Crystal chemistry of sodium in the Earth's interior: The structure of Na2MgSi5O12 synthesized at 17.5 GPa and 1700 degrees C

The crystal structure and chemical composition of a crystal of Na2MgSi5O12 garnet synthesized in the model system Mg3Al2Si3O12-Na2MgSi5O12 at 17.5 GPa and 1700 degrees C have been investigated. Quantitative analysis leads to the following formula: Na1.98Mg1.00Si5.01O12. Na2MgSi5O12 garnet was found to be tetragonal, space group I4(1)/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) angstrom, V = 1472.5(1) angstrom(3). The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 angstrom for both the sites), whereas Si is ordered at both the Y (mean: 1.793 angstrom) and Z sites (means: 1.630 and 1.624 angstrom). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na2MgSi5O12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages.