Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

Abstract

The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi3O8) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaAlSi3O8]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi3O8, grown in a Na2WO4 flux, was ordered by hydrothermal annealing below 820° C and dry annealing above 820° C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890° C 970° C. The gallium content of the T 1o site increases continuously with decreasing temperature. The gallium contents of the T 1m and T 2m sites decrease smoothly with increasing ordering while the gallium content of the T 2o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913±3° C and 937±3° C. These abrupt changes correlate with changes in the gallium content of the T 2o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGa0.5Al0.5Si3O8) occurs in the temperature range 765° C-850° C, at a temperature intermediate to the transformation in albite (50% order at about 680±20° C) and gallium albite.