Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

Abstract

Abstract. Enstatites (MgzSi206) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO6 octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg 2+ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, MgzSiO4 and fayalite, FezSiO4) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)o.6Mgl.4Si206 stable at room temperature using the 488nm or 514.5 nm lines of an Ar + ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.

Inelastic neutron scattering and lattice dynamics of minerals

“…In most of these studies, lattice dynamics calculations have been used for the planning, analysis and interpretation of experimental neutron data and for prediction of various microscopic and macroscopic thermodynamic properties. [13,14] were also carried out at the University of Washington, USA.…”

“…These studies shed light on various anomalous behaviours of minerals including pressure-induced amorphization, phonon and elastic instabilities, novel high pressure phase transitions, high pressure-temperature melting, etc. The integration of neutron scattering experiments with theoretical simulation methods [1][2][3][4][5][6][7][8][9][10][11][12][13][14] have been extremely successful in providing microscopic insights relevant to high-pressure mineral and materials sciences, seismology, geochemistry, petrology, etc. and provides a new window to explore planetary interiors.…”

“…The INS data were used to validate interatomic potentials. The models in turn were used for calculations [10][11][12][13][14] of the phase diagram (c-d), high pressure and temperature thermodynamic properties (e-i) and melting (j) which are in good agreement with experimental data [32][33][34][35][36][37][38][39][40][41][42][43] and ab initio studies [42]. The two-phase MD studies of melting in (j) are from ref.…”

“…Enstatite is the magnesium end-member of pyroxene (Mg 1−x Fe x )SiO 3 which is an important rock-forming silicate [10][11][12][13][14] and a constituent abundant in the upper mantle besides olivine (Mg,Fe) 2 SiO 4 . Enstatite has several polymorphs: orthoenstatite, protoenstatite and the low and high clinoenstatites whose structures are closely related.…”

“…Compositional modelling based on accurate data about the structure and thermodynamic properties of minerals is essential to interpret the complex seismic data. Of particular interest is an understanding of mineral behaviour in terms of its microscopic structure and dynamics [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. These characteristic properties are conveniently studied using neutron scattering-based techniques.…”