Phase transitions and decomposition relations in calcic plagioclase

A subsolidus temperature-composition diagram for the calcic portion of the system NaAlSilOrCaAl2Si2Os is derived from the following data: (a) compositions of plagioclase crystallized under a range of metamorphic conditions; (b) TEM observations of coexisting andesine (An3e) and bytownite (Anes); and (c) TEM and crystallographic studies of calcic plagioclase from the literature. The most significant feature of the diagram is a proposed existence of two tricritical points and their associated conditional spinodals. One tricritical point is a consequence of the Cl e n transition, which is inferred, from microstructures of plutonic plagioclase, to occur near An73-An75 at-900"C. The other tricritical point is a consequence of the /l = Pl transition and is inferred to be located near An75-An7s at-675"C. The Cl P /l conditional spinodal is responsible for submicroscopic tw-o-phase intergrowths in labradorites and for Bdggild intergrowths (Anqs-Anes). The 1l a Pl conditional spinodal is responsible for Huttenlocher intergrowths (Anoo-Ans7). At tempera-tures between-400 " and-575'C, plagioclase in the range of bulk composition Ana6Ane6 should produce an equilibrium oligoclase/andesine f bytownite assemblage by nucleation and growth. Intermediate compositions are interpreted to be the result of metastable plagioclase growth and/or decomposition to submicroscopic intergrowths within either of the two conditional spinodals. Even the long equilibration times associated with regional metamorphism fail to produce equilibrium plagioclase feldspar compositions