Abstract

Lamellar homogenization rates were used to determine “average” interdiffusion coefficients (D) for seven largely disordered cryptoperthites with relatively straight, coherent lamellae and <1.6 mol% An and for two highly ordered samples with diagonally associated lamellae and about 2 and 5 mol% An. The average D values for the disordered samples at 600°C agree within a factor of ten with the D values calculated from independent tracer dilfusion data using Margules parameters for the coherent solvus to estimate activity of Or. The ordered samples did not homogenize, even at 1000°C, but their average D values at 780°C agree with the calculated D values using the same tracer diffusion data and an activity of Or based on their high-temperature solvi. The results suggest that alkali selfdiffusion rates are simlar for many natural feldspars and the difference in their interdiffusion rates is due primarily to a difference in their thermodynamic properties (activities).

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