Crystalline lithium disilicate, Li 2 Si 2 O 5 (LS 2c ) is usually produced at atmospheric pressure by suitable heat treatment of the glass with the same stoichiometric composition (LS 2g ). At this pressure, the orthorhombic phase of lithium disilicate is thermodynamically stable. The heat treatment of LS 2g under high pressure (7.7 GPa) induces the crystallization of lithium metasilicate Li 2 SiO 3 (LS m ) 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 µm, about 100 µm and larger than 100 µm. 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 LS m crystals produced under high pressure remained detectable only up to 1 hour of heat treatment at 610°C at atmospheric pressure while for longer heating times, the formation of quartz and crystalline LS 2c was observed. On the other hand, for the samples containing larger LS m crystals produced under high pressure, the metasilicate phase remained detectable up to 9.5 hours of subsequent heat treatment, forming spherulitic configurations. K E Y W O R D Sglass-ceramic, high pressure, lithium disilicate, lithium metasilicate, thermal stability

The effect of high pressure and/or high temperature on the structure and mechanical properties of pyrope glass (Mg 3 Al 2 Si 3 O 12 ) was investigated. The local structure of the glass was characterized by the total pair distribution functions derived from synchrotron X-ray diffraction. The short-range order of the Si-O and Al-O pairs was not affected by densificationat 8 Gpa at room temperature. Small irreversible changes were observed in the intermediate range order of the glass, reflected in increasing density from 2.74 to 2.78 g/cm 3 , hardness from 8.1 to 8.7 GPa, and elastic modulus from 86 to 99 GPa. Thermal treatment of the parent glass at atmospheric pressure induced the crystallization of forsterite and cordierite, while thermal treatment in situ at 8 Gpa induced the crystallization of the cubic phase of pyrope, with density of 3.54 g/cm 3 , hardness of 16.4 Gpa, and elastic modulus of 195 GPa. This study helps to understand the effect of high pressure on geologically important materials and provides valuable insights for designing and manufacturing new glass-ceramics containing metastable phases with improved properties. K E Y W O R D Sglass-ceramics, high pressure, mechanical properties, pyrope INTRODUCTIONCertain easy glass-forming compositions, that is, having slow crystallization rate on cooling from the liquid state,