Coarsening kinetics of the exsolution microstructure in alkali feldspar

Abstract

A maximum microline-low albite of 33 mole percent Or was prepared by alkali ion exchange at 1000° C and subsequently annealed in air at lower temperatures. The exsolution microstructure produced in these samples was studied by transmission electron microscopy. At or above 600° C the compositional difference of the coherent lamellae increases rapidly and reaches a maximum, corresponding to the coherent solvus, within a few days. The lamellar spacing (λ) increases more slowly and the coarsening rate is given by λ=λ0+kt 1/3; where t is annealing time, k is a constant for a given temperature, and λ0 is the lamellar spacing obtained by extrapolation to zero annealing time. λ0 is 65 A at 600° C and ca. 200 A at 700° C. The latter is near the maximum temperature at which exsolution is observed. The nonzero value of λ0 and its increase with increasing temperature is evidence that the exsolution occurs by a spinodal mechanism. Data for the coarsening kinetics at other temperatures should be useful for estimating the thermal history of cryptoperthites with coherent lamellae.