RAMAN-SPECTROSCOPIC STUDY OF PYROXENE STRUCTURES FROM THE MUNNI-MUNNILAYERED INTRUSION, WESTERN-AUSTRALIA

Raman spectra were obtained for a series of clinopyroxenes, orthopyrox enes and inverted pigeonites from the Munni Munni layered mafic-ultram afic intrusion in Western Australia. Their composition varies from mg( 48) to mg(87), where mg = mg number = atomic 100 Mg/(Mg + Fe) and Fe = total Fe. The characteristic Raman bands of pyroxene are mainly due t o vibrations of the SiO4 tetrahedra and occur in the 300-400, 650-700 and 980-1020 wavenumber regions of the spectra. Clinopyroxene slows on ly one intense Raman band in the 650-700 cm(-1) region and is readily distinguished from orthorhombic pyroxene which has two intense bands i n this region. This difference is probably due to the single symmetric ally distinct tetrahedral site in the C-2/c structure, whereas orthopy roxene has two symmetrically distinct tetrahedral chains. The pyroxene s showed systematic shifts in the wavenumber of the Raman bands with i ncreasing mg number with the orthopyroxene spectra being the most sens itive to variations in mg number. The variations in wavenumber are bel ieved 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 enclos ing the Fe2+ and Mg2+ cations form wide bands which are separated by v oids or rifts parallel to the c-axis. In the monoclinic C2/c structure , however, these polyhedra are linked in a continuous layer which rest ricts structural adjustments of the polyhedra and the adjacent tetrahe dral SiO4 chains. Therefore, since Raman spectroscopy is sensitive to changes in crystal structure, it may be used to obtain a semi-quantita tive chemical analysis of pyroxene when calibrated with a suitable sol id solution series. The number of Raman bands observed and their width also gives an indication of cation order/disorder within the crystal. 1997 (C) Crown Copyright reserved.