The system CaO-MgO-SiO2-CO2-H2O is an important model for many igneous and metamorphic processes, including the generation and differentiation of carbonatite and associated silicate magmas. We have experimentally established the vapor-saturated solidus for the system CaO-MgO-CO2-H2O from 595¿C/1 kbar to <500¿C/40 kbar, about 25¿C lower than the corresponding temperatures for the CaO-CO2-H2O system. Brucite replaces periclase on the vapor-saturated solidus at about 750 bars pressure, remaining as the stable phase to pressures of at least 40 kbar; no dolomite or magnesite was encountered. At a pressure between 35 and 40 kbar, the assemblage portlandite +brucite+aragonite+vapor changes to one containing phase W, a previously unreported Ca-Mg carbonate. At 20 kbar, the vapor-saturated liquid contains at least 24 wt % H2O. The vapor-saturated solidus for the system CaO-MgO-SiO2-CO2-H2O ranges from 613¿C/1 kbar to 565¿C/10 kbar, experimentally indistinguishable from that for CaO-MgO-CO2-H2O, about 10¿C lower than that for CaO-CO2-H2O, and about 25¿C lower than that for CaO-SiO2-CO2-H2O. In the quinary system, monticellite is replaced by dellaite and an unidentified silicate on the vapor-saturated solidus about 4.1 kbar. MgO-poor liquids are similar in composition to the magma from which the Magnet Cove calcite carbonatite crystallized. More magnesian magmas would produce dolomite at moderate depths, such as at Aln¿ Island.