MgO FeO SiO2 Research Articles

Phase diagrams of oxide system FeO–MnO–MgO–SiO2. Report 2. Triple state diagrams FeO–MnO–SiO2, FeO–MgO–SiO2, MgO–MnO–SiO2

The knowledge of the thermodynamics and phase equilibria of silicate melts FeO–MnO–MgO–SiO2 and solid solutions of silicates of the MnO–MgO–SiO2 system allows us to predict the appearance of non-metallic inclusions in steel. The FeO–MnO–MgO–SiO2 system includes six double and four triple state diagrams of oxide systems. Previously, the state diagrams of binary FeO–MnO, FeO–MgO, FeO–SiO2, MnO–MgO and ternary FeO–MnO–MgO oxide systems were studied. In this work, state diagrams of the FeO–MnO–SiO2, FeO–MgO–SiO2 and MgO–MnO–SiO2 systems were constructed. The calculations used the theory of subregular ionic solutions, which takes into account the dependence of the coordination number on the composition of the melt, quite accurately describes diagrams with two immiscible liquids and is consistent with the experimental data presented in the literature. In the work, the energy parameters of this theory were selected to calculate the activities of the components of the oxide melt of each of the systems under study, as well as the parameter of the theory of regular ionic solutions to calculate the activities of the components of the magnesium-manganese orthosilicate solution |Mg2SiO4, Mn2SiO4|. The data obtained on the state diagrams of the systems included in the FeO–MgO–MnO–SiO2 system will make it possible to establish non-metallic inclusions in equilibrium with the liquid metal in the Fe–Mg–Mn–Si–O system

Visualization of Slag Data for Efficient Monitoring and Improvement of Steelmaking Slag Operation in Electric Arc Furnaces, with a Focus on MgO Saturation

Frequent slag sampling and analysis is still the most common method used to investigate and improve slag operation in electric arc furnaces (EAFs) for low-alloyed carbon steelmaking. An MgO saturation diagram for EAF slags was derived from phase equilibrium calculations in the system CaO–MgO–FeO–SiO2–5%Al2O3 to provide monitoring and interpretation of the slag data with respect to control of MgO saturation, FeO reduction, dissolution from the MgO-based refractory lining, and unusual losses of repair mixes. Examples from 14 industrial EAFs are given.

Effect of various components on the distribution of phosphorus in CaO–FeO–MgO–SiO2–MnO–TiO2–V2O5–P2O5 slag based on IMCT

ABSTRACT A thermodynamic calculation model of phosphorus distribution ratio between CaO–FeO–MgO–SiO2–MnO–TiO2–V2O5–P2O5 slags and molten metal has been developed based on the ion and molecule coexistence theory (IMCT). The calculated phosphorus distribution ratio shows a good agreement with the measured ones by the data of industrial trial, which proves the reliability of this model. Therefore, the model is used to study the effects of TiO2, V2O5, slag basicity, FeO, MgO and MnO on phosphorus distribution. The results illustrate that the effects on phosphorus distribution from V2O5 and TiO2 are non-negligible and they are almost the same, slag basicity and FeO have the most significant effects on phosphorus distribution, while MgO and MnO have the least.

Effects of valence and spin of Fe in MgSiO3 melts: Structural insights from first-principles molecular dynamics simulations

Iron (Fe) is present in terrestrial melts and at all depths inside the Earth. How Fe in its varying oxidation and spin states influences the properties of silicate melts is of critical importance to the understanding of the chemical evolution of our planet. Here, we report the results of first-principles molecular dynamics simulations of molten Fe-bearing MgSiO3 over a wide pressure range covering the entire mantle. Our results suggest that the structural properties of the host melt, such as the average bond length and coordination in Mg–O and Si–O do not differ much when compared with the pure melt. More importantly, they show that the local Fe–O structure is more sensitive to the spin state (high-spin, HS and low-spin, LS) of iron than to its valence state (Fe2+ and Fe3+). For iso-valence configurations, the average Fe–O bond length and coordination number differ by more than 10% and ∼30%, respectively, between the HS and LS states. In comparison, the corresponding differences between Fe2+ and Fe3+ for iso-spin configurations are within 5 and 15%, respectively. Ferrous iron shows lower average oxygen coordination numbers of ∼3.8 for HS and ∼3.3 for LS compared to the corresponding numbers of ∼4.1 and ∼3.7 for ferric iron at 0 GPa and 3000 K. As pressure increases, the coordination gap between the ferrous and ferric iron closes for HS but persists for LS. Our analysis of the proportions of non-bridging and bridging oxygens and the rates of bond breaking/formation events suggests an equivalent role of the ferrous and ferric iron in terms of their network forming ability. The predicted structural behavior of iron in its different oxidation states is generally consistent with the experimental inferences for MgO–FeO–SiO2 melts. Unlike other ferrosilicate compositions for which the experimental data suggest that Fe3+ increases and Fe2+ decreases the viscosity of the melt, the ferrous and ferric iron, due to their structural equivalence, are likely to have a similar influence on the dynamical behavior of deep mantle iron-bearing MgSiO3 melts.

The Carbonate Geochemistry of Enceladus' Ocean

AbstractThe plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and salt‐rich ice grains. It is found that self‐consistency between the data sets can be achieved with a derived range of 10−4.6 to 10−3.2 for the activity of CO2 in Enceladus' ocean. This range is compatible with long‐term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO–FeO–SiO2–CO2–H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2‐rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability.

Evolution of Diamond-Forming Systems of the Mantle Transition Zone: Ringwoodite Peritectic Reaction (Mg,Fe)2SiO4 (Experiment at 20 GPa)

Abstract—The peritectic reaction of ringwoodite (Mg,Fe)2SiO4 and silicate–carbonate melt with formation of magnesiowustite (Fe,Mg)O, stishovite SiO2, and Mg, Na, Ca, K-carbonates is revealed by experimental study at 20 GPa of phase relations in the multicomponent diamond-forming MgO–FeO–SiO2–Na2CO3–CaCO3–K2CO3 system of the Earth mantle transition zone. An interaction of CaCO3 and SiO2 with a formation of Ca-perovskite CaSiO3 is also detected. It is shown that the peritectic reaction of ringwoodite and melt with the formation of stishovite controls physicochemically the fractional ultrabasic-basic evolution of both magmatic and diamond-forming systems of deep horizons of the transition zone up to its boundary with the Earth lower mantle.

The fate of carbonate in oceanic crust subducted into earth's lower mantle

We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO–MgO–SiO2–CO2 (FMSC) and CaO–MgO–SiO2–CO2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of ∼35 to 90 GPa at temperatures of ∼1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO2–V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite ±Ca-perovskite + CO2 at pressures in the range of ∼40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than ∼1500 km. We also identify carbonate and CO2–V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes.

The stishovite paradox in the evolution of lower mantle magmas and diamond-forming melts (experiment at 24 and 26 GPa)

Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO2 at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO2–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.

Magmatic evolution of the material of the Earth’s lower mantle: Stishovite paradox and origin of superdeep diamonds (Experiments at 24–26 GPa)

The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO2 and periclase–wustite solid solutions, (MgO · FeO)ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO2 react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO2–CaSiO3 system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO3 (bridgmanite) + L = SiO2 + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO3 (bridgmanite) + (MgO · FeO)ss (ferropericlase), and basic minerals, (FeO · MgO)ss (magnesiowustite) + SiO2 (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.

The role of reacting solution and temperature on compositional evolution during harzburgite alteration: Constraints from the Mesoarchean Nuasahi Massif (eastern India)

We investigate the microtextural–chemical features of partially serpentinized harzburgites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India, in order to understand the role of reacting fluid composition and temperature on the phase evolution across replacement interfaces during progressive alteration. Two distinct types of pseudomorphic replacement textures are identified. Type-1 replacement texture was developed after primary orthopyroxene and is composed of talc + olivine + lizardite + tremolite + magnetite. Primary olivine was replaced by mesh-textured Mg-rich lizardite + magnetite at the center of the olivine grains and successive layers of relatively Fe-rich lizardite, magnesite, and calcite toward olivine rims, defining type-2 replacement texture. The Nuasahi harzburgite was initially out-of-equilibrium with respect to H2O–CO2-bearing reacting solution and the secondary compositions have mainly evolved in the CaO–MgO–FeO–SiO2–Al2O3–H2O–CO2 system as a result of fluid–rock interaction. The alteration process across orthopyroxene interfaces has started at relatively higher temperature conditions (400<T<675°C) than that at primary olivine interface (T≤330°C). Each replacement process across reaction interfaces was controlled via an interface-coupled dissolution–precipitation mechanism. The sequential development of different secondary compositions in these replacement rims indicates a micrometer-scale variation in silica activity and H2OCO2aq ratio in the solution across replacement interfaces. The Fe2+Mg−1 chemical exchange potential of the equilibrating system plays an important role in dictating the Fe/Mg ratio of the secondary compositions and the molar proportions of magnetite. The precipitation of tremolite and calcite in some isolated areas reflects a variation in Ca activity in the reacting fluid. The precipitation of carbonates may also be associated with an increase in pH in the interfacial solution. The pattern in phase evolution across type-1 interfaces suggests a greater extent of diffusion of solutes toward inner interfaces relative to the rim center during progressive alteration of the Nuasahi harzburgite, possibly due to an increase in permeability in replaced phases. With increasing magnitude of diffusion toward type-1 inner reacting interfaces, the widespread presence of arrested replacement features suggests an alteration event under low fluid/rock ratio and/or sluggish reaction kinetics with decreasing temperature.

Water storage capacity in olivine and pyroxene to 14 GPa: Implications for the water content of the Earth's upper mantle and nature of seismic discontinuities

Experiments were performed under water-saturated conditions in the MFSH (MgO–FeO–SiO2–H2O) and MFASH (MgO–FeO–Al2O3–SiO2–H2O) systems at 2.5, 5, 7.5 and 9GPa, at temperatures from 1175 to 1400°C and H2O initial abundance of 0.5–5wt%. One experiment was performed at 13.5GPa at a temperature of 1400°C in the MFSH system. Water contents were analyzed by Fourier transform infrared spectroscopy. Results show that Al contents in olivine and pyroxene in equilibrium with an aluminous phase decrease significantly with increasing pressure and decreasing temperature. The incorporation of Al enhances water incorporation in olivine and pyroxene, but only at pressures of 2.5 and 5GPa. At 7.5GPa (i.e. 225km depth) the pyroxene is monoclinic, indicating that in a hydrous mantle the orthoenstatite to clinoenstatite phase transition occurs at shallower depths than previously thought, which is more consistent with the Lehmann discontinuity than with the X discontinuity. The partitioning of water between pyroxene and olivine in the MFASH system decreases from a value of 2 at 2.5GPa (80km depth) to 0.9 at 9GPa (270km depth). At 13.5GPa and 1400°C, the water content of olivine is 1700±300ppmwt H2O. The water partition coefficient between coexisting wadsleyite and olivine equals 4.7±0.7. We conclude that the water storage capacity of the upper mantle just above the 410km discontinuity is of 1500±300ppmwt H2O. If we assume that the Low Velocity Layer observed near 350km is due to mantle melting, we can constrain the water content of the mantle at that depth to be ∼850±150ppmwt H2O. This new value is four times higher than previous estimates for the mantle source of Mid Oceanic Ridge Basalts.Finally, comparison of the depth ranges of the L and X seismic discontinuities and the water storage capacity of the upper mantle suggests that the L-discontinuity (180–240km) is concomitant with a kink in the water storage due to the orthorhombic to monoclinic phase transition in enstatite, while the X-discontinuity (240–340km) coincides with a kink in the water storage capacity due to dehydration of garnet.

Influence of temperature, composition, silica activity and oxygen fugacity on the H2O storage capacity of olivine at 8 GPa

Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000–1,600°C. Experiments were conducted both in a simple system (FeO–MgO–SiO2–H2O) and in a more complex system containing additional elements (CaO–Na2O–Al2O3–Cr2O3–TiO2–FeO–MgO–SiO2–H2O). Silica activity was buffered by the presence of either pyroxene (high a SiO2) or ferropericlase (low a SiO2), and $$f_{{\text{O}}_{\text{2}} }$$ was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe2O3. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C experiment, and enstatite in experiments at 1,400–1,520°C. The H2O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate the nature of hydrous defects. The H2O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H2O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm−1) peaks, generally associated with high $$f_{{\text{O}}_{\text{2}} }$$ at low pressure, was observed in the FTIR spectra of olivine from the high $$f_{{\text{O}}_{\text{2}} }$$ experiments. Our experiments show that previous high pressure H2O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth’s mantle are not likely to be compromised by the $$f_{{\text{O}}_{\text{2}} }$$ of the experiments. However, the considerable effect of temperature on H2O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H2O storage capacity.

Thermodynamic model calculations in multicomponent liquid silicate systems

A thermodynamic model developed earlier in the present laboratory for oxidic melts was applied to some multicomponent systems, namely CaO–FeO–MgO–SiO2 , Al2 O3–CaO–MgO–SiO2 , Al2 O3–FeO–MnO–SiO2 , and Al2 O3–CaO–FeO–MgO–MnO–SiO2 . Model calculations were carried out using only the parameters corresponding to the binary systems, which, in turn, were based on the available thermodynamic information for these systems. The predicted thermodynamic activities of the component oxides in higher order systems were compared with the experimental data published in the literature. In general, the agreement between the model predictions and the experimental values was found to be satisfactory within the limits of experimental uncertainties and limitations of the model calculations. Examples of model predictions for some typical slag compositions, relevant to the Swedish steel industry and used in the blast furnace, electric arc furnace, and ladle furnace are also presented.

Perovskite as a possible sink for ferric iron in the lower mantle

The lower mantle constitutes more than half the Earth's interior by volume, and is believed to consist predominantly of (Mg,Fe)SiO3 perovskite with up to approximately 20% (Mg,Fe)O. In the system FeO–MgO–SiO2, iron partitions preferentially into (Mg,Fe)O relative to the perovskite phase and has been believed to be nearly all in the form of Fe2+on the basis of experiments in the MgO–FeO–SiO2 system1,2,3,4. Here, however, we present a Mossbauer study of (Mg,Fe)SiO3 perovskite containing 3.3 mol% Al2O3, which shows that approximately 50% of the iron is Fe3+. These results, combined with evidence from other experiments, suggest that the proportion of iron present as Fe3+in the lower-mantle perovskite phase is probably much higher than is currently believed. Because the oxidation state of iron in the perovskite phase affects the electrostatic charge balance and equilibrium defect concentration, the presence of Fe3+is likely to significantly affect physical and chemical properties of the lower mantle such as sub- and super-solidus phase relations, transport properties, mechanical behaviour, trace-element partitioning and the concentration of species such as hydroxide ions.

Constitution of the mantle. 2. Petrological models of transition zone based on FMS phase diagram

Abstract A detailed picture is presented of phase transformations in the mantle transition zone, based upon the study of phase relations and topologies in the FeOMgOSiO2 (FMS) system. The (P - T)x diagram for the composition close to pyrolite (SiO2 = 40 mol.% and Fe/(Fe + Mg) = 0.12) is used in modelling the structure of the mantle transition zone, to connect phase transitions in minerals to the observed seismic discontinuities. The mineralogical nature of the first seismic discontinuity, at a depth of about 400 km, is associated either with a univariant transformation Px + α + γ → α + β + Px or with a sharp divariant transformation α + Px → α + β + Px → β + Px, which should be closer to 420 km. For the fixed composition close to pyrolite at a depth of about 650 km neither univariant nor invariant equilibria are observed in the FMS system. It has been found that, in the FMS system, two extremely narrow (1–2 km wide) stability fields of divariant mineral assemblages γ + Pv + St and γ + Pv + Mw, separated by a trivariant zone γ + Pv (about 5 km wide) might be responsible for the nature of the 650 km discontinuity. Because of the negative slope of the phase boundaries, the second seismic discontinuity in the cold subducting slab is deeper than in the surrounding mantle.

Constitution of the mantle. I. Phase relations in the FeOMgOSiO2 system at 10–30 GPa

P - T - x phase diagrams of the FeOMgOSiO 2 (FMS) system are calculated using the ideal solution model at 10–30 GPa and 1000–2000 K. A thermodynamic data base for minerals in the MgOSiO 2 and FeOSiO 2 systems is established by supplementing the calorimetric data for low-pressure phases and equations of state for all phases with data calculated from high-pressure synthesis experiments. The thermodynamic functions of virtual phases in the FeOSiO 2 system are evaluated. The topologies of calculated P - x sections of phase diagrams at 1273 K and 2000 K are in agreement with earlier experimental data. Univariant curves γ + St = Ilm + Mw, γ + Ilm = Pv + Mw + St intersect giving the nondegenerated invariant point (St + Mw + γ + Pv + Ilm) at T ≈ 1650 K, P ≈ 23 GPa and Fe/(Fe + Mg) ratios in the range 0.19–0.30. Univariant curves α + γ = β and Px + γ = β + St do not intersect in the calculated interval of P - T conditions. At temperatures higher than 1650 K, Ilm = Pv + γ + St and γ + St = Pv + Mw equilibria are stable. It is supposed that the disagreement between the earlier experimental phase diagrams is due to the experiments being carried out at temperatures above and below the invariant point.

Structure of the mantle discontinuities

Abstract The results of thermodynamic calculations in the FeO-MgO-SiO2 (FMS) system show that the elastic properties and density across the 400-km discontinuity might be consistent with a transition zone of isochemical pyrolite composition and do not require chemical stratification. However, phase changes in pyrolite do not explain the 650-km discontinuity.

Stability of RSiO3 phases in the manganese rich portion of the system MnO-MgO-FeO-SiO2-CO2.

Experimental investigation in the Mn-rich portion of the system MnO–MgO–FeO–SiO2–CO2 has been carried out to study the effect of XFe on the phase relation in the ternary system MnSiO3–MgSiO3–FeSiO3. The experiments were conducted with oxygen and/or graphite buffers in the system C–O–H at pressure-temperature conditions corresponding to normal crustal metamorphism to understand the evolution of mineral assemblages from Ca-poor Mn–Mg–Fe carbonates admixed with silica.Stability field of pyroxmangite solid solution containing Fe can appear at lower temperature than the MnSiO3 end member. The appearance of single phase pyroxmangitess is insensitive to variations in XMn of the bulk in the range ∼0.8 and above. With increasing XMg and XFe in the bulk composition, carbonatess+quartz react to produce pyroxmangitess+Mn-clinopyroxeness and Mn-clinopyroxeness successively. All the reaction boundaries are insensitive to pressure, at least up to 8 kb, and have gentle slope in temperature. These reactions are also independent of the XCO2 of the fluid phase in the region XCO2=0.21−0.53 above moderate temperatures. The slope of the lnXCO2−10−3/T K relationship of these reactions is similar to that of the pure pyroxmangite forming reaction. Mn-clinopyroxeness can be stabilized at a low temperature than pyroxmangitess from favourable bulk composition [X(Mg+Fe)=0.4]. With increasing Fe content, stability field of Mn-clinopyroxeness extends toward higher XMn in region as compared to that in Fe-free system.

Theoretical Computation of Mineral Assemblages in Pyrolite and Lherzolite

Using the thermochemical data base of Helgeson et al. (1978), missing data on several phases have been calculated from experimental phase equilibrium relations in the systems MgO–Al2O3–SiO2, CaO–MgO–Al2O3–SiO2, CaO–MgO–FeO–SiO2, diopside–jadeite–quartz, spinel–olivine, biotite–garnet and clinopyroxene–garnet. These data have been used to compute mineral assemblages (including modes and compositions of coexisting phases) in the system CaO–MgO–Al2O3–FeO–Na2O–SiO2 with pyrolite and lherzolite compositions over a broad pressure (up to 50 kb) and temperature (up to 1500 °C) range. Generalized isopleths (lines of equal composition) of A12O3 in pyroxenes, Ca in clinopyroxene and the pressure–temperature dependence of garnet–clinopyroxene distribution coefficient (Fe2+–Mg) have been graphically displayed. Depending on the temperature, the pressures of transition of a plagioclase bearing basic rock to spinel–peridotite and of the latter to garnet–peridotite lie in the ranges of 4 to 10 and 9 to 25 kb respectively.

High mobility of heavy rare gases in potassium halides at low temperatures (20–100°C)

Akimoto, S., 1972. The system MgO–FeO–SiO2 at high pressures and temperatures – phase equilibria and elastic properties. In: A.R. Ritsema (Editor), The Upper Mantle. Tectonophysics, 13 (1–4): 161–187.Advances, during the Upper Mantle Project, in investigations on phase equilibria and elastic properties of the mantle minerals in the MgO–FeO–SiO2 system are reviewed. The experimental procedure for a phase equilibrium study is described. Some advantages and disadvantages of various high-pressure, high-temperature apparatus are discussed. Necessities of establishing a pressure calibration method at high temperatures are also pointed out. A brief description on a method, recently developed for the measurements of ultrasonic wave velocities of very small samples, is given.Olivine–spinel solid solution equilibria in the system Mg2SiO4–Fe2SiO4 have been studied at several laboratories over the pressure range 40–200 kbar at 800 and 1,000°C. Ringwood and Major first discovered a peculiar mode of the high-pressure transformation in compositions close to pure Mg2SiO4, resulting from the formation of β phase. Crystal structure of the β phase was clarified as modified spinel structure through a series of studies on the high-pressure transformation of Co2SiO4, and Mn2GeO4. The appearance of the modified spinel phase in the Mg2SiO4–Fe2SiO4 system was confirmed in further investigations at the author's laboratory. The isothermal section of the phase diagram for the Mg2SiO4–Fe2SiO4 system was constructed at 800 and 1,000°C on the basis of these recent experimental results. At 800°C a continuous series of spinel solid solutions was synthesizable from Fe2SiO4 to (Mg0.9Fe0.1)2SiO4. At 1,000°C, however, all the attempts to synthesize a true spinel phase of (Mg0.9Fe0.1)2SiO4 were unsuccessful up to about 140 kbar and coexistence of the modified spinel phase with true spinel was usually identified. This suggests that the stability field of the β(Mg, Fe)2 SiO4 is highly temperature dependent. A remarkable expansion of the β(Mg, Fe)2SiO4 region is expected at the higher temperatures. Based on the cell parameters of βMg2SiO4 and the extrapolated value for γMg2SiO4 (true spinel), the density increase associated with forsterite–βMg2SiO4 transformation, and forsterite– γMg2SiO4 transformation was calculated to be 7.9% and 10.8% respectively.Experimental data on the phase equilibria of the MgSiO3–FeSiO3 system are presented. A high-pressure disproportionation of clino-pyroxene solid solutions into stishovite plus spinel solid solutions was found. It was established that the disproportionation curve for clino-ferrosilite (FeSiO3) was well represented by the boundary curve for the coesite–stishovite transformation. A preliminary phase diagram for the MgSiO3–FeSiO3 system was constructed at 800 and 1,000°C from the data by Ringwood and Major and by the author's laboratory.Experimental data on the high-pressure phase transformations of SiO2 are summarized. The boundary curve for the coesite–stishovite transformation was determined over the temperature range 550–1,200°C in the pressure range 83–101 kbar by means of a tetrahedral anvil press. The transition curve was fitted by the linear relation P(kbar) = 67 + 0.028 T(°C). This determination was found to be in reasonable agreement with the previous data.A stoichiometric compound, Fe1.000O, was synthesized at high-pressures above 40 kbar at 775°C by a reaction between wüstite, Fe0.950O, and metallic iron. The cell dimension of Fe1.000O was determined to be 4.323 ± 0.001 å.Compressional- and shear-wave velocities of the synthetic (Mg, Fe)2SiO4 olivine, Fe2SiO4 spinel, (Mg, Fe)SiO3 orthopyroxene, coesite, stishovite and Fe0.98O were measured by means of the ultra-sonic pulse transmission method. Results are represented on Birch's diagram, where the wave velocities are plotted as a function of density. It was found that the wave velocities of these ferromagnesian silicates and oxide decrease linearly with the increase of the FeO+(FeO+MgO) ratio, and that the isomorphic lines of (Mg, Fe)2 SiO4 olivine, (Mg, Fe)SiO3 orthopyroxene and (Mg, Fe)O magnesiowüstite are approximately parallel to each other. The compressional- and shear-wave velocities of the true spinel phase of Mg2 SiO4 were estimated to be 10.0 km/sec and 5.7 km/sec respectively.Compressional- and shear-wave velocities of stishovite were determined to be 11.0 km/sec and 5.55 km/sec. The bulk modulus of stishovite was calculated from these values to be 3.43 Mbar. Compressional-wave velocity of the three polymorphs of silica, αquartz, coesite and stishovite, was found to increase regularly along Birch's mean atomic weight line of M¯=21.