PHASE-RELATIONS IN NA2O-SIO2 AND K2SI4O9 SYSTEMS UP TO 14 GPA AND SI-29 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 K2SI4O9, Na2Si2O5, Na2Si3O7 and Na2Si4O9 systems up to 14 GPa. Several high-pre ssure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and Si-29 MAS NMR techniques; In the K2Si4O9 system, the wadeite (K2ZrSi3O9)-type p hase was found to be stable and melt congruently at least up to 12 GPa . In the Na2Si2O5 system, phase C, a Na2Si2O5 polymorph previously rep orted at 10-40 MPa, was observed at 2.5 GPa. However, at 5-6 GPa, a pr eviously unknown phase (epsilon-Na2Si2O5) appeared. This phase was rep laced by yet another new phase(zeta-Na2Si2O5) at 8-10 GPa. In the Na2S i3O7 system, a new high-pressure Na2Si3O7 phase was detected at about 10 GPa. In the Na2Si4O9 system, a new Na2Si3O7 phase appeared at 6-8 G Pa and it decomposed to stishovite (SiO2) plus the high-pressure Na2Si 3O7 phase at greater than or equal to 10 GPa. The Si-29 MAS NMR spectr a revealed that the epsilon-Na2Si2O5 phase contains only tetrahedral S i sites, whereas the zeta-Na2Si2O5, Na2Si3O7 and Na2Si4O9 phases conta in both tetrahedral and octahedral Si sites. Recently, Fleet and Hende rson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na2Si2O5]. J. Solid State Chem., 119: 4 00-404; Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new h igh-pressure silicate structure (Na2Si[Si2O7]). Phys. Chem. Min., 22. 383-386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na2Si3[Si9O27]). Am. Mineral., 81. 1 105-1110.] have determined the structures of the epsilon-Na2Si2O5, Na2 Si3O7 and Na2Si4O9 phases. Subsolidus phase transformations with press ure for these alkali silicate systems can be described in terms of red uction of Si-O-Si angles at lower pressures and formation of octahedra l Si through conversion of nonbridging oxygens to bridging oxygens at higher pressures. Similar structural changes might be expected for alk ali silicate glasses (and melts) within this pressure range. (C) 1998 Elsevier Science B.V. All rights reserved.