High-pressure transformations in NaAlSiO 4 and thermodynamic properties of jadeite, nepheline, and calcium ferrite-type phase

Abstract

Phase transitions in NaAlSiO 4 were examined at 3.5–27 GPa and 950–1600 °C using a multianvil-type and a Bridgman anvil-type high-pressure apparatus. The results show that NaAlSiO 4 nepheline first dissociates into an assemblage of NaAlSi 2O 6 jadeite and β-NaAlO 2 with distorted zincite structure at about 4 GPa. At about 7 GPa, this assemblage changes to jadeite plus α-NaAlO 2 with distorted rocksalt structure. Jadeite and α-NaAlO 2 react to form an NaAlSiO 4 phase with calcium ferrite structure at 18–20 GPa, and this boundary has a negative slope. Enthalpies of nepheline and calcium ferrite-type NaAlSiO 4, jadeite, and α- and β-NaAlO 2 were measured by high temperature oxide melt solution calorimetry. The enthalpy for transition of NaAlSiO 4 nepheline to jadeite plus β-NaAlO 2 was 6.2±4.4 kJ/mol and that for the β-α-transition in NaAlO 2 was 23.6±2.4 kJ/mol at 298 K. The enthalpy of formation of NaAlSiO 4 calcium ferrite from jadeite plus α-NaAlO 2 was 79.6±6.9 kJ/mol at 298 K. The calculated boundaries of these transitions from the measured enthalpy data are generally consistent with those by experimental determination, considering the errors of the measured enthalpies and thermophysical properties of the relevant phases and the uncertainty of pressure in the high-pressure experiments. The calculated boundary for dissociation of jadeite to calcium ferrite plus stishovite has a positive slope. The present calculation of the transition boundaries involving NaAlSiO 4 calcium ferrite may suggest that configurational entropy of calcium ferrite possibly due to Al–Si disorder could contribute to the slopes of the boundaries.