Exsolution by spinodal decomposition in multicomponent mineral solutions

Phase separation in a three-component system that results from the uphill diffusion of chemical components is considered. The binary decomposition model of Cahn and Hilliard is generalized to account for the interdiffusion of several chemical components with considerably different diffusion constants. Thereafter the decomposition dynamics and the phase relations of the final system state are investigated by means of finite-element modeling. Examples from a hypothetical regular solution and from ternary feldspar are addressed. Special attention is given to situations in which different diffusivities affect decomposition dynamics and the final system states. Good qualitative agreement between our modeling and petrographic observations on exsolved feldspar is achieved. Our model explains systematic deviations from equilibrium element partitioning between the two phases exsolving from an initially homogeneous ternary feldspar during slow cooling.