The Application of IR Spectroscopy to the Investigation of Minerals
Traditionally, the application of IR spectroscopy in mineralogy is reduced to the determination of wavelengths or frequencies of discrete absorption maxima. These values are brought in correspondence with normal vibrations of different chemical bonds or g
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The Application of IR Spectroscopy to the Investigation of Minerals
1.1
The Discrete Approach
Traditionally, the application of IR spectroscopy in mineralogy is reduced to the determination of wavelengths or frequencies of discrete absorption maxima. These values are brought in correspondence with normal vibrations of different chemical bonds or groups of atoms forming covalent bonds – complex anions (e.g. CO32, C2O42, SO42, PO43, SiO44, and Si2O76), polyatomic cations (H3O+, NH4+, UO22+, etc.), neutral molecules (H2O, NH3), as well as lattice vibrations of infinite chains, layers or frameworks as parts of crystal structures of minerals. Another approach, also based on the use of discrete band maxima, is based on correlations between vibration frequencies and different characteristics of minerals (hydrogen bond strengths, degree of isomorphous substitutions, etc.). In this section, we consider several examples of the application of this approach to the investigation of structural features of minerals. A customary application of IR spectroscopy for the investigation of minerals is the identification of different groups of atoms by their characteristic absorption bands. The ranges of frequencies of characteristic vibrations of most important coordination polyhedra and complex anionic groups are listed by Miller and Wilkins (1952) and Povarennykh (1978). The identification of cationic and anionic isolated groups and polyhedra containing elements with low atomic numbers (H, Li, Be, B, C, O, N) is most important because
the determination of these components by electron microprobe analysis is difficult or impossible. However, in many cases such groups can be easily determined by their absorption in characteristic IR ranges. The individuality of numerous new mineral species approved by the IMA Commission on New Minerals, Nomenclature and Classification during last decades has been first revealed by means of IR spectroscopy. Several examples are given below. Eudialyte-group minerals are trigonal zirconoand titanosilicates characterized by very complex and variable crystal-chemical features (Johnsen et al. 2003). Their general crystal-chemical formula is N(1)3N(2)3N(3)3N(4)3N(5)3M(1)6M(2)3–6M(3)M (4)Z3[Si24O72]O0 4–6X2 where N(1–5) ¼ Na, Н3О+, K, Sr, REE, Y, Ba, Mn, Ca; M(1) ¼ Ca, Mn, REE, Na, Sr, Fe; M(2) ¼ Fe, Mn, Na, Zr, Ta, Ti, K, Ba, H3O; M(3) and M(4) ¼ Si, S, Nb, Ti, W, Na; Z ¼ Zr, Ti, Nb; O0 ¼ O, OH, H2O; X(1) and X(2) ¼ Cl, F, H2O, ОН, CO3, SO4, AlO4, MnO4. Usually these minerals are Cl -dominant in the sites X(1) и X(2) situated around the axis of threefold symmetry. CO32 -dominant minerals of this group with different occupation of N-sites, mogovidite and golyshevite have been discovered recently in the Kovdor massive of alkaline-ultramafic rocks and carbonatites, Kola peninsula (Chukanov et al. 2005a). IR spectra of these minerals contain series of absorption bands in the range from 1,350 to 1,550 cm1 (Fig. 1.1). A cancrinite-group mineral kyanoxalite, Na7(Al6–5Si6–7O24)(C2O4)0.5–1·5H2O, has been
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