Disclaimer: machine translated by DeepL which may contain errors.

Running through Space-Environment-Resources from the State of Elements in Nano-Minerals

Yoshio Takahashi, Professor, Department of Earth and Planetary Science

What do you think of when you hear the word "clay mineral"? Clay minerals are minerals formed when rocks react with water (weathering reaction) and dissolved silicon and aluminum (and magnesium in extraterrestrial materials and oceanic crust) precipitate as oxides. Clay minerals have a two-dimensional layered structure like mica, and clay minerals called smectite, in particular, take in various cations between the layers (ion exchange reaction, adsorption reaction: Figure). This reaction occurs wherever there is water, but to understand it correctly, the stability of adsorbed ions depends on whether hydrated ions in water are adsorbed on clay minerals as they are (exosphere complex) or dehydrated (endosphere complex).

For example, cesium (Cs) released by the nuclear power plant accident is a large ion, so it forms an endospheric complex. The endospheric complex is stable and does not exchange with sodium (Na) ions or calcium (Ca) ions coexisting in the water. Therefore, radioactive Cs deposited on the soil surface as aerosols from the nuclear power plant hardly move from within 5 cm of the soil surface even more than 10 years after the accident. The geology of the Abukuma region of Fukushima Prefecture, which was affected by the accident, is mainly granite, and the surface soil contains many clay minerals formed by weathering of the granite. This property of clay minerals is the reason why, at the time of the accident, 5 cm of the surface layer of soil was removed to decontaminate school yards and other areas.

Another example is rare earths, which are indispensable for various advanced devices and indispensable for achieving the SDGs. The rare earth resources that have been attracting the most attention in the world recently are called ion-adsorption deposits, which are also mainly clay minerals. Rare earths are +3-valent ions, so they are relatively strongly adsorbed between clay mineral layers. However, since the ionic radius is smaller than that of Cs, they remain hydrated in the interlayer and have no direct chemical bond with clay minerals. In the ion-adsorption deposits in China, rare earth elements can be recovered and refined at room temperature and without particularly large energy consumption by flowing electrolyte solutions with high concentrations of other cations through these weathered granite formations, making them a major source of rare earth elements in the world. Thus, understanding the reaction of cations to clay mineral interlayers has a great deal to do with understanding and solving environmental and resource problems.

Research on the same reactions has also contributed to the restoration of the water environment once maintained by C-type ^{asteroids*Note 1} such as Ryuguu. In fact, Ryuguu is also mainly composed of clay minerals, and the ionic composition of the water in the past can be estimated from the ionic composition of its interlayer. Then, by analyzing the chemical equilibrium of precipitation reactions of minerals on the asteroid based on the composition, the pH and redox state (Eh) of the water at that time can be estimated. The water quality of Ryuguu was thus estimated to be reductive and alkaline. This is a condition that facilitates the polymerization of amino acids and the non-living synthesis of nucleobases, and C-type asteroids such as Ryuguu can be said to have been factories for the production of these organic substances.

Various findings are obtained across space and time by physical and chemical laws at the atomic level (molecular geochemistry). Here, from the investigation of adsorption reactions of clay minerals, we have gained knowledge of environmental problems, resource chemistry, and the evolution of asteroids.

Thus, the understanding of atomic-level interactions among clay mineral layers has led to the "environment around us," "essential human resources," and the "evolution of the solar system. The fact that the pursuit of such scientific knowledge ultimately leads to our dreams and safety and security reminds me once again of the interest and importance of science.

Note 1: Asteroids composed mainly of volatile materials such as carbon