Abstract
Steep compositional gradients in igneous plagioclase feldspar from slowly cooled intrusive bodies imply a maximum value of the intracrystalline diffusion coefficient for NaSi [unknown] CaAl exchange, Dmax ∼10-20 centimeters squared per second for temperatures in the range 1250° to 1000°C. Millimeter-sized grains cannot be homogenized in all geologic time; hence reactive equilibrium crystallization of plagioclase from the melt does not occur in dry systems.