Analytical Methods:XRD

SASSA Home Page &#8658; Analytical Methods Home Page &#8658; Specialist Techniques &#8658; X-ray diffraction

Questions
Clay mineralogy is a basic soil property that can have a strong influence over some soil physical properties. The small size of clay particles prevented scientists from elucidating their mineral structure until the technique of X-ray diffraction (XRD) was developed in the 1930s. This showed that the minerals had a plate-like structure and various mineral groups were identified from their characteristic basal spacings.

Case studies where X-ray diffraction has been used in archaeology include:
 * The palaeohydrology of Iron Age sites in the Mun River Valley, NE Thailand.

Samples
The clay sample can be a powder sample or a suspension dried onto a glass slide, which gives a preferred orientation of the plate-like crystals.

Analysis
It is required to analyse the samples both in their original sate and after thay have been subjected to pretreatments as defferation, saturation with potassium, saturation with magnesium and glycolation. The technique of XRD involves directing a beam of X-rays at a clay fraction sample. As the x-rays penetrate the plate-like crystal, a small amount of their energy is absorbed by the atoms, which become 'excited' and emit radiation in all directions. Radiation from atomic planes that is in phase will form a coherent reflective beam that can be detected by x-ray sensitive film.

Data and interpretation
A diffractogram is produced, which is composed of a series of diffraction peaks, which identify the mineral types including kaolinite, smectite and illite.