The crystal structure of the intermediate phase (3MgO·2CO₂) of synthetic nesquehonite (MgO·3H₂O) following heat treatment was solved by Monte Carlo simulation using powder X-ray diffraction data and was confirmed by Rietveld refinement. The phase is cubic with space group I4¯3m and cell constants of a = 8.516(13) Å. The unit cell consists of independent atoms such as magnesium, carbon, and two oxygen atoms. The magnesium atom is surrounded by six oxygen atoms in octahedral coordination, and four symmetrically identical MgO₆ octahedra in a edge-sharing arrangement form an Mg₄O₁₇ block in the (100) planes of a unit cell. These Mg₄O₁₇ blocks share edges with other Mg₄O₁₇ blocks to form a framework structure. A CO₃ triangle that exists on the three-fold axis connects three Mg₄O₁₇ blocks.