A subsolidus temperature-composition diagram for the calcic portion of NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 is derived from: (a) compositions of plagioclase crystallized under a range of metamorphic conditions; (b) TEM observations of coexisting andesine and bytownite; and (c) TEM and crystallographic studies from the literature. Proposed existence of two tricritical points: one tricritical point is a consequence of the C1 (super -) <--> I (super -) transition; the other tricritical point is a consequence of the I1 (super -) <--> P1 (super -) transition. The C1 (super -) <--> I1 (super -) spinodal is responsible for two-phase intergrowths in labradorites and for BOeggild intergrowths (An 45 -An 65 ). The I1 (super -) <--> P1 (super -) spinodal is responsible for Huttenlocher intergrowths (An 66 An 87 ).--Modified journal abstract.

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