The aim of this work was to investigate the formation of lithium metasilicate (Li2SiO3) under high pressure and high temperature starting from lithium disilicate glass with the stoichiometric composition Li2O·2SiO2 (LS2). Monolithic LS2 glass samples were processed at 4.25 GPa, 4.5 GPa, 5 GPa, 5.5 GPa, 6 GPa and 6.5 GPa and submitted, simultaneously, to heat treatment for nucleation and growth of the crystalline phases. The crystallization of the samples was investigated by X-ray diffraction, Raman spectroscopy and optical microscopy. In the pressure and temperature range investigated, crystalline Li2Si2O5 remained stable up to 6 GPa and the formation of the metasilicate phase Li2SiO3 started at 4.25 GPa coexisting with quartz and coesite up to 6.5 GPa. The segregation of SiO2 phases during the crystallization of Li2SiO3 was a consequence of the lithium deficiency of the starting LS2 composition.