Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO₄ + 9.5 wt% H₂O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si + 12.35 wt% H₂O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7 wt% H₂O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500 °C and 23 GPa) in a hydrous Martian mantle.     

Thermodynamic properties and hydrogen speciation from vibrational spectra of dense hydrous magnesium silicates

Infrared absorption measurements were taken from 100 to 5000 cm^?1 of a natural chondrodite and three dense hydrous magnesium silicates: phase A, phase B, and superhydrous phase B (shy-B). Raman spectra were also acquired from phase B and the chondrodite. Roughly half of the lattice modes are represented and our data are the first report of the low frequency modes. Comparison of our new spectra to symmetry analyses suggests that multiple sites for hydrogen exist for all the phases. The shy-B we examined crystallizes in P2₁ nm with two OH sites. Models for the density of states are constructed based on band assignments for the lattice modes and for the OH stretching vibrations. Heat capacity C_P and entropy S calculated using Kieffer's formulation should be accurate within 3% from 200 to 800 K. Model values for C_P at 298 K are 299.6 J/mol-K for chondrodite, 421.5 J/mol-K for phase A, 529.4 J/mol-K for shy-B, and 618.9 J/mol-K for phase B. Model values for S₂₉₈ ⁰ are 234.2 J/mol-K for chondrodite, 303.5 J/ mol-K for phase A, 377.9 J/mol-K for shy-B, and 473.3 J/mol-K for phase B. Debye temperatures are near 1000 K.     

29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates

We present NMR spectroscopic data, obtained by ¹H MAS, ¹H static spin-echo, and ²⁹Si{¹H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg₂SiO₄, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow ²⁹Si NMR peaks and display the most de-shielded Si^VI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The ¹H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The ¹H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static ¹H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved ¹H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P2₁ mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg₂SiO₄, coexisting with phases B and superhydrous B, appears to give ²⁹Si{¹H} CP-MAS signal, indicating that it contains significant H concentration. The ²⁹Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.     

Phase F,a new hydrous magnesium silicate synthesized at 1000 °C and 17 GPa: Crystal structure and estimated bulk modulus

The structure of phase F, a non-stoichiometric hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 17 GPa and 1000° C, has been solved and refined in space group P6₃cm, using synchrotron X-ray data for a single crystal of a size 18 × 24 × 30 μm. The composition and unit cell for phase F are Mg_3.35Si_5.51H_7.26O₁₈, a=5.073(3) Å, c=14.013(9) Å, 7= 312.3(5) ų. The structure contains layers with many similarities to superhydrous phase B. The layers of oxygen atoms are stacked in the ABCBAC-type double cubic closest packing arrangement. The bulk modulus of phase F was estimated from the structural and compositional relationship to superhydrous phase B and periclase.     

MgSiO3 ilmenite: Heat capacity,thermal expansivity,and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO₃ pyroxene to ilmenite gives H ₂₉₈ = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; C_p of MgSiO₃ ilmenite is 4–10 percent less than that of MgSiO₃ pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K^-1 ·mol^-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO₃ ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10^-5 K^-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg₂SiO₄ plus stishovite, Mg₂SiO₄ spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.     

Sulfide and platinum mineralization in the Merensky Reef: evidence from hydrous silicates and fluid inclusions

The base metal sulfides of the Merensky Reef are associated with hydrous silicates and intense deuteric hydrous alteration of cumulus and postcumulus silicates. Biotite and phlogopite crystallized in the vicinity of sulfides from a volatile-enriched highly fractionated intercumulus melt. Amphibole, chlorite, and talc are later alteration phases of cumulus pyroxene and intercumulus plagioclase. Biotite is often accompanied by zircon, rutile, and quartz. Accessory quartz hosts a complex suite of H₂O-NaCl-(CaCl₂)-CO₂-CH₄ fluid inclusions which have thus far not been described from the Merensky Reef. The earliest fluid inclusion compositions are NaCl-(H₂O) with less than 10 vol.% water; CO₂ coexisting with a halite daughter crystal and brine; and polyphase inclusions with up to six daughter and accidental phases and high contents of divalent cations. The maximum trapping temperature is around 730° C at 4 to 5 kb pressure. Later inclusion generations are H₂O-NaCl, CO₂-H₂O, and pure CO₂ and CH₄. The presence of Cl-rich fluids during the intercumulus stage of the crystallizing Merensky Reef is directly related to the mode of sulfide precipitation. Prior to sulfide unmixing in a hydrous magma sulfur is likely to be present as H₂S. Sulfur saturation causes reaction of H₂S with oxides of the silicate melt to form a sulfide melt plus water. During reaction the magma is enriched in water until a separate fluid unmixes. It carries all compounds with high fluid/melt partition coefficients, as well as metals capable of forming OH- and Cl-complexes. Precious metals are assumed to have fractionated into the Cl-rich fluid as Cl-complexes rather than being dissolved in the sulfide melt. During the cooling evolution of the fluid the precious elements precipitate around the periphery of sulfide melt droplets. The model proposed explains the distribution pattern of platinum-group minerals in the Merensky Reef better than any orthomagmatic mineralization concept offered so far.     

The composition of liquids coexisting with dense hydrous magnesium silicates at 11-13.5 GPa and the endpoints of the solidi in the MgO-SiO2-H2O system

High-pressure liquids in the MgO-SiO₂-H₂O (MSH) system have been investigated at 11 and 13.5 GPa and between 1000 and 1350 °C. A bulk composition more magnesian than the tie-line forsterite-H₂O was employed for the study. Rocking multi-anvil experiments were combined with a diamond trap set-up. After termination of the experiments, the liquid trapped in the diamond layer was analysed by laser ablation ICP-MS using the ‘freezing’ technique. At 11 GPa, liquids coexist with one or two of phase A, clinohumite, chondrodite, and forsterite. A marked discontinuity in the evolution of liquid compositions near 1100 °C is observed at 11 GPa. A step of ∼13 wt% H₂O and 13 wt% MgO is interpreted to result from overstepping the fluid-saturated solidus reaction mass balanced to 1.00(18) phase A + 1.07(4) fluid = 0.63(15) chondrodite + 1.44(2) melt. At 13.5 GPa liquids coexist with one or two of hydrous wadsleyite, clinohumite, superhydrous B, phase B, and forsterite. The discontinuity in liquid composition is no longer present, indicating that the second critical endpoint of the solidus has been overstepped. Thus, hydrous melts in the Mg-rich part of the MSH system (molar bulk Mg/Si > 2) are chemically distinct from aqueous fluids at pressure up to 11 GPa. Convergence of fluid and melt compositions along the solidus resulting in a supercritical liquid occurs between 11 and 13.5 GPa, at which pressure the entire MSH system becomes supercritical.     

Structure and crystal chemistry of hydrous wadsleyite, Mg1.75SiH0.5O4: possible hydrous magnesium silicate in the mantle transition zone

The crystal structure of hydrous wadsleyite, Mg_1.75SiH_0.5O₄ synthesized in an MA 8-type apparatus at conditions of 1300°C and 15.5 GPa, has been analyzed and refined in space group Imma, using the X-ray intensities measured on a 60X60X10 μm single crystal. The composition (Z=8) and unit cell are Mg_1.74Si_0.97H_0.65O₄ by E.P.M.A. analysis and a=5.663(1) Å, b= 11.546(2) Å, c=8.247(4) Å, V=539.2(5) ų. The partial M-site occupancies were determined; vacancies associated with the incorporation of water are strongly concentrated on the Mg 3 site. The OH in the structure was confirmed by Raman and FTIR spectroscopies. The result of valence sum calculation based on the refined bond lengths indicates that O1 is a hydroxyl. The formula of hydrous wadsleyite can be expressed as Mg_2-xSiH_2xO₄, where 0≤x≤0.25. When x=0.25, all of the O1 site is hydroxyl and the maximum solubility of 3.3 wt% H₂O is realized. Structural relations to other dense hydrous phases are discussed.     

Solid solution between lead fluorapatite and lead fluorvanadate apatite: mixing behavior,Raman feature and thermal expansivity

The solid solution between lead fluorapatite and lead fluorvanadate apatite, Pb₁₀[(PO₄)_6-x (VO₄) _x ]F₂ with x equal to 0, 1, 2, 3, 4, 5 and 6, was synthesized by solid-state reaction at 1 atm and 700°C for 72 h and characterized by scanning electronic microprobe, electronic microprobe analysis, micro-Raman spectroscopy, and powder X-ray diffraction. The volume-composition relationship at ambient temperature does not show significant deviation from the Vegard’s Law. The Raman spectrum data suggest that both P and V are identical on a C _s site and both end-members show no apparent factor-group effect. The Raman frequency shift of the symmetric stretching vibration is linearly dependent on the composition. High temperature X-ray diffraction data, up to 600°C, suggest that the thermal expansion coefficients α _a , α _c , and α _V also vary linearly with the compositions of the apatites.     

Equations of state of magnesium silicates anhydrous B and superhydrous B

High-pressure single crystal X-ray diffraction experiments of phase anhydrous B and superhydrous B have been carried out to 7.3 and 7.7?GPa, respectively, at room temperature. Fitting a third-order Birch-Murnaghan equation of state to the P-V data yields values of V ₀?=?838.86?±?0.04?ų, K_T,0?=?151.5?±?0.9?GPa and K′?=?5.5?±?0.3 for Anhy-B and V ₀?=?624.71?± 0.03?ų, K_T,0?=?142.6?±?0.8?GPa and K′?=?5.8?±?0.2 for Shy-B. A similar analysis of the axial compressibilities in Anhy-B reveals that the c-axis is most compressible (K_c?=?137?±?3?GPa), the b-axis is least compressible (K_b?=?175?±?4?GPa), and the a-axis is intermediate (K_a?=?148?±?1?GPa). In Shy-B, the a-axis is most compressible (K_a?=?135?±?1?GPa), followed by the b- and c-axes which have similar compressibilities (K_b?=?146?±?3?GPa; K_c?=?148?±?3?GPa). The fact that the b-axis of Shy-B is approximately 16% more compressible than Anhy-B is primarily due to differences in the O-T layer in which the H atoms are located and the linkages with the adjacent O layers. The rigid edge-sharing chains of MgO₆ and SiO₆ octahedra in the O layer control compressibility along the a- and c-axes in both structures. The net result is a reduction in the overall anisotropic compression from ～22% in Anhy-B to ～9% in Shy-B.     

Effects of defect and pressure on the thermal expansivity of Fe x O

Pressure–volume–temperature measurements have been carried out using synchrotron X-ray diffraction for wüstite at static pressures of 1.9, 2.6, and 5.4 GPa. Our results revealed that the composition change of wüstite and, hence, rearrangements of defect structures are primarily caused by the magnetite (Fe₃O₄) exsolution at temperatures of 523–723 K. Based on the isobaric volume–temperature data collected during cooling, the contribution of compositional variations to the unit-cell volumes of wüstite in the ranges of 300–673 K and 723–1073 K is negligibly small, within the experimental uncertainties. These observations suggest that the measured volume changes in the range of 300–673 K and 723–1,073 K can be attributed to the metal–oxygen bond expansion. Owing to the magnetite exsolution, thermal expansion data are obtained in each experiment at 1.9, 2.6, and 5.4 GPa for wüstite of two different compositions, Fe_0.987O and Fe_0.942O. At all three pressures, Fe_0.942O shows a thermal expansion that is about 30% larger than Fe_0.987O. Such findings represent the first experimental evidence of a substantial effect of nonstoichiometry on thermal expansivity, and based on previous thermodynamic calculations of the defect formation and interaction, this effect is likely associated with the distinct defects arrangements in iron-rich and more iron-deficient wüstite. This study also presents thermal equations of state for wüstite of two different compositions. Such volume-related properties at high temperatures are experimentally difficult to obtain in wüstite but important for thermodynamic studies in the binary Fe–O system.     

The electrical conductivity of some hydrous and anhydrous molten silicates as a function of temperature and pressure

Electrical conductivity of the following molten silicate systems (in mol%): 60SiO₂-40Na₂O; 65SiO₂-35Na₂O; 75SiO₂-25Na₂O; 78SiO₂-22Na₂O; 72SiO₂-24Na₂O-4CaO; 66SiO₂-19Na₂O-15H₂O; and an anhydrous and hydrous (4 wt% H₂O) Mt. Erebus lava, have been measured as a function of temperature (to 1000°C) and pressure (to 1.3 kbar). The anhydrous soda-rich melts have a positive pressure coefficient of conductivity to ~200 bars and beyond this pressure the pressure coefficient is small and negative. Addition of water lowers the conductivity and gives rise to a negative pressure coefficient at the highest temperatures. The conductivity of hydrous Mt. Erebus lava passed through a maximum with increasing temperature at constant pressure. These phenomena are interpreted in terms of explanations of similar phenomena found in molten salt and aqueous electrolyte solutions.     

Gas-phase molecular structure and energetics of anionic silicates

The gas-phase stabilities of linear, branched and cyclic silicates made of up to five silicon atoms were studied with density functional theory (DFT). The starting geometries for the DFT calculations at the B3LYP/6-311+G(2d,2p) level of theory were obtained from classical molecular dynamics simulations. We have observed that geometric parameters and charges are mainly affected by the degree of deprotonation. Charges on Si atoms are also influenced by their degree of substitution. The enthalpy of deprotonation of the neutral species was found to decrease with the size of the molecule, while the average deprotonation enthalpy of highly charged compounds increased with molecular size. Furthermore, the formation of rings in highly charged silicates is enthalpically preferred to chain growth. These observations result from two competing effects: the easier distribution of negative charge in silicates with low charge density and the strong intramolecular repulsions present in silicates with high charge density. As a consequence, highly charged silicates in the gas phase tend to be as small and as highly condensed as possible, which is in line with experimental observations from solution NMR.     

Sequestration of CO2 in magnesium silicates, in Western Macedonia, Greece

Carbonation of magnesium seems to be an interesting option for long term storage of captured CO₂. This paper provides an approach to sequestration of carbon dioxide in magnesium silicates using ultramafic rocks from the mountain of Vourinos, in Western Macedonia, Greece. For the experimental procedure five samples were used, consisted of dunite, hartzburgite and pyroxenite. The carbonation method chosen is the aqueous scheme. The results showed low (only about 10% of the stoichiometrically possible amount) transformation into magnesium carbonates for the majority of the samples. Insufficient reaction time, the particle size, or improper choice of reaction conditions are may be some of the reasons for the small amounts of carbonation observed. Further studies are needed in order to identify the various issues that were responsible.     

Gel synthesis of magnesium silicates: A 29Si magic angle spinning NMR study

The formation of the magnesium silicate minerals forsterite, enstatite, and roedderite by heating of amorphous “protosilicate” gels precipitated from aqueous solution has been studied by ²⁹Si MAS nmr. Gentle drying of the hydrogels at 110° C gives materials with broad nmr signals that do not differ appreciably with preparation conditions, but the minerals formed by heating at 750° C or higher are greatly dependent on the precipitation and washing conditions of the original gel. The rare mineral roedderite, best known from studies of unequilibrated enstatite chondrite meteorites, becomes a major species along with forsterite when the hydrogels are washed with sodium hydroxide solution before drying and heating to 750° C.     

“Pseudosinhalite”, a new hydrous MgAl-borate: synthesis, phase characterization, crystal structure, and PT-stability

The new synthetic phase Mg₂Al₃O[BO₄]₂(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO₄], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H₂O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P2₁/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm^?1 and 3500?cm^?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO₂(OH).     

Static elasticity of cordierite I: Effect of heavy ion irradiation on the compressibility of hydrous cordierite

The effect of ion beam irradiations on the elastic properties of hydrous cordierite was investigated by means of Raman and X-ray diffraction experiments. Oriented single crystals were exposed to swift heavy ions (Au, Bi) of various specific energies (10.0–11.1 MeV/u and 80 MeV/u), applying fluences up to 5 × 10¹³ ions/cm². The determination of unit-cell constants yields a volume strain of 3.4 × 10^?3 up to the maximum fluence, which corresponds to a compression of non-irradiated cordierite at ~480 ± 10 MPa. The unit-cell contraction is anisotropic (e ₁ = 1.4 ± 0.1 × 10^?3, e ₂ = 1.5 ± 0.1 × 10^?3, and e ₃ = 7 ± 1 × 10^?4) with the c-axis to shrink only half as much as the axes within the ab-plane. The lattice elasticity for irradiated cordierite (? = 1 × 10¹² ions/cm²) was determined from single-crystal XRD measurements in the diamond anvil cell. The fitted third-order Birch–Murnaghan equation-of-state parameters of irradiated cordierite (V ₀ = 1548.41 ± 0.16 Å³, K ₀ = 117.1 ± 1.1 GPa, ?K/?P = ?0.6 ± 0.3) reveal a 10–11 % higher compressibility compared to non-irradiated cordierite. While the higher compressibility is attributed to the previously reported irradiation-induced loss of extra-framework H₂O, the anomalous elasticity as expressed by elastic softening (β _a ^?1 , β _b ^?1 , β _c ^?1  = 397 ± 9, 395 ± 28, 308 ± 11 GPa, ?(β ^?1)/?P = ?4.5 ± 2.7, ?6.6 ± 8.4, ?5.4 ± 3.0) appears to be related to the framework stability and to be independent of the water content in the channels and thus of the ion beam exposure.     

High-temperature thermal expansion and elasticity of calcium-rich garnets

We present new high temperature elasticity data on two grossular garnet specimens. One specimen is single-crystal, of nearly endmember grossular, the other is polycrystalline with about 22% molar andradite. Our data extend the high temperature regime for which any garnet elasticity data are available from 1000 to 1350 K and the compositional range of temperature data to near endmember grossular. We also present new data on the thermal expansivity of calcium-rich garnet. We find virtually no discernable differences in the temperatureT derivatives at ambient conditions of the isotropic bulkK _S and shearμ moduli when comparing our results between these two specimens. These calcium-rich garnets have the lowest values of ¦(?K _S /?T) _P ¦ = (1.47,1.49) x 10^-2GPa/K, and among the highest values of ¦(?μ/?T) _P ¦ = 1.25 x 10^-2GPa/K, when compared with other garnets. Small, but measurable, nonlinear temperature dependences of most of the elastic moduli are observed. Several dimensionless parameters are computed with the new data and used to illustrate the effects of different assumptions on elastic equations of state extra-polated to high temperatures. We discuss how dimensionless parameters and other systematic considerations can be useful in estimating the temperature dependence of some properties of garnet phases for which temperature data are not yet available. While we believe it is premature to quantitatively predict the temperature variation ofK _S andμ for majorite garnets, our results have bearing on the amount of diopside required to explain the shear velocity gradients in Earth's transition zone.     

High-temperature phase transition and local structure of a hydrous anorthoclase

The in situ Raman spectra of a hydrous anorthoclase at temperatures of 20–800 °C have been measured using a LABRAM-HR spectrometer and Linkam TS 1500 heating stage. The frequencies of modes at 54, 99, 130 and 162 cm^?1 related to M–O vibrations decrease sharply and then increase drastically or keep steady at temperatures above 200 °C. A knee point can be clearly seen at about 200 °C for those modes. The frequency of the mode at 282 cm^?1 shows little temperature dependence. However, for the two strongest modes at 471 and 512 cm^?1, the frequencies decrease linearly with increasing temperature. From evolution of the frequencies of modes at 54, 99, 130 and 162 cm^?1 with temperature, the following conclusions can be drawn: (1) The distance of the local M–O bond shortens rather than lengthens at temperatures above 200 °C; (2) The abrupt changes of the local structure of M site induce a collapse of the framework structure and displacive phase transition at 200 °C; and (3) The H atoms incorporated in anorthoclase are located at the M site. These results are indicative for the structure and properties of anorthoclase at deep earth conditions.     

Stability of dense hydrous magnesium silicate phases in the systems Mg2SiO4-H2O and MgSiO3-H2O at pressures up to 27 GPa

We conducted high-pressure phase equilibrium experiments in the systems MgSiO₃ with 15 wt% H₂O and Mg₂SiO₄ with 5 wt% and 11 wt% H₂O at 20 ∼ 27 GPa. Based on the phase relations in these systems, together with the previous works on the related systems, we have clarified the stability relations of dense hydrous magnesium silicates in the system MgO-SiO₂-H₂O in the pressure range from 10 to 27 GPa. The results show that the stability field of phase G, which is identical to phase D and phase F, expands with increasing water contents. Water stored in serpentine in the descending cold slabs is transported into depths greater than 200 km, where serpentine decomposes to a mixture of phase A, enstatite, and fluid. Reaction sequences of the hydrous phases which appear at higher pressures vary with water content. In the slabs with a water content less than about 2 wt%, phase A carries water to a depth of 450 km. Hydrous wadsleyite, hydrous ringwoodite, and ilmenite are the main water reservoirs in the transition zone from 450 to 660 km. Superhydrous phase B is the water reservoir in the uppermost part of the lower mantle from 670 to 800 km, whereas phase G appears in the lower mantle only at depths greater than 800 km. In cold slabs with local water enrichment greater than 2 wt%, the following hydrous phases appear with increasing depths; phase A to 450 km, phase A and phase G from 450 km to 550 km, brucite, superhydrous phase B, and phase G from 550 km to 800 km, and phase G at depths greater than 800 km. Received: 4 August 1999 / Accepted: 1 March 2000