The lamellar spacings of cryptoperthite phenocrysts sampled across a 5m-wide rhyolite dike have been determined by transmission electron microscopy. The cryptoperthites have a lamellar exsolution microstructure, and the average spacing of the lamellae varies in a regular manner with position in the dike. Near the margins the average spacing is larger (500–630Å) than at the center (460–470Å). The observed spacings are essentially in agreement with the predicted values based on experimental coarsening data and reasonable values for the parameters of heat flow calculations. The larger spacings near the dike margins can be accounted for if the combined initial temperature for the dike and country rock is between 950°–1100°C, and if the latent heat of fusion is between 70–120 cal/gm-deg C. These results limit the initial lamellar spacing to < 300Å, and they are consistent with the available experimental value of 80Å.

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