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Mineral identification at Amgueddfa Cymru

X-Ray diffraction machine
The X-Ray diffraction machine at the Museum
Passing of an X-ray beam through a rock sample from the source to the detector
Passing of an X-ray beam through a rock sample from the source to the detector
Quartz crystal
Quartz crystal
Graphite
Graphite
Diamond
Diamond
Langite
Langite
wroewolfeite
wroewolfeite

One of the activities of the Geology Department at Amgueddfa Cymru is to document all the minerals known in Wales. Minerals can be identified visually, but for a more definitive confirmation a process known as X-ray diffraction analysis (XRD) is used. This technique allows natural minerals and man-made crystalline materials to be 'fingerprinted' and compared to a database of known samples.

X-ray diffraction analysis

Most minerals are crystalline, which means they are made up of a regular framework of atoms creating a unique 'crystal lattice'.

When X-rays are passed through a mineral, the atoms cause the X-rays to be diffracted, or bent, into many directions. The resulting X-ray pattern can then be recorded to produce a 'fingerprint'. Because no two minerals have exactly the same arrangement of atoms, their 'fingerprints' (or lattices diffraction patterns) are unique. These patterns can therefore be used to identify the mineral.

To analyse a mineral by XRD a small sample, usually ground into a powder, is bombarded with X-rays. The data is recorded as a graph, called a diffractogram, which is a convenient form for viewing the result.

To identify the mineral, the result is compared with a database of patterns from thousands of known minerals.

An X-ray pattern of quartz showing its unique pattern
An X-ray pattern of quartz showing its unique pattern

Identical looking minerals

Visual identification is still important, as it is possible for two different mineral species to have the same chemical composition but look very different. For example, diamond and graphite (both pure carbon) have the same chemical composition, but are clearly different not only in appearance but also in hardness and crystal form.

On the other hand, langite and wroewolfeite are two chemically identical copper minerals that both form blue needles and are consequently difficult to tell apart visually. But because they have different crystal structures and therefore produce different diffraction patterns, XRD provides a quick and reliable method for distinguishing between them.

Some minerals don't have a regular crystal structure and therefore don't produce diffraction patterns. Known as 'Amorphous minerals', they cannot be identified by XRD.

A diffractogram pattern of an amorphous sample with no identifiable peaks
A diffractogram pattern of an amorphous sample with no identifiable peaks

The application of XRD

The technique is widely used in geology and also in a range of related disciplines. For example, it is used to identify minerals in artists' pigments and the composition of corrosion on archaeological artefacts. Conservators can then devise the appropriate treatment for museum specimens.

Authors: Amanda Valentine & Jana Horak

Article Date: 7 December 2009

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