Mg₂SiO₄-SiO₂ equilibrium phase diagram. Phases at equilibrium for the chemical system Mg₂SiO₄-SiO₂ are shown as a function of chemical composition (weight percent SiO₂) and temperature. Areas that have the horizontal line pattern indicate temperatures and compositions for which two phases are in equilibrium. The orange area indicates temperatures and compositions for which a single phase, silicate liquid (magma), is present at equilibrium. The liquid only region is bounded by saturation curves: saturation with the mineral forsterite (Fo), saturation with the mineral protoenstatite (En), saturation with the mineral cristobalite (Crs), and saturation with a second liquid (Liq2). The En and Crs saturation curves meet at a eutectic point, which marks the lowest temperature that liquid can exisit in this system. The Fo and En satuation curves meet at a peritectic point where Fo reacts with the liquid to produce En as the temperature is decreased. Between 1695°C and 2150°C, there is liquid immiscibility for SiO₂-rich liquids. This diagram was drawn from data obtained in high temperature experiments by Bowen and Andersen (1914) and by Grieg (1927). The high temperature boundaries of the two-liquid region and the critical point for liquid immiscibility are not well known.    Return to Diagram List