10:45 〜 12:15
[MIS14-P05] 低温流体中におけるマグネシウムケイ酸塩ナノ粒子の水質変質の実験的研究
キーワード：溶解、ナノ粒子、水質変質、星間塵、コンドライト隕石
Sub-μm-sized interstellar silicate dust is thought to be a primary source of solar system materials [1]. However, physical and chemical characteristics of the interstellar silicates are still unclear [2]. Chondritic meteorites, on the other hand, have been studied to search for interstellar signatures in them [3], but various secondary processes that occurred to chondrites prevent us from extracting such information.
Aqueous alteration is regarded as a major secondary process to alter chondritic materials, which happened in their parent bodies [4]. Water is usually presumed as a fluid in the parent bodies [5] and therefore the fluid temperature is usually assumed higher than its freezing point. Considering its fine-grained nature, we predict that interstellar dust is easily altered even at lower temperatures in electrolyte solutions or if the water is supercooled.
In order to estimate the effect of aqueous alteration on nanometer-sized silicate particles, we carried out a preliminary experiment on the reaction of Mg-silicate nanoparticles (NPs) with distilled water at room temperature. Initial Mg-silicate NPs used in this study was produced with an experimental setup designed for laser evaporation and condensation of silicates in ambient gases, as described in our previous paper [6]. We applied a 200 W CO2 laser beam to evaporate a synthetic Mg-silicate pellet for 10 seconds in 1 atm air, and obtained floating condensates. The condensates (Mg-silicate NPs) were collected on a filter paper set in a feedthrough by evacuating the air.
We collected two equally small fractions of the NPs from the filter. One fraction was immersed in 0.03 ml distilled water and the other in 3 ml distilled water inside a separate microtube at room temperature (each solution was named S0.03 and S3, respectively). 0.1 µl of each solution was pipetted from the microtube after prescribed durations for the immersion (45, 90, and 180 mins for S0.03; 10, 25, and 60 mins for S3), placed on a Cu-grid for transmission electron microscopy (TEM), and dried. Another fraction of the initial NPs, not immersed in water, were treated in alcohol for dispersion for a minute, dried quickly on a Cu-grid.
We observed both NPs, ones reacted with distilled water and the other not reacted, with a TEM. The unreacted NPs (Fig. 1) are generally spherical in shape and have 22-210 nm in diameter; although minor in abundance, there are small (< 20 nm), and irregular NPs that coalesce to clumps. Approximately 3 % of the spherical NPs are partially coalesced.
The water-reacted NPs are shown in Figs. 2 & 3. The diameters of spherical NPs are smaller than those of the unreacted NPs, e.g., 15-180 nm for S3 immersed for 10 min (hereafter written S3/10m) and 18-175 nm for S0.03/45m. The size becomes smaller for longer immersion for S3, but it is nearly constant for S0.03. The fraction of partial coalescence among the spherical NPs reaches 100 % for S0.03/45m and for S3/25m.
We frequently find two types of layered structure for spherical NPs after the reaction: a mantle with dense core type and a mantle with a porous core type (Figs. 2 & 3). The first type appears in both S0.03 and S3, and the second type appears only in S3.
For the differences in size and structure of NPs between S0.03 and S3, we attribute its cause to the difference in the amount of distilled water. S0.03 must have become saturated with the dissolved Mg and Si in its earlier time step. In conclusion, the mode of alteration depends on the relative amounts of water to the interstellar silicates.
[1] Williams D.A. (2005) J. Phys.: Conf. Ser. 6, 1. [2] Galliano F. et al. (2018) 56:673-713. [3] Anders E. and Zinner E. (1993) Meteoritics 28:490-514. [4] Bischoff A. (1998) Meteoritics & Planetary Sci. 33:1113-1122. [5] Brearley A.J. (2006) Meteorites and the Early Solar System II, Lauretta D.S. and McSween H.Y. Jr. (eds.), 943, 584-624. [6] Nakano Y. and Hashimoto A. (2020) Prog. Earth Planet Sic. 7, 47.
Aqueous alteration is regarded as a major secondary process to alter chondritic materials, which happened in their parent bodies [4]. Water is usually presumed as a fluid in the parent bodies [5] and therefore the fluid temperature is usually assumed higher than its freezing point. Considering its fine-grained nature, we predict that interstellar dust is easily altered even at lower temperatures in electrolyte solutions or if the water is supercooled.
In order to estimate the effect of aqueous alteration on nanometer-sized silicate particles, we carried out a preliminary experiment on the reaction of Mg-silicate nanoparticles (NPs) with distilled water at room temperature. Initial Mg-silicate NPs used in this study was produced with an experimental setup designed for laser evaporation and condensation of silicates in ambient gases, as described in our previous paper [6]. We applied a 200 W CO2 laser beam to evaporate a synthetic Mg-silicate pellet for 10 seconds in 1 atm air, and obtained floating condensates. The condensates (Mg-silicate NPs) were collected on a filter paper set in a feedthrough by evacuating the air.
We collected two equally small fractions of the NPs from the filter. One fraction was immersed in 0.03 ml distilled water and the other in 3 ml distilled water inside a separate microtube at room temperature (each solution was named S0.03 and S3, respectively). 0.1 µl of each solution was pipetted from the microtube after prescribed durations for the immersion (45, 90, and 180 mins for S0.03; 10, 25, and 60 mins for S3), placed on a Cu-grid for transmission electron microscopy (TEM), and dried. Another fraction of the initial NPs, not immersed in water, were treated in alcohol for dispersion for a minute, dried quickly on a Cu-grid.
We observed both NPs, ones reacted with distilled water and the other not reacted, with a TEM. The unreacted NPs (Fig. 1) are generally spherical in shape and have 22-210 nm in diameter; although minor in abundance, there are small (< 20 nm), and irregular NPs that coalesce to clumps. Approximately 3 % of the spherical NPs are partially coalesced.
The water-reacted NPs are shown in Figs. 2 & 3. The diameters of spherical NPs are smaller than those of the unreacted NPs, e.g., 15-180 nm for S3 immersed for 10 min (hereafter written S3/10m) and 18-175 nm for S0.03/45m. The size becomes smaller for longer immersion for S3, but it is nearly constant for S0.03. The fraction of partial coalescence among the spherical NPs reaches 100 % for S0.03/45m and for S3/25m.
We frequently find two types of layered structure for spherical NPs after the reaction: a mantle with dense core type and a mantle with a porous core type (Figs. 2 & 3). The first type appears in both S0.03 and S3, and the second type appears only in S3.
For the differences in size and structure of NPs between S0.03 and S3, we attribute its cause to the difference in the amount of distilled water. S0.03 must have become saturated with the dissolved Mg and Si in its earlier time step. In conclusion, the mode of alteration depends on the relative amounts of water to the interstellar silicates.
[1] Williams D.A. (2005) J. Phys.: Conf. Ser. 6, 1. [2] Galliano F. et al. (2018) 56:673-713. [3] Anders E. and Zinner E. (1993) Meteoritics 28:490-514. [4] Bischoff A. (1998) Meteoritics & Planetary Sci. 33:1113-1122. [5] Brearley A.J. (2006) Meteorites and the Early Solar System II, Lauretta D.S. and McSween H.Y. Jr. (eds.), 943, 584-624. [6] Nakano Y. and Hashimoto A. (2020) Prog. Earth Planet Sic. 7, 47.