The ab initio study of the stability of low temperature Al/Si ordered albite, NaAlSi3O8

The energetics of different ordering schemes for NaAlSi3O8 (albite) were investigated by both empirical potential and ab initio methods. These computations indicate that the ordered structure of natural low albite, in which aluminum atoms reside on the T-10 site, is favored by 30 meV (2.9 kJ/mel) over the corresponding structure in which aluminum atoms are ordered onto the T-20 site. Per missible lattice relaxation of the T-20 structure, with an associated substantial decrease in the gamma lattice angle, is unique to the T-10 structure and appears to be responsible for its substantially lower enthalpy.