Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p51a1187n&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P51A-1187
Physics
3947 Surfaces And Interfaces, 5112 Microstructure, 5460 Physical Properties Of Materials
Scientific paper
Evaporation, which plays an important role in chemical and isotope fractionation at low pressures, is a surface reaction, and its kinetics is affected by surface conditions, such as roughness and character of dislocation outcrops. Recent experimental studies [e.g., Nagahara and Ozawa, 1999; 2000] have revealed anisotropy in evaporation kinetics for forsterite and Fe-Mg olivine. In this contribution, we discuss evaporation modes from temperature dependence of the anisotropy in evaporation rates and surface microstructures Experiments were carried out in a vacuum heating chamber. Starting materials are single crystals of Fe-Mg natural olivine (Fo~90) and synthetic forsterite, which are cut into crystallographically oriented rectangular parallelepipeds. The experimental temperature in the present and our previous studies ranged from 1300 to 1650 oC for Fe-Mg olivine and from 1500 to 1890 oC for forsterite. Surface microstructures of experimental residues were observed with FE-SEM, and average evaporation rates are obtained by sample size, weight loss and interior composition profile of residues. The evaporation rate of Fe-Mg olivine is (001)>(100)>(010) at temperatures above ~1500 oC, which is consistent with that of forsterite in the same temperature range, whereas (001)>(010)>(100) below ~1500 oC. The anisotropy is not significant at temperatures above 1750 oC. STM observation on (010) surface shows that the surface is smooth with elementary steps at temperatures below 1750 oC, suggesting layer evaporation. FE-SEM observations on the (100) surface is macroscopically rough above ~1500 oC and smooth below the temperature, although they have partially macro steps below 1750 oC, which are not recognized on the surface above 1750 oC. The crossover of the evaporation rate between (100) and (010), and the vanishing of anisotropy at high temperatures would be induced by (macroscopic) roughening at around 1500 oC for (100), and roughening transition for (100), respectively.
Kawasaki H.
Matsumoto Naoko
Nagahara Hiroki
Ozawa Kazuhito
Tachibana Shogo
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