High-pressure transformations in NaAlSiO4 and thermodynamic properties of jadeite, nepheline, and calcium ferrite-type phase

Phase transitions in NaAlSiO4 were examined at 3.5-27 GPa and 950-1600degreesC using a multianvil-type and a Bridgman anvil-type high-pressure apparatus. The results show that NaAlSiO4 nepheline first dissociates into an assemblage of NaAlSi2O6, jadeite and beta-NaAlO2 with distorted zincite structure at about 4 GPa. At about 7 GPa, this assemblage changes to jadeite plus alpha-NaAlO2 with distorted rocksalt structure. Jadeite and alpha-NaAlO2 react to form an NaAlSiO4 phase with calcium ferrite structure at 18-20 GPa, and this boundary has a negative slope. Enthalpies of nepheline and calcium ferrite-type NaAlSiO4, jadeite, and alpha- and beta-NaAlO2 were measured by high temperature oxide melt solution calorimetry. The enthalpy for transition of NaAlSiO4 nepheline to jadeite plus beta-NaAlO2 was 6.2 +/- 4.4 kJ/mol and that for the beta-alpha-transition in NaAlO2 was 23.6 +/- 2.4 kJ/mol at 298 K. The enthalpy of formation of NaAlSiO4 calcium ferrite from jadeite plus alpha-NaAlO2 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 NaAlSiO4 calcium ferrite may suggest that configurational entropy of calcium ferrite possibly due to Al-Si disorder could contribute to the slopes of the boundaries. (C) 2002 Elsevier Science B.V. All rights reserved.