Feldspar crystallisation in felsic magmas; a review
Feldspar crystallisation in felsic magmas; a review (in The second Hutton symposium on the origin of granites and related rocks; proceedings, P. E. Brown (editor) and B. W. Chappell (editor))
Special Paper - Geological Society of America (1992) 272: 399-407
Feldspar compositional paths are influenced only to a minor degree by P. The presence of H (sub 2) O, or other melt components incompatible in feldspar, has a more major effect, not only increasing the T interval over which feldspar crystallizes, but also decreasing the amount of Ab enrichment of the feldspar which occurs during crystallization. The amount and behaviour of H (sub 2) O in the magma has a pronounced influence on feldspar compositions when two feldspars are stable. Under conditions of high bulk H (sub 2) O content or constant activity of H (sub 2) O, plagioclase and alkali feldspar compositions evolve by increasing Ab content similar to the behaviour shown by the simple binary systems. If the bulk H (sub 2) O content is low, however, and the H (sub 2) O content increases during crystallization, plagioclase evolves by increasing Ab content until alkali feldspar is stabilized. From that point on, plagioclase compositions change mainly by decreasing Or content, where alkali feldspar evolves mainly by increasing Or content.