Phase relations in Na₂O-SiO₂ and K₂Si₄O₉ systems up to 14 GPa and ²⁹Si NMR study of the new high-pressure phases: implications to the structure of high-pressure silicate glasses

Preliminary studies have been made of phase relations in the K₂Si₄₉, Na₂Si₂O₅, Na₂Si₃O₇ and Na₂Si₄O₉ systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and ²⁹Si MAS NMR techniques. In the K₂Si₄O₉ system, the wadeite (K₂ZrSi₃O₉)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na₂Si₂O₅ system, phase C. a Na₂Si₂O₅ polymorph previously reported at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a previously unknown phase (ε-Na₂Si₂O₅) appeared. This phase was replaced by yet another new phase (ζ-Na₂Si₂O₅) at 8-10 GPa. In the Na₂Si₃O₇ system, a new high-pressure Na₂Si₃O₇ phase was detected at about 10 GPa. In the Na₂Si₄O₉ system, a new Na₂Si₄O₉ phase appeared at 6-8 GPa and it decomposed to stishovite (SiO₂) plus the high-pressure Na₂Si₃O₇ phase at 9j 10 GPa. The ²⁹Si MAS NMR spectra revealed that the εNa₂Si₂O₅ phase contains only tetrahedral Si sites, whereas the ζ-Na₂Si₂O₅, Na₂Si₃O₇ and Na₂Si₄O₉ phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na₂Si₂O₅]. J. Solid State Chem., 119: 400-414; Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na₂Si[Si₂O₇]). Phys. Chem. Min., 22: 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na₆Si₃[Si₉O₂₇]). Am. Mineral., 81: 1105-1110.] have determined the structures of the ε-Na₂Si₂O₅, Na₂Si₃O₇ and Na₂Si₄O₉ phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si-O-Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressure. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.