Alkali-feldspars: Which solvus?

Variation in published alkali-feldspar solvus curves is discussed in the light of the concept of complete (crystal-crystal) and exchange (crystal-fluid-crystal) equilibrium. Exchange equilibrium may lead to very regular solvus-like two-phase curves differing substantially from the true binodal solvus; certain experimental strategies tend to favour the attainment and persistence of the exchange equilibrium condition. New long duration experiments under “alkali excess” and “alkali+silica excess” conditions did not yield fledspar pairs significantly off the bracketed binodal obtained by Smith and Parsons (1974) but exchange equilibrium behaviour is shown to influence the course of equilibration in peralkaline experiments. Agreement in solvus curves obtained by bracketing experiments by several workers is excellent (±2 mol. % Or) with a straight linear relationship between T _crit and P such that dT _crit/dP≈16°/Kbar. These curves do not exhibit breaks or sensitivity to chemical environment, and there is no evidence that the solvus changes shape with increasing P.