Phase transitions and decomposition relations in calcic plagioclase

A subsolidus temperature-composition diagram for the calcic portion of the system NaAlSi₃O₈-CaAl₂Si₂O₈ is derived from the following data: (a) compositions of plagioclase crystallized under a range of metamorphic conditions: (b) TEM observations of coexisting andesine (An₃₉) and bytownite (An₈₈); 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 C1 ⇄ I1 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 I1 ⇄P1 transition and is inferred to be located near An₇₅-An₇₈ at ∼675°C. The C1 ⇄ I1 conditional spinodal is responsible for submicroscopic two-phase intergrowths in labradorites and for Bφggild intergrowths (An₄₅-An₆₅). The I1 ⇄P1 conditional spinodal is responsible for Huttenlocher intergrowths (An₆₆-An₈₇). At temperatures between ∼400° and ∼~575°C, plagioclase in the range of bulk composition An₄0-An₉₀ should produce an equilibrium oligoclase/andesine + 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.