The metastable phase 9Mg(OH)(2)center dot MgCl2.4H(2)O (9-1-4 phase) was found at the extended metastable isotherm of Mg(OH)(2) in the system MgO-MgCl2-H2O at 120 degrees C and occurs as intermediate binder phase during setting of magnesia cement due to temperature development of the setting reaction. The crystal structure of the 9-1-4 phase was solved from high resolution laboratory X-ray powder diffraction data in space group /2/m (C2/m) (a = 22.2832(3) angstrom, b=3.13501(4) angstrom, c=8.1316(2) angstrom, beta=97.753(1)degrees, V=562.86(2) angstrom(3), and Z=1). Structural and characteristical relations of the phases in the system MgO-MgCl2-H2O can be derived, with which the development of the cement or concrete qualities becomes explainable.