Basic XRD

Questions we can answer

GENERAL INFO

RIGAKU SMARTLAB

TRAINING AND REFERENCE

SERVICES AND COLLABORATION

ROWE GROUP
How XRD Works
The XRD can be used to identify single crystals, and to reveal the structure of single crystals. It can be used to identify crystals which are present in a mixture, e.g. minerals in a rock. For minerals with variable formulas and structures, such as clays, XRD is the best method for identifying them and determining their proportion within a sample. For more background on XRD techniques, see the Wikipedia article on X-ray crystallography. A fantastic program on the history of crystallography and X-ray diffraction is available from the Royal Institution for free on YouTube here.

X-ray diffraction has been used to detect the structure of crystals for over a century, and the basic method has not changed. A crystalline sample is placed in the path of an X-ray beam. X-rays diffract through the crystal and into a detector. The beam and detector are rotated through a range of angles. The angles at which the crystals diffract the beam into the detector correspond to planes of the crystals. Each crystal has a characteristic pattern of diffraction angles and corresponding intensity of the diffracted beam.
Basic setup for powder XRD

What Questions can we answer with XRD?
1. What minerals are in my sample?
Most geologists who use XRD do it to identify the minerals present in a rock sample. The sample is first powdered and packed into a sample holder, using techniques to insure that the powder is randomly oriented. A basic scan takes about 20 minutes, and the data usually requires little or no processing. The peaks in the data are compared to peaks in mineral scans from a library to identify the minerals present. This level of analysis can tell you what clay groups are present, (e.g. illite, smectite, chlorite or kaolinite) but is less reliable for specific information about clay mineralogy, due to clays' tendancy to orient themselves in the powder.

2. What are the proportions of the minerals in my sample?
Once the component minerals have been identified in step 1, a technique called Rietveld Analysis is used to model the XRD spectrum by weighing the reference spectra the user has chosen from a library. There are many sources of uncertainty in this analysis, but with many samples it is possible to estimate mineral mode to within certainties of <5%. Sources of error include: using reference spectra which don't actually match the minerals in your rock, orientation bias in clays and other minerals, and the presence of non-crystalline material in the sample.

3. What clay minerals are in my sample?
Clays present special challenges for XRD for four reasons: 1) they tend to orient themselves due to having one strong cleavage on 001, thus violating the requirement for random orientation in powder samples, and 2) they absorb and release interlayer water constantly, which changes the spacing of layers and thus affects the XRD pattern, 3) different cations may occupy interlayer sites in the same mineral, also affecting layer spacing, and 4) "different" clay minerals often form interlayers in the same crystal. See Wikipedia page on clay XRD to learn more.


GENERAL INFO

RIGAKU SMARTLAB

TRAINING AND REFERENCE

SERVICES AND COLLABORATION

ROWE GROUP