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In-situ water-immersion experiments on amorphous silicates in the MgO-SiO2 system : implications for the onset of aqueous alteration in primitive meteorites

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Title: In-situ water-immersion experiments on amorphous silicates in the MgO-SiO2 system : implications for the onset of aqueous alteration in primitive meteorites
Authors: Igami, Yohei Browse this author
Tsuchiyama, Akira Browse this author
Yamazaki, Tomoya Browse this author
Matsumoto, Megumi Browse this author
Kimura, Yuki Browse this author →KAKEN DB
Keywords: Amorphous silicate
Magnesium silicate hydrate (M-S-H)
Aqueous alteration
Carbonaceous chondrite
In-situ experiment
Issue Date: 15-Jan-2021
Publisher: Elsevier
Journal Title: Geochimica et cosmochimica acta
Volume: 293
Start Page: 86
End Page: 102
Publisher DOI: 10.1016/j.gca.2020.10.023
Abstract: Amorphous silicates, abundant in primitive carbonaceous chondrites, are among the most primitive materials from the early Solar System. They show evidence of some aqueous alteration in the meteorite parent bodies, but it is not clear how this highly reactive material changed at an early stage after contact with water. Herein, we report in-situ experiments on the aqueous alteration of amorphous silicate nanoparticles (typically 70 nm in diameter); we used two different compositions that are similar to forsterite (MgO/SiO2 = 2.02) and enstatite (MgO/SiO2 = 1.15) in the simple MgO-SiO2 system to under-stand basic reaction principles at the onset of the aqueous alteration. The experiments were performed in pure water at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM), and pH measurements. The in-situ TEM images of the nanoparticles in particular those with the forsterite composition gradually became difficult to recog- nize in water. The pH value of the water also increased with time, suggesting that preferential Mg2+ dissolution occurred from the amorphous silicates right after mixing with water. The in-situ XRD patterns showed that magnesium silicate hydrate (M-S-H), which is a poorly crystalline phase like a phyllosilicate, newly appeared. The M-S-H seems to have been formed via a dissolution-precipitation process. Its formation rate from amorphous silicates was considerably higher than from crystalline silicates, because amorphous silicates are highly metastable and have high solubility in water. M-S-H formation from the for-sterite composition, which has a highly unstable amorphous structure, is ten times faster than from the enstatite composition. The M-S-Hs show string-like or tiny fragmental textures in the final dried products that are very similar to those observed in the matrices of some primitive carbonaceous chondrites. M-S-H would have been the initial product formed in the aqueous alteration of amorphous silicates in the meteorites; thus, it is an important tracer of early aqueous activity at low temperatures in the early Solar System. By comparing the in-situ observations with those obtained after drying the experimental samples, we found two types of M-S-Hs: epigenetic M-S-Hs which have a slightly Si-rich composition formed during drying, and syn- genetic M-S-Hs formed by in-situ alteration. Carbonaceous chondrites may also contain these two types of hydrous silicates, and this should be investigated to understand the conditions for aqueous alteration in the early Solar System in more detail. The present study clearly showed the importance of Mg/Si ratio in the precursor materials, although the actual chondrites are in more complicated multi-component system. Future experiments based on the present results can extend the investigation to the system containing Fe, S, and other components as in carbonaceous chondrites. (C) 2020 Elsevier Ltd. All rights reserved.
Rights: ©2021. This manuscript version is made available under the CC-BY-NC-ND 4.0 license
Type: article (author version)
Appears in Collections:低温科学研究所 (Institute of Low Temperature Science) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

Submitter: 木村 勇気

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