Here I introduce the crystallization and texturing mechanism of nano-polycrystalline diamond (NPD) in the graphite-diamond direct transformation in laboratory and in nature. Two types of transformation processes, diffusion-controlled (nucleation and growth) process and diffusion-less martensitic process are involved in the diamond formation depending on the crystallinity of the initial starting material used. The former process is dominated when using low crystalline graphite/carbon materials and produces a granular texture, while the latter process is favorable when using high crystalline graphite and produces lamellar (layered) texture. Understanding the transformation and texturing mechanism has enabled the texture control of NPD and even opened a window to the production of new varieties of NPDs with novel microtextures. It is also helpful to understand the formation process and origin of the unique microtexture of natural NPD (impact diamond), which was recently identified from the large impact crater in Siberia, Russia. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale are the key factors for accelerating the multiple diamond nucleation and suppressing the overall grain growth to produce the unique nanocrystalline texture of natural NPD.

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I started my research as a petrology student supervised by Shohei Banno and Yoshiyuki Tatsumi at Kyoto University. I described every phenocryst in a single thin section to explain enigmatic plagioclase morphology and obtained PhD by discussion of processes in chemically-zoned magma chamber. I started high-pressure and high-temperature (HPHT) experiments as a postdoc at Ikuo Kushiro's lab at Tokyo, where I conducted partial melting experiments of hydrous mantle peridotite with Kei Hirose and duplicated andesite-dacite-rhyolite magmas by crystal fractionation of hydrous arc basalts. Then I joined the Depths of the Earth lead by John Holloway at Arizona State University (USA) and became the first Japanese who learned how to use multi-anvil type HPHT apparatus in the States. I proposed a choke point of subducting hydrous minerals, hydrous mantle transition and generation of komatiite and kimberlite magmas. After I learned Bassett-type diamond anvil cell from Helene Bureau, Nikolay Zotov, and Hans Keppler at Bayerisches Geoinstitut (Germany), I moved back to Kyoto University. With Kenji Mibe, Masami Kanzaki, Shigeaki Ono, and Kyoko Matsukage, I determined critical endpoints between various magmas and aqueous fluids by use of X-ray radiography and suggested new hypothesis for subduction zone magmatism. I have found seawater-like saline fluid inclusions in mantle xenoliths beneath Pinatubo and others, proposing the importance of being salty in subduction zone fluids.

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The equation of state (EoS) at multi-megabar condition should include a parameter at infinite pressure such as K′_∞ in Keane EoS. The Keane EoS model was adopted for the first time to extract meaningful physical properties for MgSiO₃ post-perovskite (PPv) phase. The thermal EoSs of PPv were determined by using both laser-heated diamond anvil cell and density-functional theoretical techniques, within a multi-megabar pressure range, corresponding to the conditions of a super-Earth's mantle. The experimentally determined Grüneisen parameter, which is one of the thermal EoS parameters, and its volume dependence were consistent with their theoretically obtained values. Both the experimental and theoretical EoS were also found to be in very good agreement for volumes up to 300 GPa and 5000 K, respectively. Our newly developed EoS is applicable to a super-Earth's mantle, as well as the Earth's core-mantle boundary region. On the other hand, the double stage diamond anvil cell (ds-DAC) technique was developed using a focused ion beam (FIB) system in order to generate the Tera pascal (TPa) regime corresponding to conditions of exoplanet's interior. Micro-manufacturing using a FIB system enables us to control shapes of 2^nd stage micro-anvils, process any materials, including nano-polycrystalline diamond (NPD) and single crystal diamond, and assemble the sample exactly in a very small space between the 2^nd stage anvils. This method is highly reproducible and would allow us to open a new frontier.

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Organic minerals are natural organic compounds with both well-defined chemical composition and crystallographic properties; their occurrences show traces of the high concentration of certain organic compounds in natural environments. Thus the origin and formation process of organic minerals will lead us to understand the fate and behavior of the organic molecules in the lithosphere. All of each organic mineral can be classified into the one of following two groups: ionic organic minerals in which organic anions and various cations are held together by ionic bonds, and molecular organic minerals in which electroneutral organic molecules are bonded by weak intermolecular interactions. Karpatite, a natural crystal of coronene (C₂₄H₁₂), is the most typical molecular organic minerals and its crystal/molecular structures and carbon isotopic composition suggests that this mineral was crystallized from PAHs (polycyclic aromatic hydrocarbons)-rich hydrothermal petroleum by hydrothermal activity. In the process of formation of organic minerals, the formation of structural units, such as organic acid anion and PAH molecules, precedes their migration and concentration. The first stage includes the formation or cleavage of C-C bonds, but the latter stage does not. In addition, we have investigated the influence of size, morphology, surface structure, and aggregation state on the reductive dissolution of hematite with ascorbic acid using two types of nanoparticles with average diameters of 6.8 ± 0.8 nm and 30.5 ± 3.5 nm, referred to as Hem-7 and Hem-30, respectively, in this paper. TEM (transmission electron microscope) observation revealed that previous to dissolution, Hem-7 is present as both dispersed particles and as aggregates. Dispersed particles dissolve initially before aggregates, which influences its dissolution rate. The Hem-30 hematite has nanoscale surface steps and internal defects, and its dissolution initiates from the steps, defects, or sharp edges of the crystals. This study directly shows the importance of size, surface roughness, defects, crystal morphology and aggregation states on dissolution rates of nanoparticles.

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The author and his coleage have been studying electroceramics based on the applied mineralogy. The mineralogy has long history and has been the origin of all science. The material science is also based on the mineralogy. The author studied crystal structure analysis at the Mineralogical School of the University of Tokyo, and material science at the Department of Ceramics of Nagoya Institute of Technology, so he applied mineralogy to material science. He has been studying in following area: microwave dielectrics, millimeter-wave dielectrics, multilayer ceramic condenser, piezoelectric materials and so on. In this paper, pseudo-tungstenbronze dielectrics and homologous compound series on the microwave dielectrics and indialite/cordierite glass ceramics on the millimeter-wave dielectrics has been reviewed. The pseudo-tungstenbronze solid solutions have special point of x = 2/3 on the Ba_6−3xR_8+2xTi₁₈O₅₄ (R = rare earth) that is the compositional ordering performed high quality factor based on the relationship between crystal structure and properties. Based on the knowledge of high Qf due to compositional ordering, new dielectrics with high Qf had been designed. On the homologous compounds, the relationship between the Qf properties and crystal structure due to substitute large cataion has been clarified for the design of base station resonator. On the millimeter-wave dielectrics, indialite glass ceramics are presented, which has low dielectric constant of 4.7 and extremely high Qf of more than 200 × 10³ GHz. It will be applied for resonators, patch antennas and LTCC substrates. The other materials such as multilayer capacitors and piezoelectric materials will be reviewed near future.

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