Raman spectroscopic study of pyroxene structures from the munni munni layered intrusion, Western Australia

Abstract

Raman spectra were obtained for a series of clinopyroxenes, orthopyroxenes and inverted pigeonites from the Munni Munni layered mafic-ultramafic intrusion in Western Australia. Their composition varies from mg48 to mg87, where mg = mg number = atomic 100 Mg/(Mg + Fe) and Fe = total Fe. The characteristic Raman bands of pyroxene are mainly due to vibrations of the SiO4 tetrahedra and occur in the 300-400, 650-700 and 980-1020 wavenumber regions of the spectra. Clinopyroxene shows only one intense Raman band in the 650-700 cm-1 region and is readily distinguished from orthorhombic pyroxene which has two intense bands in this region. This difference is probably due to the single symmetrically distinct tetrahedral site in the C2/c structure, whereas orthopyroxene has two symmetrically distinct tetrahedral chains. The pyroxenes showed systematic shifts in the wavenumber of the Raman bands with increasing mg number with the orthopyroxene spectra being the most sensitive to variations in mg number. The variations in wavenumber are believed to be related to the straightening of the tetrahedral chains and the corresponding changes in the Si - O bond lengths and O - Si - O angles caused by substitution of Fe for Mg. These changes are greater in the orthorhombic Pbca structure, because the edge-sharing polyhedra enclosing the Fe2+ and Mg2+ cations form wide bands which are separated by voids or rifts parallel to the c-axis. In the monoclinic C2/c structure, however, these polyhedra are linked in a continuous layer which restricts structural adjustments of the polyhedra and the adjacent tetrahedral SiO4 chains. Therefore, since Raman spectroscopy is sensitive to changes in crystal structure, it may be used to obtain a semi-quantitative chemical analysis of pyroxene when calibrated with a suitable solid solution series. The number of Raman bands observed and their width also gives an indication of cation order/disorder within the crystal. 1997 Show more