Phase Relations in the Na2CO3–ZrO2–SiO2–H2O System at 0.1 and 0.05 GPa and 450°C

The phase relations in the Na2CO3–ZrO2–SiO2–H2O system were studied at 0.1 and 0.05 GPa and 450°C using large-particle-size and nanocrystalline zirconias. Four silicates were obtained when use was made of readily soluble ZrO2(nanocr): ZrSiO4 , Na2ZrSi6O15 · 3H2O, Na2ZrSi3O9 · 2H2O, and Na4Zr2Si5O16 · H2O. In the system containing poorly soluble ZrO2(cr), only Na2ZrSi6O15 · 3H2O was found to crystallize. It is shown that the structures of all the Na–Zr silicates contain invariant six-polyhedron structural precursors, each made up of two ZrO6 octahedra and four SiO4 tetrahedra, and belong to a homologous series of structures based on the silicate Na2ZrSi2O7, which forms via direct packing of cyclic subpolyhedral precursors.