A protoplanetary disk is a dynamic system where physical and chemical processes interplay in complex manners, and it is the place where planetesimals were formed that further collided to form planets. The chemical difference among planets and planetesimals along with volatility-controlled composition of chondrites deviated from CI chondrite clearly indicate condensation and accompanying chemical differentiation are crucial. In order to gain fundamental understanding of the origin of planets, which requires integration of physics and chemistry, we have carried out evaporation and condensation experiments that enable us to describe principle behaviors of gas-solid reaction kinetics involving multiple condensed phases. Those parameters are dependent on cooling time scale and total pressure of the disk, which evolve with time and space. It is shown that phases that are not expected in equilibrium condensation appear in a dynamically cooling system, that the phases and their grain sizes are strongly dependent on the cooling time scale, and that the grains become as large as mm in size only with the chemical process, which is much larger than that usually assumed in standard models. The condensed grains with iron mantle make coagulation easier resulting in effective growth of dusts to form planetesimals.
A rare earth mineral is defined as a mineral containing rare earth elements (REE: Sc, Y and lanthanoids) as essential constituents. So far, more than 280 species of RE minerals, as independent species, have been described after the official IMA-CNMNC approval. The chemical bonds between REE and anions possess largely ionic character and the coordination polyhedra of REE are not regular, but rather distorted, in almost all cases. The REE3+ ions exhibit 7 kinds of coordination number between 6 and 12, among which 8 is the most frequently observed. The coordination numbers of the larger Ce-group REE3+ are similar to those of Ca2+ and Th4+, and are generally higher than those of the smaller Y-group REE3+, which is similar to that of U4+. Isomorphous substitutions are commonly observed between cations having similar ionic radii and coordination numbers. The difference in the cation size between the Y- and Ce-group REE results in different crystal structures, when these structures consist of isolated anionic groups, such as CO32− and PO43−. The crystal structures having infinite frameworks, e.g., chains, sheets and 3-dimensional frameworks of silicate, niobates and others, sometimes accept both of the Y- and Ce-group REE in the spaces between/among the frameworks. The isomorphous substitutions between REE3+ and the other heterovalent cations found in the crystal structures of RE minerals are coupled substitutions with charge compensation mechanisms.
Some meteorites experienced transient high-pressure and -temperature conditions on its parent-body. High-pressure polymorphs form in and around shock-melt veins of shocked meteorites. Recent developed Nano-technologies such as TEM and FIB system allow us to scrutinize high-pressure polymorphs in shocked meteorites. We here introduce the occurrences, natures and formation mechanisms of high-pressure polymorph of olivine and silica in shocked meteorites and their implications for planetary science.
Fluids in the Earth's crust play crucial roles in various geological phenomena including earthquakes, volcanism and formation of ore deposits. Mineral-filling veins are fossils of fluid-filled fractures, but it is not easy to extract information on nature of fluid flow during vein formation. Mineral veins in the metamorphic rocks show systematic variation in internal textures (grain shape, mineral distribution, crystallographic orientation, relation to the host rock) as a function of aperture size and host rock. Hydrothermal experiments for silica precipitation have revealed that the occurrences and mineralogy of silica minerals are strongly controlled by solution chemistry (supersaturation, minor elements), rock substrates (quartz distribution, grain size), fracture geometry (aperture distribution, surface roughness) as well as temperature, which results in a variation in vein texture. Combining modeling and analyses of natural and synthetic veins, vein texture is a potentially useful indicator of hydrological properties (flow velocity, flow direction, degree of supersaturation, duration of sealing etc.) in the crust.