ABSTRACT

The spinodal surface and the stable binodal surface must touch tangentially at any extremum/saddle point and along any consolute line. Because of this, the most convenient way to study immiscibility in a ternary is to calculate the spinodal surface. Other authors have accomplished this in a binary regular and subregular solution and in a ternary regular solution but the derived equations are not applicable to a ternary subregular solution or to a Kohler solution. The equation for the spinodal in a ternary Kohler solution is herein derived so that now it is possible to study immiscibility using the spinodal in any ternary Kohler solution, including a subregular ternary solution. MEIJIRING used the spinodal equation to derive relations which characterise the form of segregation in a ternary regular solution. When the “effective” regular solution parameter is substituted into these relations, they can be used as a rough approximation to characterise segregation in a subregular ternary solution; the number of peaks and saddles in the ternary solvus can be predicted but the temperature/composition of them cannot. This rough scheme is used with a graph which shows that the composition of the critical point on a subregular binary solvus is only a function of the ratio of the two subregular parameters. The temperature dependence of subregular solution parameters is capable of exerting a strong control on the unique features of a ternary spinodal, so that a more exact study of immiscibility must involve the calculation of the spinodal surface using temperature dependent solution parameters. A model spinodal for the ternary feldspar system at l kb is calculated using temperature dependent subregular solution parameters.