Since mineral dissolution and precipitation are important chemical reactions in geological processes such as metamorphism and hydrothermal alteration, they are considered to be significant for understanding the fundamental mechanisms of Earth scientific phenomena such as volcanic eruptions and earthquakes. The key points in Carbon dioxide Capture and Storage (CCS) and Carbon dioxide Capture, Utilization and Storage (CCUS) technologies can be thought of as the dissolution and precipitation of minerals. This reaction is not only a heterogeneous solid-liquid reaction through the surface layer between a mineral (solid) and water or carbon dioxide (fluid), but also a complex process involving a mixture of physical processes such as adsorption-desorption, chemical reactions, and transfer phenomena (diffusion, advection, etc.) across the interface between the mineral and fluid. When we consider mineral dissolution in terms of stoichiometry, it can be classified into congruent dissolution and incongruent dissolution. Congruent dissolution refers to dissolution that results in dissolved species consistent with the stoichiometry of the host mineral and rock, while incongruent dissolution is dissolution that shows non-stoichiometric behavior.

In many cases, the dissolution of silicate minerals is considered to be an incongruent dissolution. However, it has been suggested that the addition of chelating agents can greatly enhance the dissolution reaction, apparently exhibiting congruent dissolution.

Incongruent dissolution is one of the causes of heterogeneous solid-liquid reactions, but the results may have implications for a wide variety of geochemical processes. On the other hand, the coexistence of chelating substances such as organic acids may accelerate dissolution (weathering), which provides clues to understanding the origin of life and other processes through rock and fluid interactions.

Brilliant rainbow-colored scoriae have been recognized in deposits ejected from the B-fissure vents during the 1986 Izu-Oshima eruption. We hypothesize that the microtexture on the surface of the scoriae forms a structure that causes the colors. Based on microscopic observations, we found precipitates of approximately ∼ 1 µm thickness on the surface of the rainbow-colored scoriae. The precipitates consist of aggregates of spherulites, which are a composite mainly of polygonal Fe-oxide(s), dendritic Fe-silicate(s), and Mg-Ca-bearing mineral(s). The sizes of the spherulites range from 0.07-0.29 µm, 0.07-0.55 µm, and 0.16-0.81 µm for blue, yellow, and red areas, respectively. The areas showing metallic brilliance have larger Fe-oxide(s) areas, leading to a high reflectivity. The colors did not change for respective areas with different observation angles, indicating non-iridescent structural color from a randomly arranged component. Beneath the crystallized precipitates, Na-rich silicate glass (∼ 2 µm) was found to exist above the original silicate glass. These textural and compositional characteristics can be explained by the oxidation of basaltic glass at a high temperature in an SO₂-bearing gas and the subsequent rapid cooling of the glass. These results indicate that the structural colors are produced by randomly arranged spherulites with the size of visible wavelengths that crystallized near the glass surface. Thus, rainbow-colored scoriae record the high temperatures oxidation and rapid cooling, which can be attained inside an eruption column. The colors of scoriae can be an indicator for magma-air interactions during an eruption.

The Kawai-type multi-anvil press (KMA) is one of the powerful high-pressure appraratus to investigate the Earth’s interior. Because KMA can produce a stable pressure-temperature field in a relatively large sample volume, it provides reliable results. Here I introduce recent progress on studies of deep earth science, especially showing my research achievements. I have developed KMA techniques for ultra-high pressure generation and in-situ X-ray diffraction, which have been applied for studies on phase relations and crystal chemistry of mantle-constituent minerals up to mid-mantle conditions. I have studied crystal chemistry of bridgmanite up to 52 GPa in pyrolite and basaltic systems, and found that oxygen vacancy components in bridgmanite disappear around 40 GPa corresponding to 1000 km depth. This phenomenon can explain stagnation of subducting slabs around 1000 km depth. In-situ X-ray diffraction techniques to precisely and accurately determine phase stability have been developed and applied to determine bridgmanite-forimng reaction boundaries: Mg₂SiO₄ post-spinel, MgSiO₃ akimotoite-bridgmanite, and Mg₃Al₂Si₃O₁₂ post-garnet transitions. These studies provide insights into the structures and dynamics between 660 and 1000 km depths. In addition, I also introduce my recent studies on deep water cycle in the mantle: high-pressure phase relations and water partioning of mantle minerals under hydrous conditions. Water partitioning between nominally anhydrous minerals (NAMs) of olivine and its high-pressure polymorphs and hydrous phases has been studied. The results indicate that these NAMs are nearly dry when coexisting with hydrous minerals, resulting in factors of deep-forcus earthquakes and slab stagnation in a wet subducting plate. Phase relations of aluminous silica minerals under hydrous conditions have been also studied. I found that CaCl₂-type aluminous silica is stable even at top-lower mantle conditions and can accommodate more than 1 wt% water in the crystal structure, suggesting deep water cycle by silica minerals.