From a geological point of view, clays are the most complicated natural mineral aggregates. Not least because of their submicroscopic particle size, a direct visual observation of these minerals isn't practicable. To determine the type and portion of the containing mineral components, it is necessary to combine both radiographical and thermoanalytical methods. Prevalent methods are scanning electron microscopy (SEM), X-ray fluorescence analysis (XRF), X-ray diffraction analysis (XRD), simultaneous thermal analysis (STA ) and grain-size distribution (GSD).
The finer the matter, the more complicated and complex the analysis - this is the easy way to introduce this topic. With scanning electron microscopy (SEM) it is possible to visualize the finest mineral textures and microstructures of complex clay mineral aggregates. The SEM images below show the considerable differences between the various types of clay minerals. All SEM images were provided courtesy of the "Clay Archive of Mineralogical Society of Great Britain & Ireland and the Clay Minerals Society".
The mineral phase based research and analysis of clays can be summarized under the term of clay mineralogy. The mineralogical characteristics are of decisive importance for the technical application of clay resources. This applies both to the use in fine and coarse ceramics as well as to other purposes beside ceramics. The knowledge of clay mineralogy allows to draw first conclusions about special characteristics of a clay.
Therefore, the quantitative mineral phase analysis is still one of the most difficult tasks in mineralogy to determine type and portion of clay minerals in a mineral mixture. The work of Max von Laue can be regarded as pioneering. In 1912 he discovered the diffraction of X-rays on crystals, which provides both the evidence of the wave character of X-ray radiation and the lattice structure of crystals. For his work, he received the Nobel Prize in physics in 1914. Even today, X-ray diffraction is the most important methodic basis in clay mineralogy.
The method is based on the principle that a crystalline substance possess a characteristic atomic lattice, which is diffracted by X-rays in the same characteristic way. The resulting interference patterns are the key to identify the crystal structure. The Bragg equation is the mathematical determination of the mineral structure from the diffraction pattern obtained during the X-ray diffraction. X-ray diagrams provide interference maxima (reflexes, peaks) from whose position the diffraction angle and the associated lattice plane distance (d) can be determined. Since each crystalline substance has an individual X-ray diffractogram, X-ray diffraction can be used to identify unknown crystalline structures.
Mineral raw materials generally consist of several mineral phases. Due to the fact that the portion of the individual minerals does not correlate with the measured interference intensities, the X-ray diffraction only provides qualitative results. Furthermore, peaks of certain minerals can be overlapping each other, so that a distinct classification is often not possible. Especially the structurally very similar layered composition of clay minerals causes can lead to this issue. Therefore, the clay minerals have be separated from the remaining minerals before the actual identification to severally examine them.
This seperation is advantageously carried out by the dispersing in water and the subsequent fractionation in the centrifugal field of particles sized < 2 µm. Beside the determination of b-parameters (060 reflexes), the identification and differentiation of the individual clay mineral types are mainly based on the ascertainment of the basal face interspaces dL (001 reflexes) in c-direction. For this purpose, texture preparations with a as parallel as possible orientation of the several clay particles have to be made. Due to the fact that the base reflexes, e.g. of smectites (dL = 12 - 18 Å) and chlorites (dL = 14 Å), could coincide, a proof of swelling capacity has to be undertaken for a distinct identification. To provide this evidence a further texture preparations is evaporated with ethylene glycol in a vacuum desiccator. In case of an inner-crystalline swellable mineral, a diagnostic expansion of the crystal lattice will be detected.
The X-ray diffraction as solitary method for an exact determination of the type and portion of clay minerals phases in a mineral mixture is usually not sufficient, because only the crystalline parts can be captured. Amorphous phases such as volcanic glasses can neither be proven qualitatively nor quantitatively, so that X-ray diffractions have to be combined with additional analyses. For several year the infrared spectroscopy has proven to be a suitable method.
The method is based on interactions between infrared radiation (0.7 – 500 µm) and matter. By recording the energy values from infrared radiation, the individual molecular groups show a characteristic vibration spectrum. Since the amount of absorbed energy depends on the number of molecules in the infrared beam, quantitative data on the mineral content can be generated. The ascertainment of the minerals amount is calculated by analyzing the extinctions of the beam on selected measurement marks. Even amorphous, microcrystalline or poorly crystallized phases can be detected and analyzed by infrared spectroscopy.