Statistical mechanical modeling of the kinetics of order-disorder in omphacitic pyroxenes |
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| Authors: | Charles R Ross II |
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| Institution: | 1. Department of Earth and Space Sciences, University of California, Los Angeles, 90024, Los Angeles, CA, USA
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| Abstract: | A kinetic statistical model based upon a one-dimensional Ising lattice reflecting the nature of the pyroxene octahedral strip, and with nearest-neighbor and mean-field interactions, is applied to the order-disorder transformation in omphacitic pyroxenes. The equilibration temperature of a naturally ordered omphacite and the annealing data of Carpenter (1981a) on omphacites from the same sample, when applied to the model, provide time- temperature-transformation relations for both long-range and short-range ordering. Results indicate that cooling times on the order of tens of millions of years are necessary for the development of significant degrees of long-range order under optimum conditions, whereas short-range order is developed on much shorter time scales. These results are in agreement with observations of naturally ordered and disordered omphacites with the exception of omphacites ordered in a low-temperature subduction-zone regime; the results support the hypothesis that these omphacites crystallize in a highly ordered state. |
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