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Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

Abstract

Enstatites (Mg2Si2O6) 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 Mg2+ 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, Mg2SiO4 and fayalite, Fe2-SiO4) 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)0.6Mg1.4Si2O6 stable at room temperature using the 488 nm 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.

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References

  1. Chaplot SL (1978) A computer program for external modes in complex molecular-ionic systems, Report 972: Bhabha Atomic Research Centre, Bombay, India

  2. Choudhury N, Chaplot SL, Rao KR, Ghose S (1988) Lattice dynamics of MgSiO3 perovskite, Pramana (J. Phys, India) 30:423–428

  3. Choudhury N, Chaplot SL, Rao KR (1989) Equation of state and melting point studies of forsterite. Phys Chem Min 16:599–605

  4. Choudhury N, Chaplot SL, Rao KR, Ghose S (1993) Phonon dispersion relation in protoenstatite Mg2Si2O6 (to appears)

  5. Cohen RE (1987) Elasticity and equation of state of MgSiO3 perovskite. Geophys Res Lett 14:1053–1056

  6. Damen TC, Porto SPS, Tell B (1966) Raman effect in zinc oxide. Phys Rev 142:570–574

  7. Dowty E (1987) Vibrational interactions of tetrahedra in silicate glasses and crystals: calculations on ideal silicate-aluminategermanate structural units. Phys Chem Min 14:80–93

  8. Durben DJ, Wolf JH, McMillan PF (1991) Raman scattering study of the high temperature vibrational properties and stability of CaGeO3 perovskite. Phys Chem Min 18:215–223

  9. Ghose S, Hastings JM, Choudhury N, Chaplot SL, Rao KR (1991) Phonon dispersion relation in fayalite, Fe2SiO4. Physica B174:83–86

  10. Ghose S, Choudhury N, Chaplot SL, Rao KR (1992) Phonon density of states and thermodynamic properties of minerals. In: Saxena S (ed) Thermodynamic Data: Advances in physical geochemistry Springer New York p 283–314

  11. Hemley RJ, Jackson MD, Gordon RG (1987) Theoretical study of the structure, lattice dynamics and equation of state of perovskite type MgSiO3 and CaSiO3. Phys Chem Min 14:2–12

  12. Kieffer SW (1985) Heat capacity and entropy: systematic relations to lattice vibrations. In: Kieffer SW, Navrotsky A (eds) Microscopic to Macroscopic. Rev in Min 14:65–126

  13. Murakami T, Takeuchi Y, Yamanaka T (1982) Transition of orthoenstatite to protoenstatite and the structure at 1080 °C. Z Kristall 160:299–312

  14. Pickett WE, Cohen RE, Krakauer H (1991) Lattice instability, isotope effect and high Tc superconductivity in La2−x BaxCuO4. Phys Rev Lett 67:228–231

  15. Price DL, Ghose S, Choudhury N, Chaplot SL, Rao KR (1991) Phonon density of states in fayalite, Fe2SiO4. Physica B174:87–90

  16. Purton J, Jones R, Catlow CRA, Leslie M (1993) Ab initio properties for the calculation of the dynamical and elastic properties of α-quartz. Phys Chem Min 19:392–400

  17. Rao KR, Chaplot SL, Choudhury N, Ghose S, Hastings JM, Corliss LM, Price DL (1988) Lattice dynamics and inelastic neutron scattering from forsterite Mg2SiO4: phonon dispersion relations, density of states and specific heat. Phys Chem Min 16:83–97

  18. Sasaki S, Takeuchi Y, Fujino K, Akimoto S (1982) Electron density distributions of three orthopyroxenes Mg2Si2O6, Co2Si2O6 and Fe2Si2O6. Z Kristall 158:279–297

  19. Sharma SK (1989) Applications of advanced Raman spectroscopic techniques in earth sciences. In: Bist HD, Durig JR, Sullivan JF (eds) Vibrational spectra and structure. Elsevier Science Publishers BV, Amsterdam Vol 17B: 513–568

  20. Smyth JR (1971) Protoenstatite: a crystal structure refinement at 1100° C. Z Kristall 134:262–274

  21. Smyth JR, Ito J (1977) The synthesis and crystal structure of a magnesium-lithium-scandium protopyroxene. Am Min 62:1252–1257

  22. Vaughan MT, Bass JD (1983) Single crystal elastic properties of protoenstatite, a comparison with orthoenstatite. Phys Chem Min 10:62–68

  23. White WB (1975) Structural interpretation of lunar and terrestrial minerals. In: Kerr C (ed) Infrared and Raman spectroscopy of lunar and terrestrial minerals. Academic Press, New York, p 325–358

  24. Wolf GH, Bukowinski MST (1985) Ab initio structural and thermoelastic properties of orthorhombic MgSiO3 perovskite. Geophys Res Lett 12:809–812

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Ghose, S., Choudhury, N., Chaplot, S.L. et al. Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6 . Phys Chem Minerals 20, 469–477 (1994). https://doi.org/10.1007/BF00203216

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Keywords

  • Olivine
  • Forsterite
  • Enstatite
  • Fayalite
  • Mg2SiO4