Recent studies of structural changes of silicate minerals under high temperature and high pressure were reviewed. Recently it was found that several high pressure phases transform to amorphous phase or metastable phase during decompression. Examples are MnTiO₃ perovskite/LiNbO₃ phase transition and C2/c /P2₁/c transition in MgSiO₃ enstatite. In order to study these unquenchable phases it is essential to use in-situ observation technique. Existing phase diagrams based on quench experiments should be reinvestigated. For better understanding of the earth's mantle, structure and physical properties of mantle minerals must be studies at its stable P-T conditions rather than at ambient condition.