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The phase boundary between wadsleyite and ringwoodite in Mg₂SiO₄ composition was determined by in situ observation using synchrotron X‐ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X‐ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=10.32(28)+0.00691(9)×T, where P is pressure in gigapascals and T is temperature in degrees Celsius.