X-Ray Diffraction for Geology and Minerals Analysis – XRD

X-Ray Diffraction (XRD) is an extensively used analytical technique for the determination of the phase assemblage of crystalline materials. It is highly relevant to minerals and geology where it can be applied to determine the mineralogical composition of geological samples including sediments. Generally speaking, it is a bulk analysis technique (analysing several square millimetres), although specialised instruments are available that can perform XRD on very small regions i.e. microdiffraction.

How Does XRD Work?

X-ray diffraction is based on the principle that when X-rays pass through a crystal lattice, they interact with the atoms in the lattice, causing constructive and destructive interference of the X-ray waves. This interaction results in a pattern of diffracted X-rays that can be detected and analysed and is based on the Bragg’s Law principle originally proposed by father and son William and Lawrence Bragg in 1913. Learn more

Every (crystalline) mineral contains atoms arranged in a specific and repeating structure. The structure of every mineral is different and can be influenced by the atoms present and their specific arrangement. XRD can be used to determine the dimensions i.e. lattice parameters, which in turn are like a fingerprint unique to each mineral and thus identifications can be made.

XRD Systems

XRD systems range in size depending on what your specific application requires. For instance, you can get a small benchtop system that could be easily moved from lab to minesite. There are also compact systems suited to high-throughput continuous operation with automation solutions minimising operator input and at the top end are high-flux systems suited to the most challenging or highest throughput applications. These larger systems can also take magazines of samples that can also be set to run in batches.

Anatomy of an X-Ray Diffractometer

Regardless of size, the basic components of an XRD are the same i.e.

X-ray source
Sample holder
Detector
Goniometer – that mounts the source, sample holder and detector relative to each other and controls the geometry

Nowadays the operation of an XRD is all controlled by software which will allow you to do everything from measurement to analysis.

XRD, X-ray diffractometer goniometer assembly showing main components i.e. X-ray source, sample holder and detector.

Sample Preparation

Rock samples are crushed and ground down to a article size of approximately 100µm. This helps to create a test sample that is more indicative of the whole sample. The resultant powder is carefully packed into a suitable sample holder.

The way in which the powder is packed into the sample holder is important as minerals with certain morphologies e.g. clays with plate-like particles, can be inadvertently oriented, i.e. packed with preferred orientation, which can skew the quantitative results.

Identification and Quantification

Within the software of XRDs there will be search/match facilities that interface with databases of known minerals such as ICDD and COD that will facilitate the identification of minerals based on the XRD pattern generated by your sample.

Example XRD patterns for albite and and albite/microcline mixture.

Quantitative analyses can be derived based on the relative intensity of the peaks of each phase/mineral present. Using Rietveld analysis, the relative proportions of each phase can be determined.

Connemara marble samples geological materials

Connemara marble samples before pulverising.

XRD pattern for Connemara marble sample.

Quantitiative XRD analysis of Connemara marble sample.

These images came from the Application Note – Fast Phase Analysis of Mineral Powder

XRD vs XRF

While both techniques use X-rays and have similar acronyms, they both produce different, but complimentary datasets. As mentioned XRD determines phase assemblages and can identify what minerals are present. Conversely, XRF which is also commonly used for geological applications determines elemental compositions.

So, XRF can determine the presence of elements such as Al and Si and what percentages they are present in, often down to the ppm level. It cannot however reveal how these elements are present e.g. as SiO₂, Al₂O₃ or Al₂O₃·2SiO₂·2H₂O (kaolinite) or combinations thereof for instance.

Example XRF analysis with measured vakues compared to certified values.

Example XRF analysis with measured values compared to certified values for a silicate rock sample.

On the other hand, XRD can determine if the phases SiO₂, Al₂O₃ or Al₂O₃·2SiO₂·2H₂O are present and using Rietveld analysis, it can also quantify how much of each phase is present.
It should also be noted that:

XRF and XRD data are complimentary and can be combined to make identification of unknowns simpler, i.e. potential XRD matches can be culled by only looking at those containing elements identified via XRF
Quantitative XRD can be refined using XRF data to produce more accurate results
XRF is insensitive to light elements such as Li, Be, B etc.
XRD however can easily identify minerals containing light elements such as spodumene

Summary

In summary, XRD is a powerful tool in geology that helps researchers and geologists understand the mineralogical composition of geological materials. This information is of commercial relevance in everything from exploration geology, through process metallurgy to quality control of final products.

The AMI is equipped to provide XRD services and analysis of geological materials. Contact us to find out how we can assist you using XRD.