Mechanism of forsterite evaporation as inferred from surface microstructures

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Evaporation mechanism of synthetic pure forsterite and natural iron-bearing forsterite was experimentally studied in vacuum in the temperature range of 1400-1800°C. Forsterite, which is one of the most important minerals in the planetary system, shows anisotropy of evaporation regarding evaporation rates and surface microstructures. The surfaces develop evaporation pits emanating from dislocation outcrops and smooth or stepped evaporation surface of which evaporation is not enhanced by dislocations. The shape of pits is controlled by anisotropy in evaporation rate and/or that in the surface free energy. Although forsterite and iron-bearing olivine have different type, orientation, and density of dislocation, the characteristics of evaporation pits are common for three crystallographically oriented surfaces. It suggests the larger contribution of anisotropy of surface free energy than the stress energy of dislocations on the shape of the evaporation pits. The estimated evaporation rates, on the contrary, indicate the importance of dislocation, which changes the overall evaporation rate by several factors at the experimental temperature. The change of evaporation rate affects the estimation of timescale of chemical fractionation in the solar nebula.
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