High-pressure phase relations and thermodynamic properties of hexagonal aluminous phase and calcium-ferrite phase in the systems NaAlSiO 4–MgAl 2O 4 and CaAl 2O 4–MgAl 2O 4

Abstract

Phase relations in the system NaAlSiO 4–MgAl 2O 4 were determined at 11–30 GPa at 1273–1873 K, using multianvil apparatus. At 1873 K, calcium-ferrite solid solution in the compositional range of (1 − x)NaAlSiO 4· xMgAl 2O 4 (0 ≤ x ≤ 0.3) is formed above 17 GPa, and hexagonal aluminous phase is stable in the compositional range of 0.5 ≤ x ≤ 0.7 above 13.5 GPa. The hexagonal aluminous phase becomes nonstoichiometric with increasing MgAl 2O 4 component from x = 0.5 due to substitution mechanisms involving cation vacancy. In the composition of 0.3 ≤ x ≤ 0.5, Na-rich calcium-ferrite and Mg-rich hexagonal aluminous phase coexist. In 50%NaAlSiO 4 50%MgAl 2O 4 composition (mol%), MgAl 2O 4 spinel + NaAlSi 2O 6 jadeite + α-NaAlO 2 reacts to form a single hexagonal phase (see Reaction (1)) at 13–14 GPa at 1273–1873 K. In 67%MgAl 2O 4 33%CaAl 2O 4 composition, MgAl 2O 4 spinel + CaAl 2O 4 calcium ferrite changes to a single hexagonal phase (see Reaction (2)) at 13–14 GPa at 1273–1673 K. The two hexagonal phases of Na 0.5Mg 0.5Al 1.5Si 0.5O 4 and Mg 0.67Ca 0.33Al 2O 4 are stable up to at least 30 GPa at 1873 K. By high-temperature drop-solution calorimetry, enthalpies at 298 K of Reactions (1) and (2) to form hexagonal phases were obtained to be 54.6 ± 1.6 and 36.8 ± 2.3 kJ/mol, respectively. Isobaric heat capacities ( C p ) and entropies ( S° 298) of hexagonal phases of Na 0.5Mg 0.5Al 1.5Si 0.5O 4 and Mg 0.67Ca 0.33Al 2O 4 were calculated by Kieffer models, based on Raman spectra and C p measured by a differential scanning calorimeter. The calculated S° 298 of hexagonal phases of Na 0.5Mg 0.5Al 1.5Si 0.5O 4 and Mg 0.67Ca 0.33Al 2O 4 are 86.7 and 88.0 J/(mol K), respectively. Using the above enthalpies and entropies, P– T boundaries for formation of Na 0.5Mg 0.5Al 1.5Si 0.5O 4 and Mg 0.67Ca 0.33Al 2O 4 hexagonal phases from the low-pressure phase assemblages were calculated. The calculated boundaries are generally consistent with high-pressure experimental data within the errors. The measured enthalpies and molar volumes suggest that hexagonal phase of Na 0.5Mg 0.5Al 1.5Si 0.5O 4 would transform to calcium ferrite at pressure in the upper half of the lower mantle.