Abstract
The adiabatic elastic moduli of single-crystal MgSiO 3 in the tetragonal garnet structure have been measured at ambient conditions using Brillouin spectroscopy. The results (in GPa) are: C 11 = 286.4 ± 1.3; C 33 = 280.1 ± 1.8; C 44 = 85.0 ± 0.7; C 66 = 93.2 ± 1.1; C 12 = 83.0 ± 2.9; C 23 = 104.9 ± 2.4; C 16 = 1.4 ± 1.3. The isotropic properties of MgSiO 3 majorite are K VRH = 159.8 ± 4.4 GPa and μ VRH = 89.7 ± 0.6 GPa. An empirical relationship describing garnet elasticity along the pyrope-majorite join indicates that the bulk modulus increases by about 8% and the shear modulus by about 2% going from majorite to pyrope. The tetragonal garnet structure is characterized by ordering on the octahedral site; axial elastic moduli differ as a result of differing axial arrangements of MgO 6 and SiO 6 units. The overall softening of the garnet structure with increasing majorite content is attributed to the presence of weak magnesium octahedra and may also reflect the extensive cross-linking that is typical of garnets. Two factors contribute to the observed elasticity of majorite: coupled substitution of aluminum by magnesium + silicon, and structural ordering on the octahedral site. In this case, the compositional effect is probably minor, but until we have elasticity measurements on completely disordered ‘cubic’ MgSiO 3 garnet, the relative importance of the two components cannot be easily distinguished.
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