Study of Mineral Composition

The method for the study of crystal structure, including mineral composition, is x-ray diffraction. It is a semiquantitative method (Klug and Alexander 1954 Warren 1999) for the study of mineral composition. Powder samples of rocks are studied, and the place of the peaks (in nanometer or diffraction angle) and their relative intensity are observed. The diffractogram is usually compared to that of known substances that can be found in databases (ICDD 2009). In order to accurately quantify the rock samples, x-ray diffraction must be complemented with other quantitative techniques, including chemical analysis (Section 4.1.1), thermal, infrared analyses (Chapter 2, Section 2.1.2) and surface area (Chapter 1, Section 1.3.4.1.5 Section 4.1.3) measurements. 

Thermal analysis is suitable for the study of the changes under heat, including the changes in mass and enthalpy (Smykatz-Kloss and Warne 1991 Liptay 1973). In addition, when the minerals are qualitatively or semiquantitatively known from x-ray diffraction studies, their quantity can be determined more precisely on the basis of characteristic thermal changes of the minerals. Thermal analytical methods give very useful information on the types of water in clay minerals and on the valency of the interlayer cation (Chapter 2, Section 2.1.2 Foldvari 1991 Foldvari et al. 1998). 

Thermal analysis, together with other methods (x-ray diffraction, x-ray photoelectron spectroscopy), provides more accurate information on metal speciation than extraction methods (Section 4.3). 

Infrared spectra, including near-infrared spectra, also give useful information on the crystal structures of minerals and functional groups of soil organic matter (Farmer 1974 Petit 2008). Infrared spectra are usually interpreted empirically because the infrared absorption bands depend on several factors the atomic mass, length and forces of chemical bonds, and the symmetry of the overall unit cell and each atom in the unit cell. 

Near-infrared spectroscopy (NIR) works in the 800 nm-2.5 pm (12,500— 4,000 cm-1) range. The advantage of NIR is that it can typically penetrate much farther into a sample than the mid-infrared radiation (30-1.4 pm, 4,000-400 cm-1)- It can be used for the quantitative measurement of organic functional groups of soil organic matter, especially O—H, N—H, and C=0 (Siesler et al. 2002). In addition, the structural modifications under the effect of chemical treatments (e.g., acidic treatments) can also be studied by NIR (Madejova et al. 2009). 

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