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There has been a lack of data available for Si-rich garnets along the Py-Mj join until recently. A project undertook to synthesize a range of samples across the Py-Mj join using recently developed techniques, characterizing elastic behaviour. New sound velocity data for the pyrope end-member of garnet is reported and compared with previous studies.

We performed in-situ Raman spectroscopy and X-ray diffraction experiments at high temperature and ambient pressure to investigate the intrinsic anharmonic properties of [Beta]-[Mg.sub.2]Si[O.sub.4] and the mechanism and kinetics of its back-transformation to forsterite. High-temperature Raman spectra of [Beta]-[Mg.sub.2]Si[O.sub.4] and its back-transformed products were recorded up to 1200 K. [Beta]-[Mg.sub.2]Si[O.sub.4] persists metastably up to 800-900 K, and the Raman frequency shifts with temperature were determined. Between 800 and 1000 K, new peaks are observed at about 670 and 1020 [cm.sup.-1]. Above 1000 K, a direct transformation to forsterite occurs. The peaks that appear between 800 and 1000 K are attributed to a defective spinelloid that forms as an intermediate phase during the back-transformation of [Beta]-[Mg.sub.2]Si[O.sub.4] to forsterite. Similar features are observed in the Raman spectrum of partially transformed [Gamma]-[Ni.sub.2]Si[O.sub.4] heated at 1073 K and ambient pressure for 10 min. These results indicate that a two-step mechanism, possibly martensitic, is operative in the back-transformation of the [Beta]- and [Gamma]-phases to olivine at low to moderate temperatures and for a large overstepping of the equilibrium conditions. The kinetics of the [Beta]- to [Alpha]-[Mg.sub.2]Si[O.sub.4] back-transformation were monitored between 1023 and 1120 K at ambient pressure using X-ray powder diffraction. For the kinetic data obtained in air, two regimes are evident from an Avrami analysis. The first regime is characterized by an exponent n [approximately equal to! 2 for a low transformed fraction (X < 0.5); the second has n [approximately equal to] 1 for higher transformed fractions. For this second regime, an activation energy of 432 [+ or -] 64 kJ/mol is derived for the growth process from the kinetic data. A smaller data set collected in vacuum indicates much slower transformation rates and suggests a significant effect of the [O.sub.2] or [H.sub.2]O partial pressures on the kinetics. Intrinsic mode anharmonic parameters can be calculated from the Raman frequency shifts with temperature and used to correct vibrational heat capacity models for intrinsic anharmonic effects. These corrections are slightly higher for the [Beta]-phase than for forsterite but the difference is within the experimental error. This indicates that, within the resolution of our experiments, no significant effect of intrinsic anharmonicity on the location and slope of the [Alpha]-[Beta] phase transition can be predicted.

The pressure dependence of the elastic wave velocities for hot-pressed, elastically isotropic polycrystals of the Β (modified spinel) phase of magnesium orthosilicate ([Mg.sub.2][SiO.sub.4]) has been determined at room temperature to [...]

Highlights: [•] Velocities of grossular garnet to 10GPa and 1000K. [•] Ultrasonic interferometry in conjunction with synchrotron X-radiation. [•] Garnet elasticity not sensitive to Ca content except for shear modulus. [Copyright &y& Elsevier]

Abstract: Compressional (P) and shear (S) wave velocities have been measured for two synthetic polycrystalline specimens of pyrope–majorite garnets [Py60Mj40 and Py50Mj50] by ultrasonic interferometry to 8GPa and 1000K, in a DIA-type cubic anvil high pressure apparatus (SAM-85) interfaced with synchrotron X-radiation and X-ray imaging. Elastic bulk (K S) and shear (G) moduli data obtained at the end of the cooling cycles were fitted to functions of Eulerian strain to third order yielding pressure derivatives of the elastic moduli (∂K S/∂P) T =4.3 (3); (∂G/∂P) T =1.5 (1) for Py60Mj40 garnet and (∂K S/∂P) T =4.4 (1); (∂G/∂P) T =1.3 (1) for Py50Mj40 garnet. Both (∂K S/∂P) T and (∂G/∂P) T are identical for the two garnet compositions and are also consistent with Brillouin scattering data for polycrystalline Py50Mj50. Moreover, the new pressure derivatives of the elastic moduli are equal within experimental uncertainties to those of end-member pyrope garnet from ultrasonic studies [Gwanmesia, G.D., Zhang. J, Darling, K., Kung, J., Li, B., Wang, L., Neuville, D., Liebermann, R.C., 2006. Elasticity of polycrystalline pyrope (Mg3Al2Si3O12) to 9GPa and 1000°C. Phys. Earth Planet. Inter. 155, 179–190] and from Brillouin spectroscopic studies [Sinogeikin, S.V., Bass, J.D., 2002a. Elasticity of majorite and majorite–pyrope solid solution to high pressure: implications for the transition zone. Geophys. Res. 9(2), 1017], thereby demonstrating that the pressure derivatives of the elastic moduli are independent of the physical acoustics technique employed and unaffected by substitution of Si for Mg and Al within the Py–Mj solid solution in the range (Py100–Py50) of the present measurements. Temperature dependence of the elastic obtained from linear regression of entire P–T–K and P–T–G data are (∂K S/∂T) P =−14.6 (4)MPa/K; (∂G/∂T) P =−9.4 (4)MPa/K for Py60Mj40 garnet, and (∂K S/∂T) P =−14.6 (4)MPa/K; (∂G/∂T) P =−9.33 (2)MPa/K for Py50Mj50 garnet. These values are essentially identical for the two compositions and are also in excellent agreement with the Brillouin scattering data of [Sinogeikin, S.V., Bass, J.D., 2002b. Elasticity of pyrope and majorite–pyrope solid solutions to high temperatures. Phys. Earth Planet. Inter. 203, 549–555] for Py50Mj50 garnet; however, they are higher than those for pyrope by about 23% for (∂K S/∂T) P and 11% for (∂G/∂T) P [Gwanmesia, G.D., Zhang. J, Darling, K., Kung, J., Li, B., Wang, L., Neuville, D., Liebermann, R.C., 2006. Elasticity of polycrystalline pyrope (Mg3Al2Si3O12) to 9GPa and 1000°C. Phys. Earth Planet. Inter. 155, 179–190]. [Copyright &y& Elsevier]

Abstract: New, high quality data are presented on the elastic properties of β-Mg2SiO4 (wadsleyite) from 295 to 660K at ambient pressure. Elasticity measurements were carried out on a sintered polycrystal using resonant ultrasound spectroscopy (RUS). Room temperature values for the adiabatic bulk (K S) and shear (G) moduli are 170.2(1.9) and 113.9(0.7)GPa, respectively. The K S data exhibit linear dependence on temperature (T) with (∂K S/∂T) P =−1.71(5)×10−2 GPaK−1. Our result for (∂K S/∂T) P is consistent with a relatively high magnitude for this derivative which contrasts with −1.20×10−2 to −1.30×10−2 GPaK−1 reported in some earlier studies. The average (∂G/∂T) P =−1.57(3)×10−2 GPaK−1 over the temperature range studied. This result is consistent with most earlier measurements of (∂G/∂T) P for wadsleyite. The (∂K S/∂T) P and average (∂G/∂T) P for wadsleyite over 295–660K are not measurably affected by the presence of iron as seen from comparing our results with those from a RUS study on β-(Mg0.91Fe0.09)2SiO4. Further, our results for (∂K S/∂T) P and average (∂G/∂T) P are consistent with olivine content of 44–54% at 410-km depth in Earth, given other assumptions made in recent studies about properties of the α- and β-olivine phases for P, T conditions at that depth. Our G(T) data appear to exhibit small, but persistent, nonlinear behavior; the magnitude of (∂G/∂T) P increases with T. We discuss the important implications on upper mantle mineralogy if this nonlinear effect is confirmed and is demonstrated to apply when extrapolating beyond the temperature range of 295–660K. [Copyright &y& Elsevier]

Abstract: Acoustic wave velocities for synthetic polycrystalline pyrope (Mg3Al2Si3O12) were measured to 9GPa and temperatures up to 1000°C by ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil DIA-type apparatus (SAM-85). Specimen lengths at high pressures (P) and temperatures (T) are directly measured by X-radiographic methods. Elastic wave travel times and X-ray diffraction data were collected after heating and cooling at high pressures to minimize effect of non-hydrostatic stress on the measurements. A linear fit to the high P and T data set yields the elastic bulk and shear moduli [K S =175 (2)GPa; G =91 (1)GPa] and their pressure and temperature derivatives [; G′=1.7±0.2 and (∂K S /∂T) P =−18 (2) MPa/K; (∂G/∂T) P =−10 (1) MPa/K]. In a separate analysis, the pressure–volume–temperature data collected during these acoustic experiments were fit to a high temperature Birch–Murnaghan (HTBM) equation [with K′ fixed at 3.9] and to each isothermal P–V–T data yielding (∂K T /∂T) P =−22 (2) MPa/K and (∂K T /∂T) P =−20 (5) MPa/K, respectively. Comparison of Py100 data with those other Py–Mj compositions indicates that the thermo elastic properties are insensitive to majorite content in the garnet along the pyrope–majorite join. [Copyright &y& Elsevier]