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

Details 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 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

Apr 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998pepi..107....9k&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 107, Issue 1-3, p. 9-21.

Physics

4

Scientific paper

Preliminary studies have been made of phase relations in the K2Si4O9, 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 (ɛ-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 >=10 GPa. The 29Si MAS NMR spectra revealed that the ɛ-Na2Si2O5 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: 400-404 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 ɛ-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 pressures. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.

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