Final answer:
As a magma cools, the first plagioclase crystals to form are high in calcium, following Bowen's reaction series from calcium-rich to sodium-rich compositions.
Step-by-step explanation:
The first plagioclase crystals to form as a hot melt cools are high in calcium. This can be understood by considering the Bowen's reaction series, which describes the sequence of mineral crystallization from a magma. Plagioclase feldspars are part of this series, and as they crystallize, they follow a continuous reaction series from calcium-rich to sodium-rich compositions. Initially, as the temperature of the magma begins to decrease, the plagioclase that forms is higher in calcium, with successive crystals becoming progressively more sodium-rich as cooling continues.