method with many references
- Less accurate in trace amounts and with larger crystalline structures
- Hard to identify unknown heterogeneous or multi phase materials
- X-rays do not interact very strongly with lighter elements
Different types of XRD variations:
- X-ray diffraction (XRPD)
- Rietveld method
XRD method is well documented and widely used.
In GTK’s mineralogical laboratory, analytical services are carried out. Examples of such studies include:
- Characterization of bentonitic clays
- Determination of the components in dry cement
- Identification of industrial and ore minerals
- Determination of the mineral composition of ballast material and sand
- Identification of the components in technical products
- Characterization of clays met with during tunnelling
- Identification of the components in ash from biogenic fuels
Bish D.L. & Post J.E. (editors) 1989, Modern Powder Diffraction. Mineralogical Society of America.
Räisänen M.L. 1996, Geochemistry of podzolized tills and the implications for aluminium mobility near industrial sites: a study in Kuopio, eastern Finland. Geological Survey of Finland, Bulletin 387.