1) The document describes a phase diagram for plagioclase feldspars that explains the transition from calcium-rich to sodium-rich plagioclase crystals with decreasing temperature. 2) The phase diagram shows the liquidus and solidus lines that separate the "all liquid", "liquid + crystal", and "all crystal" fields, and indicates how temperature and composition of crystals and melt change during crystallization. 3) If crystallization occurs slowly enough for equilibrium, the composition of crystals and melt will move in tandem down the diagram. But rapid cooling can result in zoned crystals with calcium-rich cores surrounded by progressively more sodium-rich zones.