The width (W) of plagioclase crystallizing from magmas is given by the equation log graphic where t is the crystallization time calculated from Jaeger's model and η is the viscosity of a magma. The values log 2η−1/3 for basaltic and granitic magmas under hydrous and anhydrous conditions are −0.1 and −1.0 for basaltic magmas, and −1.5 and −3.5 for granitic magmas, respectively. The equation can be explained by coarsening, thermal diffusion, and (or) material diffusion processes.

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