Phase relations in Na 2O–SiO 2 and K 2Si 4O 9 systems up to 14 GPa and 29Si NMR study of the new high-pressure phases: implications to the structure of high-pressure silicate glasses

Abstract

Preliminary studies have been made of phase relations in the K 2Si 4O 9, Na 2Si 2O 5, Na 2Si 3O 7 and Na 2Si 4O 9 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 K 2Si 4O 9 system, the wadeite (K 2ZrSi 3O 9)-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na 2Si 2O 5 system, phase C, a Na 2Si 2O 5 polymorph previously reported at 10–40 MPa, was observed at 2.5 GPa. However, at 5–6 GPa, a previously unknown phase ( ε-Na 2Si 2O 5) appeared. This phase was replaced by yet another new phase ( ζ-Na 2Si 2O 5) at 8–10 GPa. In the Na 2Si 3O 7 system, a new high-pressure Na 2Si 3O 7 phase was detected at about 10 GPa. In the Na 2Si 4O 9 system, a new Na 2Si 4O 9 phase appeared at 6–8 GPa and it decomposed to stishovite (SiO 2) plus the high-pressure Na 2Si 3O 7 phase at ≥10 GPa. The 29Si MAS NMR spectra revealed that the ε-Na 2Si 2O 5 phase contains only tetrahedral Si sites, whereas the ζ-Na 2Si 2O 5, Na 2Si 3O 7 and Na 2Si 4O 9 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 [Na 2Si 2O 5]. J. Solid State Chem., 119: 400–404; Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na 2Si[Si 2O 7]). Phys. Chem. Min., 22: 383–386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na 6Si 3[Si 9O 27]). Am. Mineral., 81: 1105–1110.] have determined the structures of the ε-Na 2Si 2O 5, Na 2Si 3O 7 and Na 2Si 4O 9 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 pressures. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.