RESUMEN
Jeffbenite (having the same chemical composition of pyrope, ~ Mg3Al2Si3O12, and also known as TAPP phase) is a mineral inclusion only found in diamonds formed between about 300 and 1000 km depth) and is considered a stable phase in the transition zone (410-660 km depth) and/or in the shallowest regions of the lower mantle (around 660-700 km depth). This rare and enigmatic mineral is considered to be a pressure marker for super-deep diamonds and therefore it has a key role in super-deep diamond research. However, the pressure-temperature stability fields for Mg3Al2Si3O12 jeffbenite is unknown and its actual formation conditions remain unexplored. Here we have determined the thermodynamic pressure-temperature stability field for the jeffbenite Mg-end member and surprisingly discovered that it is stable at low pressure-temperature conditions, i.e., 2-4 GPa at 800 and 500 °C. Thus, Mg3Al2Si3O12 jeffbenite is not the high-pressure polymorph of pyrope and is likely a retrogressed phase formed during the late ascent stages of super-deep diamonds to the surface.
RESUMEN
The simple chemistry and structure of quartz together with its abundance in nature and its piezoelectric properties make convenient its employment for several applications, from engineering to Earth sciences. For these purposes, the quartz equations of state, thermoelastic and thermodynamic properties have been studied since decades. Alpha quartz is stable up to 2.5 GPa at room temperature where it converts to coesite, and at ambient pressure up to 847 K where it transforms to the beta phase. In particular, the displacive phase transition at 847 K at ambient pressure is driven by intrinsic anharmonicity effects (soft-mode phase transition) and its precise mechanism is difficult to be investigated experimentally. Therefore, we studied these anharmonic effects by means of ab initio calculations in the framework of the statistical thermodynamics approach. We determined the principal thermodynamic quantities accounting for the intrinsic anharmonicity and compared them against experimental data. Our results up to 700 K show a very good agreement with experiments. The same procedures and algorithms illustrated here can also be applied to determine the thermodynamic properties of other crystalline phases possibly affected by intrinsic anharmonic effects, that could partially invalidate the standard quasi-harmonic approach.
RESUMEN
The seven main crystal surfaces of forsterite (Mg2 SiO4 ) were modeled using various Gaussian-type basis sets, and several formulations for the exchange-correlation functional within the density functional theory (DFT). The recently developed pob-TZVP basis set provides the best results for all properties that are strongly dependent on the accuracy of the wavefunction. Convergence on the structure and on the basis set superposition error-corrected surface energy can be reached also with poorer basis sets. The effect of adopting different DFT functionals was assessed. All functionals give the same stability order for the various surfaces. Surfaces do not exhibit any major structural differences when optimized with different functionals, except for higher energy orientations where major rearrangements occur around the Mg sites at the surface or subsurface. When dispersions are not accounted for, all functionals provide similar surface energies. The inclusion of empirical dispersions raises the energy of all surfaces by a nearly systematic value proportional to the scaling factor s of the dispersion formulation. An estimation for the surface energy is provided through adopting C6 coefficients that are more suitable than the standard ones to describe O-O interactions in minerals. A 2 × 2 supercell of the most stable surface (010) was optimized. No surface reconstruction was observed. The resulting structure and surface energy show no difference with respect to those obtained when using the primitive cell. This result validates the (010) surface model here adopted, that will serve as a reference for future studies on adsorption and reactivity of water and carbon dioxide at this interface.
