The coarsening of the coherent lamellar microstructure in a high sanidine-high albite cryptoperthite of Or35 bulk composition was determined experimentally between 560° and 470°C. The lamellar spacing (λ) was measured by transmission electron microscopy. It increases as a function of the isothermal annealing time (t) and is expressed by λ = λ0 + kt1/3, where k is a rate constant and λ0 is the initial lamellar spacing. The maximum temperature for the development of this microstructure is about 570°C, and the lowest temperature practical for experimental study is about 470°C where the lamellae coarsen only 60A/year. Annealing times were for ≤548 days and the experiments were done dry at one atmosphere. An experiment at 560°C and a water pressure of 2 kbar yielded a similar result to the dry experiments. The kinetic data are summarized by k (A/day1/3) = 1.78 × 108 exp (−25000/RT) where R is the gas constant and T is the absolute temperature. These data predict an increase in the lamellar spacing of 5500A for exsolution starting at 560°C and a linear cooling rate of 0.01 degree/year. The potential application of these kinetic data to cryptoperthites in volcanic and shallow intrusive rocks is considered; one of the most important factors is the size of the initial lamellar spacing in natural samples.

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