LATTICE-DYNAMICS AND RAMAN-SPECTROSCOPY OF PROTOENSTATITE MG2SI2O6

Enstatites (Mg2Si2O6) are important rock forming silicates of the pyro xene group whose structures are characterised by double MgO6 octahedra l bands and single silicate chains. Orthoenstatite transforms to proto enstatite above 1273 K with a doubling of the a axis and a rearrangeme nt of the silicate chains with respect to the Mg2+ ions. Lattice dynam ical calculations based on a rigid-ion model in the quasi-harmonic app roximation provide theoretical estimates of elastic constants, long wa velength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of p rotoenstatite. The computed elastic constants are in good agreement wi th experimental data. The computed density of states of a chain silica te such as protoenstatite is distinct from that of olivines (forsterit e, Mg2SiO4 and fayalite, Fe2SiO4) with isolated silicate tetrahedra. T he band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal v ibrations of the isolated silicate group, whereas in protoenstatite th ese gaps are filled by the vibrations of the bridging oxygens of the s ilicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature f actors. Validity of these calculations are supported by Raman scatteri ng measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,SC)0.6Mg1.4Si2O6 stab le at room temperature using the 488 nm or 514.5 nm lines of an Ar+ io n laser and a micro-Raman spectrometer with backscattering geometry. T he Raman spectra were analysed and interpreted based on the lattice dy namical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement wit h those obtained from lattice dynamical calculations.