Upper and lower critical points: Thermodynamic constraints on the solution properties of silicate melts

Thermodynamic stability analysis both classical and nonclassical is applied to silicate melts to examine the possible occurrence of two liquid fields with upper and/or lower critical temperatures. The conditions for the existence of an upper critical point in a binary silicate melt are that the second derivatives of the enthalpy and entropy with respect to composition are both negative in supercritical melts. The necessary condition for the existence of a lower critical point is that the excess entropy is negative in near critical melts. The excess entropy of olivine-rich melts is likely to be negative, implying that a two-liquid field with a lower critical point may be encountered in superliquidus peridotite melts.