English: Phase diagram for the ternary system MgO - MgCl₂ - H₂O at 23 °C. Recomputed and redrawn from information in: Ladawan Urwongse and Charles A. Sorrell (1980): "The System MgO‐MgCl₂‐H₂O at 23°C". Journal of the American Ceramic Society, volume 63, issue 9-10, pages 501-504; ]

The stable oxychloride phases at 23 °C are P5 or 5:1:8 = 5 Mg(OH)₂ · MgCl₂ · 8 H₂O and P3 or 3:1:8 = 5 Mg(OH)₂ · MgCl₂ · 8 H₂O. At his temperature, the magnesium chloride crystallizes as the hexahydrate MgCl₂·6H₂O

The dark blue region comprises the stable clear solutions. Its vertices are pure water, the saturated Mg(OH)₂ solution SH, the triple equilibrium compositions S1,S2, S3, and the solid magnesium chloride hexahydrate SC. As mass fractions (not molar fractions), they are

SH = 0.0044 MgO + 0.9956 H₂O (Sol:Mg(OH)₂)

S1 = 0.008 MgO + 0.170 MgCl₂ + 0.822 H₂O (Sol:Mg(OH)₂:P5)

S2 = 0.010 MgO + 0.222 MgCl₂ + 0.768 H₂O (Sol:P5:P3)

S3 = 0.012 MgO + 0.345 MgCl₂ + 0.643 H₂O (Sol:P3:MgCl₂·6H₂O)

SC = 0.469 MgCl₂ + 0.531 H₂O (Sol:MgCl₂·6H₂O)

The light blue region is the approximate range of compositions of homogeneous metastable clear solutions and gels that form initially and eventually crystallize as P5, P3, or mixtures with some saturated solution and Mg(OH)₂ or MgCl₂·6H₂O.