We have experimentally determined the equilibrium boundary of the reaction forsterite (Fo) + periclase (Per) = anhydrous phase B (Anh-B) at 900¿--1600¿C, and we used the retrieved Gibbs free energy of formation of Anh-B to calculate the stability field of Anh-B + stishovite (Sti) with respect to the Mg2SiO4 polymorphs. The results suggest the possibility of the reaction sequence wadsleytite → Anh-B + Sti→ ringwoodite with increasing pressure at temperature below ~600¿C. This might lead to an eye-shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab, such as would prevail in a 140 Myr old slab subducting at an angle of ~60¿ with a rate of ≥ 14 cm/yr (e.g., Tonga). The equilibrium boundary of Anh-B + Sti with respect to the wadsleyite and ringwoodite has, respectively, positive and negative slope in the P-T space. The reaction of Fo + Per to Anh-B seems to offer a viable explanation of the so-called X discontinuity that has been observed at 275--345 km depth in several subcontinental and subduction zone environments. It is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in MgO due to the incongruent melting of olivine at pressures of 6--8 GPa.