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

Abstract

Abstract.The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi308) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaA1Si3Os]) 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 NaGaSi308, grown in a Na2WO4 flux, was ordered by hydrothermal annealing below 820~ 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~ 970 ~ C. The gallium content of the Tao site increases continuously with decreasing temperature. The gallium contents of the Tam and T2m sites decrease smoothly with increasing ordering while the gallium content of the 7"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 T2o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGao.sAlo.sSi3Os) 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.