Single crystals of Mg2GeO4 spinel, a stable analogue to mantle ringwoodite at atmospheric pressure up to 800°C, were synthesized from Mg2GeO4 olivine in hydrous conditions at 1.9 GPa and ∼1000°C. Infrared (IR) spectra show OH-stretching peaks at 3531 and 3502 cm–1. Quantification of IR spectra reveals hydrogen in the order of 5 to 10 ppm wt. H2O. A complete hydrogen/deuterium exchange was achieved at 700°C leading to a diffusion coefficient of deuterium of ∼3x10–14 m2/s. This value is of the same order as diffusion coefficients observed in other mantle minerals, such as garnet and diopside.

The O–O distance of ∽ 2.9 Å calculated from the spectra of Mg2GeO4 spinel is consistent with the GeO4 tetrahedra edge length known from crystal-structure data. Whereas the hydration mechanism and the site occupancies involved are still in discussion for mantle spinel, the IR measurements on Mg2GeO4 confirm the idea of protonation at the tetrahedral edges.

Kinetic models and rate equations for polymorphic reconstructive phase transformations in polycrystalline aggregates are usually based on the assumptions that (a) the product phase nucleates on grain boundaries in the reactant phase and (b) growth rates of the product phase remain constant with time at fixed P-T. Recent observations of experimentally-induced transformations between (Mg,Fe)2SiO4 olivine (α) and its high pressure polymorphs, wadsleyite (β) and ringwoodite (γ), demonstrate that both these assumptions can be invalid, thus complicating the extrapolation of experimental kinetic data. Incoherent grain boundary nucleation appears to have dominated in most previous experimental studies of the α–β–γ transformations because of the use of starting materials with small (<10–20 µm) grain sizes. In contrast, when large (0.6 mm) olivine single crystals are reacted, intracrystalline nucleation of both β and γ becomes the dominant mechanism, particularly when the P-T conditions significantly overstep the equilibrium boundary. At pressures of 18–20 GPa intracrystalline nucleation involves (i) the formation of stacking faults in the olivine, (ii) coherent nucleation of γ-lamellae on these faults and (iii) nucleation of β on γ. In other experiments, intracrystalline nucleation is also observed during the β-γ transformation. In this case coherent nucleation of γ appears to occur at the intersections of dislocations with (010) stacking faults in β, which suggests that the nucleation rate is stress dependent. Reaction rims of β/γ form at the margins of the olivine single crystals by grain boundary nucleation. Measurements of growth distance as a function of time indicate that the growth rate of these rims decreases towards zero as transformation progresses. The growth rate slows because of the decrease in the magnitude of the Gibbs free energy (stored elastic strain energy) that develops as a consequence of the large volume change of transformation. On a longer time scale, growth kinetics may be controlled by viscoelastic relaxation.

The equation of state of Fo90 hydrous ringwoodite has been measured using X-ray powder diffraction to 45 GPa at the GSECARS beam line at the Advanced Photon Source synchrotron at Argonne National Laboratory. The sample was synthesized at 1400°C and 20 GPa in the 5000 ton multi anvil press at Bayerisches Geoinstitut in Bayreuth. The sample has the formula Mg1.70Fe0.192+ Fe0.023+H0.13- Si1.00O4 as determined by electron microprobe, Fourier transform infrared and Mössbauer spectroscopies, and contains ~0.79% H2O by weight. Compression of the sample had been been measured previously to 11 GPa by single crystal X-ray diffraction. A third-order Birch-Murnaghan equation of state fit to all of the data gives V0 = 530.49±0.07 Å3, K0 = 174.6±2.7 GPa and K' = 6.2±0.6. The effect of 1% H incorporation in the structure on the bulk modulus is large and roughly equivalent to an increase in the temperature of ∼600°C at low pressure. The large value of K' indicates significant stiffening of the sample with pressure so that the effect of hydration decreases with pressure.