Clay minerals in soils can be identified using one or more of the following methods: 1. X-ray diffraction (XRD) 2. Differential thermal analysis (DTA) 3. Transmission electron microscopy 4. Scanning electron microscopy 5. Infrared spectroscopy.
X-ray diffraction (XRD) technique is one of the most commonly used methods to identify the minerals present in a soil, although it is difficult to determine their quantity in the soil. Minerals may be considered to be made up of regularly repeating unit cells, each unit cell having dimensions of a, b, and c along the three coordinate directions x, y, and z. In clay minerals, the c spacing, which is the dimension of the unit cell across the thickness of the layer, usually varies from mineral to mineral, while the other two dimensions remain more or less constant among different clay minerals.
Friedrich and Knipping performed the first X-ray diffraction experiment using a crystal of copper sulfate in 1912. Bragg diffraction was first proposed by English physicists W.H. Bragg and his son W.L. Bragg in 1912 as a means of analyzing the structure of crystals.
They were awarded the Nobel Prize in physics in 1915 for their work in determining crystal structures. It was found later that every crystalline substance gives a pattern; the same substance always gives the same pattern; and in a mixture of substances each produces its pattern independently of the others.
Differential thermal analysis, DTA, is the simplest and most widely used thermal analysis technique for the identification of clay minerals.
DTA consists of heating a test sample and a thermally inert substance simultaneously at a constant rate of about 10°C/min and continuously measuring difference in temperature between the sample and the inert material. Alumina and silicon carbide are commonly used as the thermally inert reference materials.
The results of DTA are presented in the form of a thermogram, which is a plot of the temperature (T) on the x-axis and the difference in temperature between the sample and the inert material (ΔT) on the y-axis.
The transmission electron microscope was the first type of electron microscope developed and is patterned exactly on the light transmission microscope except that a focused beam of electrons is used instead of light to “see through” the specimen. It was developed by Max Knoll and Ernst Ruska in Germany in 1931.
The scanning electron microscope is one of the most versatile instruments available for the examination and analysis of the microstructure, morphology, and chemical composition characterization of soils and other materials.
Electron microscopes are scientific instruments that use a beam of energetic electrons to examine objects on a very fine scale. Zworykin et al. first described a true SEM in 1942 with a resolving power of 50 nm. Modern SEMs can have resolving power better than 1 nm.
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. It involves the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam.
Infrared (IR) spectroscopy deals with the infrared region of the electromagnetic spectrum with a longer wavelength and lower frequency than visible light. Infrared radiation spans a section of the electromagnetic spectrum having wavenumbers from roughly 13000 to 10 cm–1, or wavelengths from 0.78 to 1000 µm. It is bound by the red end of the visible region at high frequencies and the microwave region at low frequencies.
The IR region is commonly divided into three smaller areas: near IR, mid IR, and far IR. For IR spectroscopy, the most frequently used is the mid IR region having wave number between 4000 and 400 cm–1 (or wave length between 2.5 and 25 µm). IR absorption positions are generally presented as either wavenumbers or wavelengths. Wavenumber defines the number of waves per unit length usually expressed as number per cm (cm–1).