X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy
Due to the regular atomic structure of crystalline material (the crystal lattice) each mineral has its typical X-ray diffraction (XRD) pattern which follows the Bragg equation:
nλ = 2d*sinθ
Consequently, X-ray diffraction can be used to decipher the composition of rocks and is in particular useful for very fine grained rocks rich in clay minerals. This is true, in particular, because clay minerals as phyllosilicates have very strong and easily identifiable XRD patterns. XRD is a semi quantitative method. It uses the height of the individual 2θ peaks in relation to each other to determine the rough amount of the individual minerals present in the sample.
The FTIR spectroscopy is a physical process which is based on the interactions of infrared radiation (electromagnetic radiation with wavelengths of approx. 0.7-500 µm) with matter. In the identification of unknown substances, infrared spectroscopy represents a quantitative method of frequent application. As certain functional groups such as the SiO44-, CO32- or OH group have specific absorption ranges of which the wave numbers are largely independent of the remainder of the molecule, an unknown specimen can often also be identified already on the basis of the absorption frequencies of such groups.
By the combination of XRD and FTIR it is possible to record the modal (true) composition of the geological materials with adequate accuracy and also to correlate them with the chemical composition.
A further method for deciphering qualitatively, but fast and cheap, the phyllosilicates within a specimen is DCM (Dielectric Constant Measurement).