Measurements of concentration gradients in the boundary layers adjacent to plagioclase-volcanic glass interfaces indicate that plagioclase crystallization rate is diffusion controlled during certain stages of growth. A modified version of the theory of Harloff (1927) is offered as the most likely explanation of oscillatory zoning in plagioclase. Clouded plagioclase may result from the co-precipitation of plagioclase and iron and magnesium rich phases due to a buildup of iron and magnesium in the boundary layer during plagioclase growth. The petrologic significance of plagioclase-magmatic liquid equilibrium pairs is considered.

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