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Abstract
Some of the most common pyroxenes (bronzite, hypersthene, diopside, hedenbergite, augite, omphacite, aegirine and spodumene) and pyroxenoids (wollastonite, rhodonite, fowlerite and charoite) have been investigated by non-contact Raman spectroscopy. In order to complete the information on the vibrational modes, the IR spectrum of augite was devised. In the Raman spectrum of aegirine, the bands appear at lower wavenumbers than in the rest of the pyroxenes. The bands assigned to the Si-O vibrational modes are split due to the deformation of the tetrahedra. In the rhodonite and zinc-rich variety (fowlerite) spectra, differences were noted; in fowlerite, some bands are shifted at higher frequencies, compared with rhodonite. The pyroxenoids presented the same typical bands of the single-chain structure. The spectrum of charoite shows two new lines at 2367 cm^sup -1^ and 2403 cm^sup -1^, which are due to the modes of the N-H bonds.
Key words: Raman spectra, IR, pyroxenes, pyroxenoids, single-chain silicates, aegirine, fowlerite, charoite.
Introduction
Pyroxenes are one of the most abundant rock-forming minerals on Earth (Deer et al., 1997). Studies showed that these minerals are also the major silicates on the surface of Mars (Wang et al., 2001). Many Raman studies on pyroxenes and pyroxenoids were made (Etchepare, 1971; Sharma et al., 1983; McMillan, 1984; Mernagh and Hoatson, 1997; Swamy et al., 1997; Huang et al., 2000; Mills et al., 2004; Pommier et al., 2005; Makreski et al., 2006; Katerinopoulou et al., 2007), but less information was found about aegirine and charoite minerals.
The structure of pyroxenes is built up by the linkage of SiO4 tetrahedra sharing two out of four corners forming continous chains along the c axis. Two tetrahedra repeat in these chains. Cations are located between them, in sites labelled M1 and M2. Pyroxenes include two groups: orthopyroxenes (ortorhombic symmetry) and clinopyroxenes (monoclinic symmetry). When the cation in the M2 site is six-fold coordinated, the minerals are orthorhombic (enstatite-ferrosilite), and when it is in eight-fold coordination they are monoclinic. In clinopyroxenes, the M2 site is occupied by Ca^sup 2+^, Na^sup +^, Li^sup +^ cations (diopside, hedenbergite, augite, aegirine, spodumene). The M1 site is six-fold coordinated and occupied by Fe^sup 3+^, Al^sup 3+^, Fe^sup 2+^, Mg^sup 2+^, Mn^sup 2+^. The orthopyroxenes studied for the present paper (bronzite and hypersthene) have...