Congruent melting kinetics of albite: Theory and experiment

Congruent melting of albite, NaAlSi3O8, is found to be a surface-initiated process, with melt nucleating at external surfaces and internal cracks and cleavage planes. Melting rates are found to be within a factor of 5 of rate s calculated by a normal growth model. The melting point of albite is shown to be in the 1100 degrees to 1120 degrees C range. Melting rates are found to be proportional to the amount of superheat and inversely proportional t o the viscosity of the melt phase. The strong dependence of melting rates o n viscosity is related to the configurational entropy by the Adam ann Gibbs [1965] theory. The normal growth model is shown to be appropriate for desc ribing the congruent melting kinetics of silicates and oxides at moderate a mounts of superheat. Homogeneous melting models are probably predicting the temperature of mechanical instability of the solid, rather than the true m elting point. Surface premelting is discussed and shown to not be related t o the premelting effects described by Richer er al. [1994]. Surface premelt ing is predicted for albite.