Measurements of Evaporation Rates of Sulfur from Iron-iron Sulfide Melt

Introduction: We carried out evaporation experiments on Fe-FeS melt and obtained evaporation rates of sulfur from Fe-FeS melt. The Fe-FeS eutectic temperature is about 988°C (1261K), and this temperature could be achieved in the inner region of the solar nebula, and it should be also achieved during the chondrule formation. It is of importance to understand evaporation kinetics of Fe-FeS melt for discussion on the origin of troilite in meteorites, especially in chondrules. Experimental: Evaporation experiments were done in a gold image furnace (ULVAC-RHVE44VHT), in which samples were heated by infrared light focused with gold mirrors [1]. Troilite powder synthesized from pyrrhotite (Fe0.886S) was used as a starting material. The powder was pressed into a pellet (3mm in diameter and ~2mm in height), and its weight was adjusted to 50 ± 0.1mg. An alumina ring (3mm in inner diameter) was used to hold a melt droplet instead of the conventional Pt-wire loop method because Pt reacts with Fe. Experiments were carried out at 1100° and 1200°C and at p(H2)=10 –4 and 10bar for different duration from 0 to 150min. Heating duration was measured after the furnace temperature achieved to the experimental temperature. Results: The weight of the melt droplet decreases with heating duration. The residual sulfur content was calculated from the weight loss assuming that only sulfur evaporates from the melt. The higher temperature, the larger evaporation rates of Fe-FeS melt. Within the compositional range of total melt (34.2–29.3 wt% of S at 1100°C, 36.5–27 wt% of S at 1200°C), the higher sulfur content in the melt, the larger evaporation rates. Discussion on evaporation kinetics: If the chemical reaction on the surface of the melt droplet controls the evaporation, the evaporation rate of sulfur is expressed by the Hertz-Knudsen equation;