The influence of local stress state and crystallinity of graphite on the graphite-diamond transformation process was investigated using laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics.

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High-temperature synchrotron X-ray powder diffraction experiments and differential scanning calorimetry for Ca-poor orthopyroxene and Ca-free orthoenstatitethe have been carried out in the present study to clarify the stability field of high-temperature orthopyroxene by drawing P-T projection in enstatite-diopside system. Our results show that high-temperature orthopyroxene, which is stable only around 1400 C at atmospheric pressure, have stability fields above 1200 C at the pressure that protoenstatite is no longer stable.

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In this study, synthesis of anorthite crystal has been tried by using the flux of CaO-V₂O₅ with Li₂O and/or B₂O₃ additive. In one type of flux, the crystal of ca. 8x3x1 mm was obtained by the cooling of 0.5˚C/h, from 1200 to 1050˚C. In order to obtain better and larger crystals, the synthesis in this type of flux has been continued with changing temperature and cooling conditions. In this case, however, the flux around the crystal could not be removed by water. On the other hand, in another type of flux the anorthite crystal could be separated by rinsing and ultrasonic cleaning with hot water, although it seemed to be not suitable for growing larger crystals. Crystals obtained until now are smaller than a few millimeters. Nevertheless, it is significant since we can select the flux system on the purpose.

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In order to evaluate the sorption-behaviour of Pu to colloidal ferrihydrite in the Lake Karachai area, we introduce thermodynamic parameter, the mole fraction ratio, assuming that the concentration of Pu is very small. The observed data of Pu on ferrihydrite1 and our thermodynamic discussion have revealed that Pu is kept sorbed to ferrihydrite for long time.

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The origin and formation process of aragonite in six-sided prisms are studied by observing the external forms and internal textures. The results show that the external forms and textures can be explained by the nucleation and growth of orthorhombic lattices on the surface of an initial hexagonal crystal, followed by the transformation and distortion of the initial crystal.

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The concept of the rapid incorporation path is applied to analyze the origin of habit change among polyhedra. The result shows that the habit change can occur when the activation energy increases.

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Oxygen extraction from Ca₁₂Al₁₄O₃₃(C12A7) has been investigated in association with cement clinker fabrication using moon regolith simultant. Many bubbles, consisting of oxygen, were observed with the fusion of this phase even in vacuum.

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Mn3V2Si3O12 garnet was found in the Kurase, Fujii, and Hokkejino mine. Now this garnet is applying as a new mineral named ''momoiite''. In natural condition, momoiite contains Ca in 8-fold dodecahedral site and Al in 6-fold octahedral site respectively, and composition of end-member has never been occurred. To clarify the conditions for the formation of end-member Mn3V2Si3O12, we performed the synthetic experiments along Mn3V2Si3O12-Mn3Al2Si3O23 and Mn3V2Si3O12-Ca3Al2Si3O12 using hydrothermal apparatus. The P.T. conditions for Mn3V2Si3O12 garnet will be reported.

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A dynamic light scattering (DLS) method was applied for monitoring initial stage of crystal growth of calcium carbonate and we observed impurity effects of lanthanum ion on crystal growth. The crystal growth rate in the lanthanum-doped condition considerably decreased in comparison with lanthanum-free condition. The obtained sample of DLS measurement was observed by SEM. The size of particles was in the same range with results of DLS.

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About low crystalline clay and amorphous aluminum-silicate complex (HAS-Clay), the generation process was examined from XRD, FTIR and so on, by changing the heating time in the synthesis. From the result of XRD pattern, the peak derived from amorphous aluminum-silicate was seen in the synthesis time until two hours, and the peak derived from low crystalline clay was also seen in the synthesis time over three hours. In FTIR spectrum, one peak only observed in amorphous aluminum silicate, and three peaks observed in low crystalline clay and amorphous aluminum-silicate complex.

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Peritectic solidification was studied using Sr(NO₃)₂-H₂O system where Sr(NO₃)₂ (α phase) , Sr(NO₃)₂.4H₂O (β phase) and H₂O (γ phase) crystallize: the phase relations of the first and the last two are peritectic and eutectic, respectively. Three solutions with composition C1, C2 and C2 where α, β and γ respectively equilibrate with liquid phases were kept at several temperatures to observe the solidification processes under an optical microscope. With decreasing temperature, α, β and γ phases nucleated in this order in the solution C1. It is noted that β phases nucleated heterogeneously on the preexisting α phases. In the three solutions with the lower temperatures, γ and then α and occasionally β phases appeared to complete solidification. It is concluded that the favorable growth of β phase on α phase results in peritectic texture in any solution although the texture consisting of β and γ phases only is not formed.

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Most biominerals contain organic macromolecule, which alters morphology and mechanical properties of the crystals. In the present study, we have investigated some biominerals composed of calcite. The amount of the intra-crystalline organic substance in the prismatic layer of pearl oyster (Pinctada fucata) was estimated ca. 2.8wt.% using TG-DTA analysis. We can get values of lattice strain in the crystals derived from the peak width of the reflections in the pattern obtained by XRD analysis. This analysis indicated that prismatic calcite had more strain than inorganic one (Iceland spar). It also showed that those calcite crystals are anisotropic. These crystallographic features of organic calcite must be related to the interaction between the crystal and the intra-crystalline macromolecule.

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Monohydrocalcite is rare mineral in geological settings. It found as biogenic calcium carbonate. In order to understand the stability relationships of calcium carbonate minerals under low temperature, we determined MHC solubility.The solubility of MHC increases with the temperature. MHC is stable at lower temperature. In addition, the solubility of MHC becomes lower than vaterite at the temperatures lower than 5 celcius degree. It indicates that MHC is possibly third stable calcium carbonate at lower temperature.

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Metamorphosed calcium carbonates are found from sea-impact silica glass LDSG by bulk data and in-situ observation. The Lybian desert silica glass LDSG includes NaCl-bearing SiC and carbon-bearing calcites in cavities which are remained during its rapid formation event. The present results of sea-impact with calcium carbonates are supported by the continental drift computed map of its formation age to show wide carbonate sea-coasts as its impact-site area.

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Abiotic aragonites commonly contain repeated twins with {110} as the twin plane (Bragg, 1924). In biogenic aragonite, the twinning was also previously reported in several researches (e.g. Kobayashi and Akai, 1994). In this study, we investigated the shell structure composed of aragonite crystals, larval shell of Crassostrea nippona and Lottia kogamogai. It was found that they also contain {110} twinnings densely like previous researches. However, in the study of gastropod nacre, it was revealed that tablets constituting nacre are twin-free aragonite crystal. This result indicates that the aragonite tablets are controlled by the specific growth mechanism.

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Adsorption of abiotic amino acids by allophane was carried out at pH 4, 6 and 8. D-, L-norvaline, D- and L-norleucine were used in this work. The isotherm for adsorption of D-amino acid was compared with that of L-amino acid. On norvaline, the isotherm of D-norvaline was almost the same as that of L-norvaline at pH 4, 6 and 8. In the case of norleucine, at pH 6 and 8, the isotherm of L-norleucine was slightly higher than that of D-norleucien. At pH 4, the isotherm of D-norleucine is similar to that of L-norleucine. Allophane might discriminate between D- and L-norleucine at pH 6 and 8.

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It is important to understand the rate and mechanism of mass transport and reaction in rock pores to quantitatively treat the water-rock interaction. In the present study, permeability, diffusion coefficient and dissolution rate of a rhyolite in Kozushima, Japan, were measured in the laboratory and an analysis using a reaction-transport equation was conducted. The result was compared with the weathering rate obtained from the field study and the mechanism of weathering in the field was considered. It was revealed that in the field the solute concentration in rock pores becomes high even assuming the highest flow rate and diffusion rate, which results in slow dissolution rate due to the effect of saturation. This is consistent with that the dissolution rate obtained from the field analysis (∼5E-19 mol cm-2 s-1) is significantly smaller than the rate under the far from equilibrium condition measured in the laboratory (∼2E-17 mol cm-2 s-1).

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Dissolution experiments of olivine were carried out at different pH, temperature and PO2 to make a kinetic model that describes Fe redistribution during dissolution. The model considers dissolution rate, Fe(II) oxidation rate, water flow rate, pH and temperature, and calculates the behavior of dissolved Fe(II) at low PO2, which has a potentiality to predict a PO2 level from Fe(II)/Fe(III) concentrations of a weathering profile. Our results indicated that the Fe(II) oxidation rates are higher at lower PO2 than expected from the classic rate law and that one will overestimate the PO2 values if the classic rate law of Fe(II) oxidation is applied to paleosols.

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Iron oxides occur ubiquitously on the Earth surfaces and have strong affinity with various toxic metals, which govern the mobility of these metals in the environment. In general, the iron oxide occurs as nano-sized particle. In order to understand the effect of size on the association with toxic metals, we first establish the synthesis method of size-controlled hematite nanoparticles with targeted diameter <10 nm (S-hemNP) and ∼40 nm (L-hemNP). In addition, co-precipitation with As was tested and the preliminary results show that more than a few 100 ppm of As associate with hematite nanoparticles analyzed by X-ray adsorption analysis.

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Sorption of actinides, particularly plutonium, onto submicrometer-sized colloids increases their mobility, but these plutonium colloids are difficult to detect in the far-field. We identified actinides on colloids in the groundwater from the Mayak Production Association, Urals, Russia; at the source, the plutonium activity is ∼1000 becquerels per liter. Plutonium activities are still 0.16 becquerels per liter at a distance of 3 kilometers, where 70 to 90 mole percent of the plutonium is sorbed onto colloids, confirming that colloids are responsible for the long-distance transport of plutonium. Nano–secondary ion mass spectrometry elemental maps reveal that amorphous iron oxide colloids adsorb Pu(IV) hydroxides or carbonates along with uranium carbonates.

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The release of arsenic from anthropogenic sources is a serious environmental concern around the world. One of the possible sources of arsenic is from acid mine drainage (AMD). In Horobetsu sulfur abandoned mine, high concentration of As (9mg/L) was detected in the AMD with a pH of 1.8. The need to develop new measures in minimizing the release of As must be established. In this study, the mineralogical association of arsenic and its leachability in the arsenic-bearing ore was examined. The mineralogical characteristics of arsenic-bearing ores was analyzed by XRD, XRF. The specific mineral phases on the micro part were further analyzed by mineralogical microscope and EPMA. Arsenic extraction from the iron mineral phases was carried out by TAO and CDB solutions. The solubility test was batch experiment with distilled water.

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IR spectra of coccolith,Discoaster spp. were measured.

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Microorganisms play an important role in the geochemical cycling of metals, and capable of actively solubilizing and precipitating gold. Native gold grains from Sado island were examined by SEM, and bacteria-like morphologies were found in the surface. Culture experiments using Sulfate Reducing Bacteria and Diatoms were conducted Biomineralization of gold was found on there microorganisms by SEM and TEM, the gold grains were in nano-size. Therefore, It was suggest that both of diatom and SRB have ability of precipitation of gold.

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In order to understand the relations between rhabdophane and other RE phosphates in Pronto paleosol (2.45 Ga age), thermal behaviors of Y-analogue of rhabdophane mineral (Takai and Uehara, 2007) and textures and chemistries of RE phosphates in paleosol were investigated. Some results suggest the formation mechanism of the monazite and xenotime at the rims of apatite: the formation of rhabdophane during weathering and subsequent decomposition into monazite and xenotime during metasomatism. Therefore, Ce contents in rhabdopahe and monazite can be useful for the estimation of atmospheric oxygen evolution.

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Apatite (Ca5(PO4)OH,F,Cl) is an useful mineral to immobilize Pb. In general, Pb immobilization by apatite is considered to occur via ion exchange or precipitation of low solubility Pb phase. In order to understand the nano-scale phenomena in the Pb immobilization process, powder HAP and bulk single crystal of fluorapatite (FAP) were reacted with Pb solution (2 mM) at pH of 5.0 at room temperature. In case of FAP, cross sectional TEM reveals that Ca selectively leached from the surface, while the secondary Pb-phosphate, chlorpyromorphite (Pb5(PO4)3Cl, CPY), precipitates on the surface. In case of HAP, the secondary Pb-phosphate, hydroxypyromorphite (Pb5(PO4)3OH, HPY), precipitates epitaxially in parallel to b axis of HAP. Two types of mechanism were observed in the HPY precipitation; (i.) HPY precipitate on HAP from the solution supersaturated with respect to HPY; (ii.) HAP is replaced with HPY synchronizing with HAP dissolution.

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We applied Terrace-ledge-kink model to evaluate the selective adsorption of aspartic acid onto steps and kink sites on calcite surface. In this model, step retreat process progresses trough two steps. First step is a nucleation of double kink site. Then a pair of single kink sites appears. Second step is detachments of the single kink sites. The morphological change caused by Aspartic acid can be explained from the relation among the rates of double kink nucleation and detachments of single kink sites. In other words, we can determine the rates, which reflect the influence of aspartic acid onto each site, from the morphological changes. From the results, we concluded that L-Asp prefer [481] steps to [441] steps, and kink site on [441] step to kink site on [481] step. In the case of D-Asp, the preferences were just opposite to the case of L-Asp.

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High-pressure and high-temperature behavior of Fe-Xe system has been studied up to about 180 GPa and 3000 K using laser-heated diamond anvil cell combined with synchrotron radiation. No new compound was formed even under such extreme conditions but the possibility of dissolving some amount of Xe into Fe was suggested.

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We found a new nickel sulfide phase with Ni3S composition. We were carried out in situ observation of the Ni3S phase to study its structure and stability filed by using the MAX80 system installed at AR NE5C at PF. Powder X-ray diffraction revealed that the new Ni3S phase has the Fe3P-type structure. As the Fe3S phase also takes the Fe3P structure, the Fe3S and the Ni3S is thought to make a complete solid solution. The Ni3S phase is stabilized above 5 GPa, and it breaks down into the Ni and the Ni3S2 below 5 GPa. The Ni3S phase melts incongruently into Ni and liquid around 700 K at 6 GPa and its melting temperature gradually increases with pressure. The addition of Ni to Fe3S must affect not only the stability field but also the physical properties such as bulk modulus.

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The pressure-volume equation of state of iron-nickel-silicon alloy Fe-9.8 wt% Ni-4.0 wt% Si and iron-silicon alloy Fe-3.4 wt% Si has been investigated using diamond anvil cell up to 374 GPa and 252 GPa, respectively. These alloys have the hcp structure at all pressure studied. The compression data were fitted to a third order Birch–Murnaghan equation of state. The density of these alloys indicates that the inner core has a composition of Fe-10 wt% Ni-5.4-7.7 wt% Si assuming that the inner core contains the chondritic iron and nickel ratio.

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The high-pressure study of second-order phase transition of metal dioxides has been performed by both high-pressure experiments and first-principle calculations. In the case of iridium dioxide, the second-order phase transition from the rutile-type to the CaCl2-type structure was not confirmed, because the pyrite-type structure is stable relative to the CaCl2-type structure. In the case of ruthenium dioxide, our study showed that the CaCl2-type structure is metastable, although the second-order phase transition from the rutile-type to the CaCl2-type structure was observed in previous experimental study. The observation of the metastable phase in the previous experiments is due to a significant differential stress in the sample.

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Using Mie-Gruneisen-Debye type treatment, we derive temperature-pressure-volume equation of state (T-P-V EOS) of Au based on the combined use of the observed data from shock compression up to 240 GPa, volume thermal expansion up to 1300 K at 0 GPa, and high temperature elasticity at 0 GPa from ultrasonic measurements. The EOS parameters are successfully optimized to be dK/dP = 6.0 and q = 1.6 with fixed K = 167 GPa, Gruneisen parameter = 2.97 for Au. Based on previous simultaneous volume measurements of Au and MgO, Pt and MgO, and Au and Pt, we find the present Au EOS is fully consistent with either MgO or Pt EOS developed previously without relying any pressure scale.

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To estimate the spin state of iron in Mg-perovskite in the lower mantle, the spin state of ferric iron in synthesized Mg-perovskite was measured at pressures up to 200 GPa at room temperature by X-ray emission spectroscopy at SPring-8. The result indicates that the spin number of ferric iron in Mg-perovskite began to decrease at pressure around 120 GPa with increasing pressure, but remained intermediate between high spin and low spin at 200 GPa. This result indicates that the spin state of ferric iron in Mg-perovskite does not become low spin in the lower mantle if the temperature gradient of the spin transition is positive.

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We have investigated that the phase relation of aegirine at high-pressure and high-temperature. The high-pressure X-ray diffraction experiments were performed using LHDAC and synchrotron radiation. Fine powdered natural or synthesized aegirine was loaded with fine NaCl or KCl powder. They were compressed with a rhenium gasket and diamond anvils. Angle-dispersive XRD patterns were recorded repeatedly at room temperature before and after heating at high pressure. Above 15 GPa, aegirine dissociated into an assemblage of SiO2-stishovite and unknown cubic phase after heating. Above 38 GPa, this assemblage changed to SiO2 phase and calcium ferrite-like structure phase. Above 70 GPa, the diffraction pattern corresponding to orthorombic perovskite structure was observed. This phenomenon indicates that perovskite including sodium and ferric iron was synthesized.

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It is thought that a calcium ferrite phase may exist in the Earth's lower mantle as a high-pressure phase of mid-ocean ridge basalt. Calcium ferrite-type NaAlSiO4 is one of major endmembers of the calcium ferrite phase. In this study, its heat capacity measurement was performed using a differential scanning calorimetry in the temperature range of 125-400 K. Furthermore, Raman and IR spectroscopy were made to obtain information of the lattice vibration. Heat capacities excluding observed data were estimated by the Kieffer model calculation based on the lattice vibrational information. Lattice vibrational entropy at 298 K was obtained to be 86.7 J/mol.K. Configurational entropy was calculated to be 11.5 J/mol.K by assuming a perfect random distribution of Al and Si in the same cation sites. Thus, entropy of calcium ferrite-type NaAlSiO4 at 298 K was determined as 98.2 J/mol.K.

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The post-corundum phase transition has been investigated in Ti2O3 on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The Th2S3-typed Ti2O3 was found at about 10 GPa, and this phase was stable at least up to about 80 GPa. The structural feature of the Th2S3-type Ti2O3 is that the Ti atom in this structure is coordinated with seven oxygens, while in corundum-type of Ti2O3 each Ti atom has a six-fold coordination.

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Equations of state and axial compressions of postperovskite phases are investigated in the MgSiO3–FeSiO3–FeAlO3 system. The b-axis is found to be the most compressible axis for both phases, while the c-axis is slightly less compressible. The incompressible behavior of the c-axis is likely to be attributable to repulsion among oxygen atoms arranged along the c-axis. The bulk sound velocity significantly change not only by the postperovskite transition but also by enrichment in the FeSiO3 and FeAlO3 components.

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Phase relations, element partitioning, and density changes in pyrolite have been studied at pressures to 50 GPa along a typical geotherm on the basis of multianvil apparatus using sintered diamond anvils combined with synchrotron in situ X-ray measurements. Electron microprobe, Mossbauer, and EELS measurements have been made on the recovered samples, and we found that the content of ferrous iron in magnesiowustite increases at pressures greater than ~40GPa, consistent with the high- to low-spin transition in this phase. The bulk density of pyrolite agrees well with those of typical seismological models, suggesting that pyrolite is a reasonable composition for the lower mantle.

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Preliminary deformation experiments of wadsleyite were conducted at conditions of mantle transition zone using a deformation-DIA apparatus (D-DIA) and 6-6 type multi-anvil assembly in order to study rheological properties of wadsleyite. Deformation of wadsleyite was achieved at 17-18 GPa, 1700 K and strain rate of 5.6 x 10-5 s-1 to strain of 15 % in uniaxial deformation geometry.

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We conducted the albite decomposition experiments at 1.4-3.6 GPa and 673-973K under differential stresses using multi-anvil type deformation apparatus (D-CAP) with synchrotron X-ray. The polycrystalline albite was deformed up to 35% strain at constant strain rates of 10^(-5)-10^(-6)/s during the transformation. Stresses and transformation rates were observed by the time-resolved 2-dimensional X-ray diffraction patterns and axial strains were measured from the transmission X-ray images of the samples.

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Grain growth kinetics of forsterite (Fo) and enstatite (En) in fine grain aggregates of Fo + En are examined as a function of volume fraction of En.

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Creep experiments on forsterite aggregates have been established.

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Diffusion experiments were conducted between wadsleyite aggregates and basalt at 16 GPa 1400-1600°C in order to investigate the mobility of the incompatible elements in wadsleyite aggregates derived from basalt source and to understand the behaviors of the these elements in the transition zone. Elemental depth profiling data obtained by LA-ICPMS are successfully fitted to a semi-infinite diffusant model equation. The effective diffusion coefficient of Al, Na, K, Rb, Sr, Ba, Nb are 7.8*10^-14∼5.6*10^-13 am2/sec at 16 GPa and 1500°C.

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In this study, we determine the diffusivity of silicon in forsterite at high pressure and temperature to understand the effect of pressure on diffusivity.We performed high-pressure experiments using a Kawai-type multianvil apparatus. In the experiments, we used single crystal of forsterite as starting material with coating of 29Si-enriched Mg2SiO4 on the polished surface. Thick coating layer (200-300 nm) can maintain the isotope composition constant at the surface during diffusion annealing at pressures of 3, 8, 13 GPa and temperatures of 1600 and 1800 K for 3-60 hours. After high-pressure experiments, the recovered samples were analyzed by a secondary ion mass spectroscopy (SIMS) to obtain the variation of isotope concentration with depth from the crystal surface. Self-diffusion coefficient at 13 GPa and 1600 K is preliminarily estimated to be D=7x10-21 m2/s.

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Here we report new experimental results on the back transformation kinetics of majoritic garnet into pyropic garnet + clinopyroxene by time-resolved in-situ X-ray diffraction measurements. We found that the majoritic garnet could never survive in the upper mantle conditions on time scales of normal mantle plume flow. Accordingly, exists of untransformed majoritic garnet in diamond suggests that the diamond have been transported to the earth's surface directly from the mantle transition zone by deep and rapid ascent of kimberlite magma.

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CO2 hydrate is a clathrate hydrate consisting of water molecules forming cages and of guest molecules included in the cage. High-pressure and low-temperature experiments of CO2 hydrate were performed in the pressure range from 0.2 to 3.3 GPa and 100 to 300 K using diamond anvil cells. X-ray diffractometry and Raman spectroscopy revealed that stability field developed at least to 60 K below 1 GPa. Decomposition predicted by a theoretical study was not found in the region examined. Raman spectra showed softening of intramolecular vibration modes with increasing pressure. That may be related to instability of CO2 hydrate under room temperature and high-pressure.

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Hydrogen hydrate is considered to be one of the major constituents of protostar, icy planets and their moons. A filled ice Ic structure of hydrogen hydrate is maintained to at least 80.3 GPa. This stability was considered to be induced by intermolecular interactions. In this study, high pressure experiments of deuterated hydrogen hydrate were performed. Then, the isotopic effects and the intermolecular interactions in the filled ice Ic structure were examined. In the XRD measurements, the compressibility of deuterated hydrogen hydrate was different with that of H2-H2O hydrogen hydrate. This isotopic effect might be induced by the differences of the intermolecular interactions.

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Melting experiments were conducted on pyrope (Mg3Al2Si3O12)-enstatite(MgSiO3)-H2O system at pressures of 5 GPa and temperatures between 1173 K and 1473 K in two water contents (18.7 wt% and 11.3 wt%). As the results, the solidus temperature of pyrope-enstatite-H2O system was determined as 1373 K at 5 GPa by means of the textural observation, and the discontinuous change of degree of melting and the water content in melt/fluid were observed. This means that the liquid immiscibility exist at 5 GPa in the pyrope-enstatite-H2O system.

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We have demonstrated in-situ high-pressure XRD on the structure of hydrous albite melt with DIA-type press and polychromatic X-ray. In the radial distribution function, the peak of Al-O atomic pair merged with Si-O up to 2.3 GPa. However, by the further compression up to 6.1 GPa, the Al-O peak shift to longer distance with pressure from 3.6 GPa and is obviously separated from Si-O peak, suggesting that the coordination number of Al increases.

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In order to refine the D atom position in Ca(OD)2 at ambient conditions, neutron powder diffraction measurement was performed at the High Resolution Powder Diffractometer (HRPD) beamline in JRR-3, Ibaraki, Japan. Quality of obtained diffraction data is good enough to refine the D atom position by Rietveld analysis. An ordered model with D located at the 2d crystal site on the 3-fold rotation axis and a statically disordered model with D located at 6i sites with occupation factor 1/3 were refined. The disordered model gave a better fit to the observed pattern than the ordered model (Rwp=6.80, S=1.1773 for the disordered model and Rwp=8.41, S=1.4557 for the ordered model).

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Hydroxides with the CdI₂ layered structure are interesting model systems for the investigation of the high-pressure behavior of hydrogen bonding in solids. Powder sample of Ca(OH)₂, exhibits a reversible pressure-induced amorphization at 11 GPa. In contrast, a single crystal undergoes a pressure-induced phase transition at 6 GPa and room temperature due to the effect of grain size. The mechanism of high-pressure phase transition of Ca(OH)₂ remains unknown. We investigated the high-pressure behavior of both powder and single crystal samples under quasi-hydro static conditions by using DACs. Moreover, the isotope effect on the pressure-induced responses was studied for the purpose of deciphering the phase transition mechanism. Split peaks were found in the Raman and IR spectra at 6~8 GPa. The observed XRD pattern showed unknown peaks before amorphization at 12 GPa. Additionally, these unknown peaks were quite different between different pressure transmitting media. These observations suggest that there are several new high-pressure phases, and both hydrostatic condition and grain size may affect on the mechanism of the transitions.

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