High-pressure phase relations in the composition of albite NaAlSi3O8 constrained by an ab initio and quasi-harmonic Debye model, and their implications

Phase relations in the composition of albite NaAlSi3O8 at pressures up to 40 GPa were constrained by a theoretical method that combines the ab initio calculation and quasi-harmonic Debye model. First, the P-T dependence of the thermodynamic potentials of the individual phase, stishovite (SiO2; St), the calcium-ferrite type NaAlSiO4 (Cf), jadeite (NaAlSi2O6; Jd) or the hypothetical hollandite-structured NaAlSi3O8 (Na-Holl) was derived. Our results are generally in consistent agreement with available experimental data and previous theoretical predictions. Second, the Gibbs free energy of the hypothetical Na-Holl phase was compared with that of the phase assemblage Jd + St (JS) or Cf + 2St (CS). Our results show that the Na-Holl phase is not a thermodynamically stable phase over the studied P-T conditions of 0–40 GPa and 100–600 K, which rules it out as a possible intermediate phase along the transition path from the JS phase assemblage to CS phase assemblage. Our calculations have predicted that the JS phase assemblage transforms into the CS phase assemblage at about 33.6 GPa at 0 K, and the Clayperon slope of this phase transition is about 0.014 GPa/K. This study implies that lingunite (Na-Holl), found in some meteorites, is not possibly a thermodynamically stable high-P phase, and the Cf phase probably plays an important role in maintaining the sodium budget and hosting the large-ion lithophile elements in the deep interior of the Earth.