Microstructures and symmetries of synthetic and natural（Mg, Fe）SiO3 garnet（majorite）have been investigated by transmission electron microscopy. Modulated and twin structures commonly observed in the synthetic majorite suggest a cubic-tetragonal phase transition due to cation ordering in its octahedral sites during quenching. Natural tetragonal majorite, which was newly discovered in a shock meteorite, has less-ordered octahedral cations than those in the synthetic samples. The order-disorder state of majorite provides important constraints not only on shock metamorphism of meteorites, but also on the deep Earth's mineralogy.
The crystallization process of diamond in the graphite-diamond transformation was studied through detailed microtextural observation of synthetic and natural nano-polycrystalline diamonds （NPDs） by transmission electron microscopy. The transformation occurs by two types of processes, diffusion-controlled （nucleation and growth） process and diffusion-less （martensite） process, which produce different types of microtextures, typically, granular and lamellar （layered） textures. We found that the transformation process and the microtexture of diamonds are largely influenced by the crystallinity of the graphite starting material. This allows us to produce a variety of NPDs with novel microtextures by using graphitic starting materials with various degrees of crystalinity. We also studied impact diamonds from a large meteorite crater as a natural counterpart of NPD and revealed that they formed mainly by the martensitic transformation from single crystalline graphite which were originally contained in the host metamorphic rock at the impact site. Shock-induced fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD.
Various mineral species are produced by geological process. Dynamical processes in the Earth expand varieties of mineral species, and their textures and crystal structures. Mineral textures include historical information about the growth, transition and deformation of the minerals in the Erath. Hence, analyses of mineral textures are conducted macro- to nano-scale using optical and electron microscopies. I present our study about lutecite, which is a kind of aggregate of fine quartz and mogánite crystals, and discuss metastable precipitation processes of minerals in geological conditions.
Structural studies on analogs of the Earth's interior materials can give important insights into the understanding for material construction, physical properties and dynamics in the Earth's interior. In particular, CaGeO3 high-pressure perovskite phase is an excellent low-pressure analog of MgSiO3 bridgmanite, the most dominant constituent in the Earth's lower mantle, with the orthorhombic Pbnm perovskite-type structure. I here review our recent structural research of CaGeO3 perovskite as a function of temperature. The research leads to the earth-scientific implication as to the phase transition of MgSiO3 bridgmanite in the Earth's lower mantle.
High-pressure single-crystal X-ray diffraction measurement of cuprospinel CuFe2O4 was performed at beamline BL10A at the Photon Factory, KEK, Japan. With increasing pressure, the Cu position in the octahedral site moves toward the center of the octahedral coordination, which approaches to the regular octahedral geometry. This geometrical modification enhances the repulsion between eg orbital from the Cu2＋ and electrons from the ligand oxygen atoms. At 4.6 GPa, the cuprospinel with cubic structure is consequently transformed to a tetragonally distorted structure, in which the octahedral coordination is elongated along the c-axis with the Jahn-Teller effect of Cu2＋.
Inelastic X-ray scattering is a powerful tool to measure phonon properties of materials, even under high pressure conditions. This technique has been applied to determining single crystal elasticity. In this paper, we will introduce recent studies to determine elastic stiffness constants of some Earth's lower mantle materials and gold as a pressure marker. Current and future challenges are discussed.
It has been frequently shown that water has important effects on the dynamics of deep Earth interiors. In order to know how water is transported into the Earth's deep interiors and estimate the effects of water on the dynamics, we have been investigating the hydrous minerals under lower mantle conditions. Our first principles calculations have successfully predicted the existence of new hydrous minerals under extreme high pressure conditions. These new hydrous phases suggest the possibility of the transportation of water into the deepest part of Earth's mantle.
Gas hydrates, clathrate compounds, consist of hydrogen-bonded water molecules forming cages and of guest species included in the host cages. A wide variety of gas hydrate exhibits phase changes depending on pressure and guest size, and finally transforms to filled ice structures. Upon phase transition between cage structures, e.g. sⅠ to sH of methane hydrate, a characteristic “cage recombination mechanism” using an analogy from genetic recombination was observed, while upon sH to filled ice structure, typical reconstructive mechanism was observed. At low temperatures and high pressures phase changes induced orientational ordering of guest molecules occurs for methane hydrate and hydrogen hydrate. At high temperatures and high pressures, methane hydrate decomposes to solid methane and ice Ⅶ. The stability region and decomposition curve was obtained.