Water probably has played significant roles in the origin and evolution of the Earth. During the accretion period, an H₂-H₂O atmosphere would have been formed by impact degassing of planetesimals, which may have resulted in the formation of magma ocean (hence promoted metal-silicate separation) and condensed to form the proto-ocean. During the evolution of the Earth, water would have circulated between the surface and mantle reservoirs. This global water cycle may have controlled the ocean mass and water content in the mantle as it coupled with thermal evolution of the Earth through mantle rheology. It may also have oxidized the mantle and the atmosphere in the Earth's history.

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“Water” of various chemical forms which occur in hydrous and nominally anhydrous minerals have important roles in properties and behaviors of minerals. Major interests on “water” in minerals are hydrolytic weakening of quartz, promotion of various reaction of mineral under small amounts of “water”, amounts and chemical form of “water” in magma and mantle, the relation between optical anomaly and OH- ordering in minerals, etc. Infrared (IR) spectroscopy is an available method to analyze the chemical form of “water” in minerals. In this review, several topics on the detection of “water” in minerals under IR spectroscopy, such as “water” analysis in naturally deformed and recrystallized quartz, behav-iors of “water” in quartz at low and high temperature, orientation determination of OH dipole in crystal, and IR absorption spectra of brucite under high pressure with diamond anvil, are introduced.

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The author reviews some aspects of the role of dense hydrous magnesium silicates (DHMSs) in conjunction with the compositions and the dynamic processes of the mantle and subducting slabs. Petrological observations suggest the presence of only a small amount of water (<1000 ppm), which is mostly accommodated in pyroxenes, in the upper mantle. At depths of the mantle transition region, modified spinel is the major host of water, and thus none of the DHMSs are likely to be predominant over the depth interval of the average upper mantle - mantle transition region. One important aspect of the effect of water in this region is that a small amount of water may seriously affect the sharpness of 410-km seismic discontinuity, as suggested on both thermodynamical and experimental grounds. On the other hand, major hydrous minerals that constitute the upper parts of the subducting slabs, such as serpentine, would transform to form high-pressure phase assemblages including some DHMSs. This may provide a certain amount of water deep into the mantle if the temperatures of these slabs are substantially cooler than the surrounding mantle, which should have important implications for the dynamic processes of the deep mantle.

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Rheology of rocks and minerals in the earth interior is strongly controlled by temperature, pressure, grain size and H₂O contents. Activation energies for creep of silicates range from 100 to 530 kJ/mol, and activation volumes seem to range from less than 1 cc/mol for Si to 7 cc/mol for O. The effect of H₂O on rheology of silicates depends on the defect structure of H in the lattice. Considering that the dominant defect of H is 3Hsi showing 3H replacing Si, solubility limits of H in quartz, forsterite, modified spinel, spinel, pyroxene, and perovskite are calculated as a function of water fugacity by means of defect chemistry. Compared with the experimental results, the author obtained their solubility limits under excess H₂O condition at very high pressures. It shows that olivine may contain 20000ppm(=H/106Si) at 15GPa and modified spinel and spinel may contain 220000ppm at 15GPa and 300000ppm at 24GPa. On the other hand, it seems that perovskite may contain at most 1000ppm under 100GPa. Combined with dislocation motion and diffusivity of defects, flow laws for above minerals are deduced as the function of f_H2O and f_O2 in this paper.

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Formation and transformation of clay minerals occur ubiquitously at the interface between mineral and water in the upper part of the Earth crust. The solution-mediated reactions pertaining to clay minerals are characterized by a consecutive transformation from metastable phases toward final stable phases based on the Ostwald's step rule and by the phase transformations via dissolution and recrystallization processes. Two examples have been introduced in this communication: one is the allophane→halloysite→kaolinite reaction series in weathering and the other is the smectite→illite/smectite interstratified minerals→illite reaction series in diagenesis. Understanding of the kinetics of these reactions in natural open systems are only qualitative at present; it is necessary in future to accumulate more data in terms of the rates and mechanisms of dissolution and growth of these minerals in order to understand quantitatively the dynamics of the solution-mediated geochemical reactions.

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Fe oxyhydroxide particles are formed by mixing of hot, H₂S-rich, oxygen-poor, low-pH hydrothermal vent fluid with cold, oxygen-rich seawater in the hydrothermal plumes associated with seafloor hydrothermal systems. These Feoxyhydroxide particles derive their compositions from scavenging of dissolved constituents (e.g. PO₄^-3 V, As, rare earth elements, Th) from seawater during transport away from the vents. The scavenging processes of Fe oxyhydroxide playan important role in balancing the oceanic geochemical budgets of most rare earth elements and oxyanion species in seawater. The estimated scavenging rates are 6×10¹⁰ to 1.1×10¹¹ tmol/year for P, 4.3×10⁸ to 1.2×10⁹ mol/year for V and 1.8×10⁸ to 1.2×10⁹ mob/year for As. These Fe oxyhydroxide particles are actively produced at the superfast-spreading (15cm/year) East Pacific Rise (EPR). During the RN Melville cruise, Fe-enriched hydrothermal plumes (Fe<730nm/l) were observed at S13°K50' to S17°00' of the East Pacific Rise (EPR). Fe-enriched plumes are dominated by Fe oxyhydroxide particles scavenging Si, P, and Ca from seawater. Room temperature Mossbauer spectra and FT-IR spectra of these Fe oxyhydroxide particles indicate that these Fe oxyhydroxide particles are dominated by ferrihydrite.

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