Cation diffusion in plagioclase feldspar.
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, D(max) approximately 10(-20) centimeters squared per second for temperatures in the range 1250 degrees to 1000 degrees 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.
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