Thermal stability of lithium metasilicate produced under high pressure from lithium disilicate glass

Crystalline lithium disilicate, Li2Si2O5 (LS2c) is usually produced at atmospheric pressure by suitable heat treatment of the glass with the same stoichiometric composition (LS2g). At this pressure, the orthorhombic phase of lithium disilicate is thermodynamically stable. The heat treatment of LS2g under high pressure (7.7 GPa) induces the crystallization of lithium metasilicate Li2SiO3 (LSm) phase, which remains metastable after pressure release. In this work, distinct heat treatments under high pressure were chosen to produce lithium metasilicate crystals with different grain sizes embedded in the glass matrix: smaller than 10 mu m, about 100 mu m and larger than 100 mu m. Thermal stability of this set of samples was investigated during subsequent heat treatments at atmospheric pressure. All samples were analyzed by X-ray diffraction, Raman spectroscopy and optical microscopy. Small LSm crystals produced under high pressure remained detectable only up to 1 hour of heat treatment at 610 degrees C at atmospheric pressure while for longer heating times, the formation of quartz and crystalline LS2c was observed. On the other hand, for the samples containing larger LSm crystals produced under high pressure, the metasilicate phase remained detectable up to 9.5 hours of subsequent heat treatment, forming spherulitic configurations.