1. n. [Formation Evaluation]
A technique for the semiquantitative mineralogical analysis of a sample of rock by measuring the diffraction peaks in X-rays diffracted by the sample. The position of the diffraction peaks is a measure of the distance between discrete crystallographic diffracting planes within minerals, while their intensity indicates the quantity of the mineral. The technique is only semiquantitative because the size and shape of the diffraction peak are strongly influenced by the geometry of the measurement, for example orientation of the minerals, and sample preparation. Fine particles such as clays must be separated from larger particles and measured separately if they are to be detected properly. To reduce errors associated with preferred orientation of minerals, samples are most commonly ground to a powder before analysis, a technique known as powder X-ray diffraction.