Abstract
In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg2SiO4. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg2SiO4 only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg2SiO4, a treatment more sophisticated than the two-shell model is needed.
Similar content being viewed by others
References
Anderson DL (1984) The earth as a planet: paradigms and paradoxes. Science 223:347–355
Birch F (1978) Finite strain isotherms and velocities for single-crystal and polycrystalline NaCl at high pressures and 300 K. J Geophys Res 83:1257–1268
Boyer LL, Mehl MJ, Feldman JL, Hardy JR, Flocken JW, Fong CY (1985) Beyond the rigidion approximation with spherically symmetric ions. Phys Rev Lett 54:1940–1943
Brown GE (1982) Orthosilicates. Rev in Mineral 5:275 2nd ed, (Min Soc Am)
Catti M (1981) The lattice energy of forsterite. Charge distributions and formation enthalpy of the SiO −44 ion. Phys Chem Minerals 7:20–25
Hazen RM (1976) Effect of temperature and pressure on crystal structure of forsterite. Am Mineral 61:1280–1293
Hazen RM, Prewitt CT (1977) Effect of temperature and pressure on interatomic distances in oxygen based minerals. Am Mineral 62:309–315
Hemley RJ, Jackson MD, Gordon RG (1985) First-principles theory for the equations of state of minerals at high pressures and temperatures: Application to MgO. Geophys Res Lett 12:247–250
Hill RJ, Newton MD, Gibbs GV (1983) A crystal chemistry study of stishovite. J Solid State Chem 47:185–200
Jackson MD (1986) Theoretical investigation of chemical bonding in minerals. PhD Thesis, Harvard University
Jackson MD, Gibbs GV (1987) A modeling of the coesite and feldspar topologoes of silica as a function of pressure using MEG methods. J Phys Chem (in press)
Jackson MD, Gordon RG (1987a) MEG investigations of some phases of MgO. Submitted to Phys Rev B
Jackson MD, Gordon RG (1987b) MEG investigations of low pressure silica-shell model for polarization. Phys Chem Minerals (in press)
Jeanloz R, Thompson AB (1983) Phase transitions and mantle discontinuities. Rev Geophys Space Phys 21:51–75
Mahan GD (1980) Polarizability of ions in crystals. Solid State Ionics 1:29–45
Matsui M, Busing WR (1984) Computational modeling of the structure and elastic constants of the olivine and spinel forms of Mg2SiO4. Phys Chem Minerals 11:55–59
Mulhausen CW, Gordon RG (1981) Electron gas theory for ionic crystals, including many-body effects. Phys Rev B 23:900–923
Parker VB, Wagman DD, Evans WH (1971) Selected values of chemical thermodynamic properties. Nat Bur Stand (U.S.) Tech Note 270-6
Pearson EW, Jackson MD, Gordon RG (1984) A theoretical model for the index of refraction of simple ionic crystals. J Phys Chem 88:119–128
Post JE, Burnham CW (1986) Ionic modeling of mineral structures and energies in the electron gas approximation: TiO2 polymorphs, quartz, fortsterite, diopside. Am Min 71:142–150
Price CD, Parker SC (1984) Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4. Phys Chem Minerals 10:209–216
Sasaki S, Prewitt CT, Sato Y (1982) Single-crystal X-ray study of γ-Mg2SiO4. J Geophys Res 87:7829–7832
Van der Wal RJ, Vos A, Kirfel A (1987) Conflicting results for the deformation properties of forsterite. Mg2SiO4. Acta Crystallogr B 43:132–142
Waldman M, Gordon RG (1979) Scaled electron gas approximation for intermolecular forces. J Chem Phys 71:1325–1329
Weidner DJ, Sawamoto H, Sasaki S, Kumazawa M (1984) Single-crystal elastic properties of the spinel phase of Mg2SiO4. J Geophys Res 89:7852–7856
Wyckoff RWG (1963) Crystal Structures, 2nd Ed, Wiley, New York
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Jackson, M.D., Gordon, R.G. A MEG study of the olivine and spinel forms of Mg2SiO4 . Phys Chem Minerals 15, 514–520 (1988). https://doi.org/10.1007/BF00311022
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00311022