The stability of magnesian calcites was reexamined, and new results are presented for 28 natural inorganic, 12 biogenic, and 32 synthetic magnesian calcites. The magnesian calcite solid-solutions were separated into two groups on the basis of differences in stoichiometric solubility and other physical and chemical properties. Group I consists of solids of mainly metamorphic and hydrothermal origin, synthetic calcites prepared at high temperatures and pressures, and synthetic solids prepared at low temperature and very low calcite supersaturations () from artificial sea water or NaClMgCl₂CaCl₂solutions. Group I solids are essentially binary s of CaCO₂ and MgCO₂, and are thought to be relatively free of structural defects. Group II solid-solutions are of either biogenic origin or are synthetic magnesian calcites and protodolomites (0–20 and ∼ 45 mole percent MgCO₃) prepared at high calcite supersaturations () from NaClNa₂SO₄MgCl₂CaCl₂ or NaClMgCl₂CaCl₂ solutions. Group II solid-solutions are treated as massively defective solids. The defects include substitution foreign ions (Na⁺ and SO₄²⁻) in the magnesian calcite lattice (point defects) and dislocations (~2 · 10⁹ cm⁻²). Within each group, the excess free energy of mixing, G^E, is described by the mixing model

, where x is the mole fraction of the end-member Ca_0.5Mg_0.5CO₃ in the solid-solution. The values of A₀and A₁ for Group I and II solids were evaluated at 25°C. The equilibrium constants of all the solids are closely described by the equation ln

, where K_C and K_D are the equilibrium constants of calcite and Ca_0.5Mg_0.5CO₃. Group I magnesian calcites were modeled as sub-regular solid-solutions between calcite and dolomite, and between calcite and “disordered dolomite”. Both models yield almost identical equilibrium constants for these magnesian calcites. The Group II magnesian calcites were modeled as sub-regular solid-solutions between defective calcite and protodolomite. Group I and II solid-solutions differ significantly in stability. The rate of crystal growth and the chemical composition of the aqueous solutions from which the solids were formed are the main factors controlling stoichiometric solubility of the magnesian calcites and the density of crystal defects. The literature on the occurrence and behavior of magnesian calcites in sea water and other aqueous solutions is also examined.