Formation of a thermodynamically metastable phase instead of a stable phase during crystallization has been interpreted by the Ostwald step rule. The rule was applied to natural crystallization of minerals in complex systems in the present paper. Phase transition from a metastable to stable phases and/or two dimensional nucleation of a stable phase on a pre-existing metastable phase are possible during growth of a metastable phase. Disappearence of a metastable phase is therefore strongly related to these processes, resulting in the formation of complex textures in grown crystals. The complex texture in cordierites from hornfels was then interpreted based on the growth and transformation of a metastable phase.

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Comprehension of metastable states in the course of phase transition, decomposition, melting, crystallization is one of significant subjects in the solid state earth dynamics. The following categories in the electronic, atomic or lattice transformation must be taken into account for the metastable phenomena of the above structure changes; (A) kinetic factor such as nucleation rate, growth rate, rate of compression and depression, heating rate and reaction duration, (B) environmental physical and thermodynamical parameters. In addition to experimental approaches to clarify metastable states, atomistic computer simulations offer informations of the precursor phenomena of structure changes under the desired physical conditions. For an example of these calculations, present MD calculation simulates the mechanism of pressure-induced amorphization of GeO₂ and its polymorphic phase transformation of post-rutile phase under hydrostatic and nonhydrostatic conditions.

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At low pressure conditions of the solar nebula, condensation and evaporation are the major phase transitions that caused chemical and isotopic fractionation, which may be responsible for planetary and meteoritical chemical fractionation. In order to describe the fractionation quantitatively as a time-dependent process, kinetic factors such as evaporation rates, fractionation factors, nucleation rates, reaction rates between gas and solids, and diffusion rates of elements in major minerals and rarely in melts should be obtained. Experimental work on evaporation of minerals and melts has enabled us to understand the roll of various kinetics during evaporation, and their parameters have been obtained for most important reactions. Theoretical treatment of kinetic processes has been also developed for chemical and isotopic fractionation due to evaporation with or without contribution of diffusion in condensed phases. Combination of experimental and theoretical work along with solar nebula modeling becomes successful in describing the development of chemical and isotopic fractionation in the solar system. The recent progress in laboratory evaporation experiments and theoretical studies is reviewed in this paper.

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Dissolution and precipitation of silica polymorphs (amorphous silica, cristobalite and quartz) are coupled with geothermal reservoir simulator (simulator for hydrodynamic fluid flow in porous media) based on reaction kinetics. Reaction kinetics are simplified as follows; silica polymorphs are always precipitated from or dissolved into the solution without direct solid phase transition among them. Only inorganic processes are considered and the effect of salinity is neglected. Initial reaction surface area is assumed to be unique for both precipitation and dissolution of all polymorphs. Based on these assumptions and published experimental data of rate constants and solubilities, two dimensional numerical simulations of geothermal system development were performed. Alteration zoning of silica polymorphs was reproduced in the calculations as observed in natural geothermal systems. Sensitivity of silica alteration zoning to formation permeability and reaction surface area was also studied. The present model is preliminary, but the results show that numerical simulations incorporating chemical processes are promissing for geothermal modeling, especially for reducing the non-uniqueness of the models.

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Geospeedometer proposed so far to estimate cooling history of rocks has a limit in terms of the maximum temperature above which we cannot deduce the history. The limit is governed by kinetic processes for geospeedometry and cooling rates; for example the olivine-spinel geospeedometry based on kinetics of Mg-Fe²⁺ exchange between olivine and spine! with the maximum grain size of a few millimeters cannot record any cooling history above 800°C if the cooling rate is <∼10^-4°C/year. In order to extend the limit, a larger distance scale for geospeedometry is needed. By using mineralogical variations forming rock structures, whose distance scale is up to a meter order or more, we can in principle estimate thermal history of rocks up to their so!idus temperatures, if effects of polycrystalline and multi-phase nature and the initial conditions for a large scale rock system are correctly assessed. The olivine-spinel geospeedometry was extended to deduce high temperature thermal history of a dunite-chromitite layer sampled from the Iwanai-lake peridotite mass, Hokkaido. The Fe-Mg variation over 6 cm in dunite from the contact is analyzed by a 1-D diffusion model with a composition-dependent diffusivity and a changeable cooling rate. The dunite-chromitite geospeedometry indicates that the Iwanai-lake mass was cooled from ∼1000°C to 900°C with increasing cooling rate from <0.01°C/year to -0.3°C/year. This suggests that the slower cooling after the melting event was followed by the ascent of the mass that resulted in the inferred rapid cooling at ∼900°C.

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Metal droplets separating in the magma ocean have the radius less than 1cm, and it takes about 106 seconds to separate in the magma ocean. Metallic liquid accumulated in the bottom of the magma ocean starts to fall into the lower mantle when the metal drops grow into the radius of about 1km. The present theoretical analysis implies that the small metal droplets falling into the molten magma ocean are thermally and chemically in equilibrium with the silicate magma ocean, whereas the large metal pools falling in the lower mantle are disequilibrium with the surrounding silicates. Therefore it may be possible to estimate the depth of the magma ocean from the conditions of the metal-silicate equilibrium. Abundances of Ni and Co in the mantle may be accounted for by the chemical equilibrium between metal and silicate at 40 GPa and 2500°C, suggesting existence of the primordial terrestrial magma ocean with the depth of 1200 km.

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