## Abstract

The single-crystal elastic moduli, c_{ij} x, of the olivine (α) and spinel (γ) polymorphs of nickel orthosilicate have been measured at atmospheric pressure and 20° C by Brillouin spectroscopy. The results are (Mbar), Ni_{2}SiO_{4} olivine: c_{11}=3.40(2), c_{22}=2.38(2), c_{33}=2.53(2), c_{44}=0.71(1), c_{55}=0.87(1), c_{66}=0.78(1), c_{12}=1.09(2), c_{13}=1.10(4), c_{23}=1.13(3), Ni_{2}SiO_{4} spinel: c_{11}=3.66(3), c_{44}=1.06(1), c_{12}=1.55(3). In comparing these results with extant elasticity data for olivine- and spinel-type compounds we find distinctive elastic characteristics related to crystal structure, and systematic trends due only to compositional variation. For silicate olivines, the longitudinal moduli decrease in the order c_{11}>c_{33}>c_{22}, regardless of composition. The moduli c_{55} and c_{66} are approximately equal, and greater than c_{44}. The former relationship is related to differences in polyhedral linkages along the crystallographic axes, whereas the latter may result from rotational freedom of SiO_{4} tetrahedra in response to different directions of shear. Composition affects elasticity most directly through the relative magnitudes of {Mathematical expression} and {Mathematical expression}. When transition-metal cations are six-coordinated by oxygen {Mathematical expression}, and when alkaline-earth cations are six-coordinated {Mathematical expression}. The longitudinal moduli along and normal to the close-packed directions of spinels are similar, reflecting the framework-like arrangement of octahedra. These longitudinal moduli exhibit little compositional dependence upon tetrahedral cations but vary dramatically with octahedral substitution. Our data indicate that tetrahedral cations affect elastic properties more as the oxygen positional parameter, u, decreases. The u parameter is also directly related to elastic anisotropy. While γ-Ni_{2}SiO_{4} (u=0.244) is elastically isotropic, anisotropy increases rapidly as u approaches a limiting value near 0.27, and may be related to mechanical stability of the spinel structure. The longitudinal wave velocities along close-packed directions in α and γ Ni_{2}SiO_{4} are equal. Thus, for an α-γ polymorphic pair, the assumptions of elastic isotropy of the γ phase and equal velocities in close-packed directions of α and γ allows the c_{ij}'s and shear modulus of a spinel-structure silicate to be estimated from c_{11} of the corresponding α phase and the bulk modulus of the γ phase.

Original language | English (US) |
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Pages (from-to) | 261-272 |

Number of pages | 12 |

Journal | Physics and Chemistry of Minerals |

Volume | 10 |

Issue number | 6 |

DOIs | |

State | Published - May 1984 |

Externally published | Yes |

## ASJC Scopus subject areas

- Materials Science(all)
- Geochemistry and Petrology