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Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters |
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| Authors: | Nobuyuki Aikawa Mineo Kumazawa Masayasu Tokonami |
| Institution: | 1. Department of Geosciences, Faculty of Science Osaka City University, Sugimoto, 558, Osaka, Japan 2. Geophysical Institute, Faculty of Science University of Tokyo, Yayoi, 113, Tokyo, Japan 3. Mineralogical Institute, Faculty of Science University of Tokyo, Hongo, 113, Tokyo, Japan |
| Abstract: | The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo67Fa33, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, K D=(Fe+2/Mg) M1/(Fe+2/Mg) M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe+2 and Mg is nearly random at low temperatures, whereas Fe+2 shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine. |
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