X-Ray Diffraction


X-ray diffraction is a method of identifying materials by their crystalline structure. The crystalline structure of minerals is a function of the spacing and arrangement of their constituent atoms and can be thought of as regularly spaced planes or layers of atoms. In x-ray diffraction analysis, x-rays are produced when electrons bombard a target made usually of copper or iron although other metallic elements can also be used for targets as well. These x-rays are referred to a monochromatic, that is they are characterized by a sharply defined wavelength that is closely similar to the spacings of the planes or layers of mineral crystals. The x-rays are aimed onto a specimen, and the layers of atoms from the specimen then reflect or diffract the x-rays. At the point when the reflected x-rays have a maximum intensity relative to their crystalline planes, an x-ray intensity maximum or peak is produced. The reflected x-rays are then picked up by a detector. Thus, in x-ray diffraction analysis minerals are identified by characteristic spacing of their atomic lattices. For use in identification, the characteristic x-ray peaks are compared with internationally accepted mineral standards.

Why should archaeologists be interested in X-ray diffraction?
X-ray diffraction is a very precise way to identify minerals especially when they represent very small units, such as clays. X-ray diffraction has been used to successfully characterize different types of pipestone (argillite) in the southern Great Plains. Another application would be the identification of ground mineral-based pigments (such as hematite) through the identification of the minerals present in pigments.

What kind of samples are required for X-ray diffraction?
One gram or less of finely powered material is usually required for most routine analyses

What else is important to know about X-ray diffraction analysis?
X-ray diffraction is not a quantitative technique but rather a forensic one in that this technique allows a person to know what minerals are present in a given substance but not how common they are relative to one another. Also, complex materials such as pottery can have a very large number of different minerals present both as inclusions and as clay minerals. Given this compositional complexity and the fact that most minerals have multiple characteristic peaks, the identification of individual minerals can be very difficult and time consuming. X-ray diffraction is the technique of choice for the identification of clay minerals. However, clays lose their crystalline structure and hence their ability to diffract x-rays when fired. However, if fired at a low enough temperature (below 800 degrees Celsius) some clay minerals will regain their structure over time.