Phase transitions among the CaGeO₃ polymorphs (wollastonite, garnet, and perovskite structures): Studies by high‐pressure synthesis, high‐temperature calorimetry, and vibrational spectroscopy and calculation

Abstract

Phase boundaries for the reactions CaGeO3 (wollastonite → garnet) and CaGeO3 (garnet → perovskite) were determined at high pressure and temperature. The enthalpies of these transitions were measured by high temperature calorimetry. These studies indicate a positive dP/dT for the wollastonitegarnet transition and a negative dP/dT for the garnet‐perovskite transition. These PT slopes are further supported by calculations of lattice vibrational entropies based on Kieffer's model and the infrared and Raman spectra of the three polymorphs.

Raman scattering study of the high-temperature vibrational properties and stability of CaGeO3 perovskite

“…The specimens used for this study were taken from the same batch of material that was used for previous calorimetric and spectroscopic studies by Ross et al (1986) and McMillan and Ross (1988). For the present study we selected optically clear specimens (approximately 3~50 gm in linear dimension), that appeared to be single crystals under the petrographic microscope.…”

“…Both compounds crystallize in chain tetrahedral structures at room pressure and display similar transformation sequences to garnet and perovskite phases with increasing pressure (Ringwood and Major 1967;Susaki et al 1985;Ross et al 1986). On decompression, both MgSiO3 and CaGeO3 perovskites are metastable and have ambient structures that are isotypic with GdFeO3 perovskite (Pbnm, D~ 6) (Sasaki et al 1983).…”

Test of the vibrational modelling for the ?-type transitions: Application to the ?-? quartz transition

“…Indeed, the temperature dependent part of the internal energy, and consequently the heat capacity of minerals are essentially controlled by their vibrational properties. Therefore, vibrational spectroscopy is an appropriate escape to calculate the heat capacities of minerals at high-pressure and hightemperature, that are necessary when modelling phase transformations likely to occur withim the mantle (Akaogi et al 1984;Ross et al 1986). However, lattice dynamic calculations are done within the framework of the quasiharmonic approximation whose limitations at high-temperature must be assessed.…”

Melting phase relations in the systems Mg2SiO4–H2O and MgSiO3–H2O and the formation of hydrous melts in the upper mantle

“…The experiments at 13 GPa were carried out only with the 5000 tonne multianvil apparatus. The pressure calibration at 6 GPa was performed using the CaGeO 3 garnet to perovskite phase transition (Ross et al, 1986) for the 500 tonne multianvil apparatus, while the pressure calibration described by Frost et al (2004) (Frost et al, 2004). However, higher uncertainty (100 °C) is considered for experiments where the thermocouple reading failed and temperature was determined based on power-temperature calibrations.…”