Feldspar crystallisation in felsic magmas: A review

Understanding the controls on the evolution of natural feldspars is greatly assisted by coupling experimental determinations of feldspar/melt equilibria with thermodynamic modelling and the calculation of crystallisation paths. Such a combined approach permits the evaluation of the influence of intensive and extensive variables on feldspar compositions. Feldspar compositional paths are influenced only to a minor degree by pressure. The presence of H₂O or other melt component incompatible in feldspar has a more major effect, not only in increasing the temperature interval over which feldspar crystallises, but also in decreasing the amount of Ab enrichment of the feldspar which occurs during crystallisation. The amount and behaviour of H₂O in the magma has a pronounced influence on feldspar compositions when two feldspars are stable. Under conditions of high bulk H₂O content or constant activity of H₂O, plagioclase and alkali feldspar compositions evolve by increasing Ab content similar to the behaviour manifested in the simple binary systems. If the bulk H₂O content is low, however, and the H₂O content increases during crystallisation, plagioclase evolves by increasing Ab content until alkali feldspar is stabilised. From that point on, plagioclase compositions change mainly by decreasing Or content, while alkali feldspar evolves mainly by increasing Or content.