Phase relations in the system NaAlSiO4-NaGaSiO4 |
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| Authors: | Suhua Chen Michael E Fleet Yuanming Pan |
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| Institution: | 1. Department of Earth Sciences, University of Western Ontario, N6A 5B7, London, Ontario, Canada
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| Abstract: | Phase relations in the system NaAlSiO4-NaGaSiO4 to 945° C at 1 kbar P(H2O) are dominated by stability of Na(Al,Ga)SiO4 with the beryllonite-type structure. The nepheline structure is restricted to NaAlSiO4-rich compositions at moderate and high temperature. Structure-composition relationships are controlled by space-fitting requirements of both framework and cavity cations, as in related systems. The two-phase (nepheline-type+beryllonite-type) field has been delineated from the end-member NaAlSiO4 composition up to the peritectic point at about 945° C (and 60 mol% NaGaSiO4), using a volume-composition relationship for the beryllonite-type phase, phase appearance, and electron microprobe analysis. At end-member NaGaSiO4 composition, the beryllonite-type phase is stable to the melting point (902±5° C). At end-member NaAlSiO4 composition, the beryllonite-type?nepheline-type transformation occurs at 348±2° C, and is associated with an increase in molar volume of 2.4% and enthalpy of 5170±40J·mol?1. Thus, end-member NaAlSiO4 nepheline, and probably all sub-potassic nephelines as well, are metastable at very-low geological temperatures. |
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