11:15 AM - 11:30 AM
[PCG21-09] Evaporation experiments of troilite (FeS) under protoplanetary disk conditions
Keywords:troilite, evaporation, elemental fractionation, protoplanetary disk, kinetics
Introduction
In different chemical groups of chondrites, the abundance of sulfur, one of the major elements, is known to be lower than that in CI chondrites, which have a chemical composition close to the solar elemental abundance [1]. This depletion is thought to be due to the volatile nature of sulfur. The primary sulfur-bearing mineral in chondrites is troilite (FeS), and its incongruent evaporation (selective evaporation of sulfur) lead to sulfur loss from solid materials. It would thus be possible that sulfur fractionation in chondrites occurred due to the incongruent evaporation of troilite in the Sun’s protoplanetary disk before the accretion of their parent bodies. The kinetics of incongruent evaporation of sulfur from troilite was investigated by Tachibana and Tsuchiyama (1998) [2], but their experiments were conducted at higher temperatures (1173 K) than those expected for FeS evaporation in protoplanetary disks. Thus, extrapolating their experimental data to disk conditions may involve considerable uncertainty. In this study, we conducted evaporation experiments on troilite under lower temperature (<1173 K) and lower hydrogen pressure (<5 Pa) conditions than in previous studies. The goal of this study is to determine the incongruent evaporation rate of troilite in protoplanetary disk and to understand its temperature and hydrogen pressure dependence.
Methods
The starting material used in this study was troilite from the Canyon Diablo IAB iron meteorite. The troilite was ground into a fine powder using an agate mortar, and approximately 1 mg (0.7-1.5 mg) was used for an experiment. The sample was placed in a silica glass tube vacuum chamber, evacuated to approximately 10^-4 Pa using a turbomolecular pump, and then filled with hydrogen gas to adjust the chamber pressure to either 1 Pa or 5 Pa. The sample was then heated externally using a Kanthal A-1 coil (1123 K for a heating duration of 3-50 hours). The starting material and experimental products were analyzed using SEM-EDS (JEOL JCM-7000) to observe surface morphology and polished cross-sections and to determine chemical compositions. Phase identification of the experimental products was performed using XRD (Rigaku MiniFlex600-C). Since the evaporation residue was found to be metallic iron (α-Fe), a calibration curve was prepared based on the d-spacing of the FeS (114) plane and the intensity of the α-Fe (110) plane. The amount of α-Fe in the experimental products was determined from this calibration curve, allowing for a quantitative evaluation of the evaporation degree. The evaporation degree was also estimated from weight changes before and after heating, assuming that only sulfur was evaporated.
Results and Discussion
Even under lower temperature conditions than in previous studies [2], selective sulfur loss from troilite occurred, leaving metallic iron as an evaporation residue. The residual metallic iron did not completely cover the entire troilite surface but exhibited a partial coverage structure, which is also consistent with the observations in [2]. This suggests that the evaporation process is rate-limited by the decomposition reaction at the FeS surface. The time variation of the evaporation amount estimated from XRD and weight measurements was also consistent with a surface reaction-limited evaporation model. The evaporation rate obtained at a hydrogen pressure of 1 Pa agreed well with the extrapolated low-temperature rate from [2]. The evaporation rate obtained at a hydrogen pressure of 5 Pa was approximately twice that at 1 Pa, showing a weaker dependence on hydrogen pressure than predicted by the reaction equation (proportional to hydrogen pressure). A similarly weak hydrogen pressure dependence was reported in [2], suggesting that under the present experimental conditions, not only the thermodynamically predicted evaporation as H2S but also evaporation as S2 may be occurring.
[1] Lodders K. (2021) Space Sci. Rev. 217, 44. [2] Tachibana S. and Tsuchiyama A. (1998) GCA 62, 2005.
In different chemical groups of chondrites, the abundance of sulfur, one of the major elements, is known to be lower than that in CI chondrites, which have a chemical composition close to the solar elemental abundance [1]. This depletion is thought to be due to the volatile nature of sulfur. The primary sulfur-bearing mineral in chondrites is troilite (FeS), and its incongruent evaporation (selective evaporation of sulfur) lead to sulfur loss from solid materials. It would thus be possible that sulfur fractionation in chondrites occurred due to the incongruent evaporation of troilite in the Sun’s protoplanetary disk before the accretion of their parent bodies. The kinetics of incongruent evaporation of sulfur from troilite was investigated by Tachibana and Tsuchiyama (1998) [2], but their experiments were conducted at higher temperatures (1173 K) than those expected for FeS evaporation in protoplanetary disks. Thus, extrapolating their experimental data to disk conditions may involve considerable uncertainty. In this study, we conducted evaporation experiments on troilite under lower temperature (<1173 K) and lower hydrogen pressure (<5 Pa) conditions than in previous studies. The goal of this study is to determine the incongruent evaporation rate of troilite in protoplanetary disk and to understand its temperature and hydrogen pressure dependence.
Methods
The starting material used in this study was troilite from the Canyon Diablo IAB iron meteorite. The troilite was ground into a fine powder using an agate mortar, and approximately 1 mg (0.7-1.5 mg) was used for an experiment. The sample was placed in a silica glass tube vacuum chamber, evacuated to approximately 10^-4 Pa using a turbomolecular pump, and then filled with hydrogen gas to adjust the chamber pressure to either 1 Pa or 5 Pa. The sample was then heated externally using a Kanthal A-1 coil (1123 K for a heating duration of 3-50 hours). The starting material and experimental products were analyzed using SEM-EDS (JEOL JCM-7000) to observe surface morphology and polished cross-sections and to determine chemical compositions. Phase identification of the experimental products was performed using XRD (Rigaku MiniFlex600-C). Since the evaporation residue was found to be metallic iron (α-Fe), a calibration curve was prepared based on the d-spacing of the FeS (114) plane and the intensity of the α-Fe (110) plane. The amount of α-Fe in the experimental products was determined from this calibration curve, allowing for a quantitative evaluation of the evaporation degree. The evaporation degree was also estimated from weight changes before and after heating, assuming that only sulfur was evaporated.
Results and Discussion
Even under lower temperature conditions than in previous studies [2], selective sulfur loss from troilite occurred, leaving metallic iron as an evaporation residue. The residual metallic iron did not completely cover the entire troilite surface but exhibited a partial coverage structure, which is also consistent with the observations in [2]. This suggests that the evaporation process is rate-limited by the decomposition reaction at the FeS surface. The time variation of the evaporation amount estimated from XRD and weight measurements was also consistent with a surface reaction-limited evaporation model. The evaporation rate obtained at a hydrogen pressure of 1 Pa agreed well with the extrapolated low-temperature rate from [2]. The evaporation rate obtained at a hydrogen pressure of 5 Pa was approximately twice that at 1 Pa, showing a weaker dependence on hydrogen pressure than predicted by the reaction equation (proportional to hydrogen pressure). A similarly weak hydrogen pressure dependence was reported in [2], suggesting that under the present experimental conditions, not only the thermodynamically predicted evaporation as H2S but also evaporation as S2 may be occurring.
[1] Lodders K. (2021) Space Sci. Rev. 217, 44. [2] Tachibana S. and Tsuchiyama A. (1998) GCA 62, 2005.