MgO SiO2 System Research Articles

Equilibrium experiments in the system MgO-SiO 2 -H 2 O (MSH): stability fields of clinohumite-OH [Mg 9 Si 4 O 16 (OH) 2 ], chondrodite-OH [Mg 5 Si 2 O 8 (OH) 2 ] and phase A (Mg 7 Si 2 O 8 (OH) 6 )

The water-pressure and temperature stability fields of clinohumite-OH, chondrodite-OH and phase A were determined in reversed equilibrium experiments up to 100 kbar within the system MgO–SiO2–H2O. Their PT-fields differ from results from former synthesis experiments. Bracketing experiments on the reaction phase A + low P-clinoenstatite ⇆ forsterite + water resulted in a slightly steeper dP/dT-slope compared to earlier experiments for this equilibrium. Clinohumite-OH and chondrodite-OH both have large stability fields which extend over pressure ranges of more than 80 kbar. However, they are hardly relevant as hydrous minerals within the subducted oceanic lithosphere. Both are too Mg-rich for a typical mantle bulk composition. In addition, the dehydration of subducted oceanic lithosphere – due to (forsterite + water)-forming reactions – will occur before the two humite-group phases even become stable. Restricted to the cool region of cold subducting slabs, phase A, however, might be formed via the reactions phase A + low P-/high P-clinoenstatite ⇆ forsterite + water or antigorite + brucite ⇆ phase A + water, before dehydration of the oceanic lithosphere occurs.

Melting experiments in the systems CaO-MgO-Al2O3-SiO2and MgO-SiO2at 3 to 15 GPa

The results of multi-anvil melting experiments are reported for a range of compositions in the system CaO-MgO-Al 2 O 3 -SiO 2 . The liquidus crystallization fields for forsterite, orthopyroxene, clinopyroxene, and garnet have been mapped out at 10 GPa, as have their intersections at various cotectics. The composition of the liquid that is multiply saturated in forsterite, orthopyroxene, clinopyroxene, and garnet has been also determined to within + or -0.5-1.0 wt% (2sigma ), and the result is in excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al 2 O 3 and increase MgO and SiO 2 in magmas formed by the melting of garnet lherzolite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO 2 also have been performed to constrain how pressure affects the compositions of liquids that are saturated in harzburgite [L+Ol+Opx]. Experiments in both CaO-MgO-Al 2 O 3 -SiO 2 and MgO-SiO 2 demonstrate that there is a maximum normative olivine content to liquids formed by initial or advanced melting of peridotite in the upper mantle, and this occurs at 7 to 8 GPa. For most peridotite that undergo decompression melting in a plume, clinopyroxene and garnet are the first crystalline phases to melt out and, with a few important but rare exceptions, the experimentally constrained liquids are unlike most volcanic rocks. Advanced anhydrous melting will yield liquids with a residual harzburgite mineralogy [L+Ol+Opx], and these liquids are similar in composition to most komatiites with Cretaceous and 2700 Myr Archaean ages.

Energy barriers to viscous flow and the prediction of glass transition temperatures of molten silicates

The results of multi-anvil melting experiments are reported for a range of compositions in the system CaO-MgO-Al2O3-SiO2. The liquidus crystallization fields for forsterite, or­thopyroxene. clinopyroxene. and garnet have been mapped out at 10 GPa. as have their intersections at various cotectics. The composition of the liquid that is multiply saturated in forsterite. orthopyroxene, clinopyroxene. and garnet has been also determined to within ±0.5-1.0 wt% (2σ). and the result is in excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al2O3 and increase MgO and SiO2 in magmas formed by the melting of garnet lherzolite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO2 also have been performed to constrain how pressure affects the compositions of liquids that are saturated in harzbiugite [L + Ol + Opx]. Experiments in both CaO-MgO-Al2O3-SiO2 and MgO-SiO2 demonstrate that there is a maximum normative olivine content to liquids formed by initial or advanced melting of peridotite in the upper mantle, and this occurs at 7 to 8 GPa. For most peridotites that undergo decompression melting in a plume, clino­pyroxene and garnet are the first crystalline phases to melt out and. with a few important but rare exceptions, the experimentally constrained liquids are unlike most volcanic rocks. Advanced anhydrous melting will yield liquids with a residual harzburgite mineralogy [L + Ol + Opx]. and these liquids are similar in composition to most komatiites with Cre­taceous and 2700 Myr Archaean ages.

Experimental Determination of Second Critical Endpoints in Fluid/Melt Systems

Fluid phases play an important role during many processes in the upper mantle and in the crust. They work as efficacious transport agents during mantle metasomatism and depress the solidus of silicates towards lower temperatures. Consequently, they have a prominent influence on the velocities of seismic wave propagation and on the rheological properties of rocks. However, even in modem literature the term 'fluid' is not adequately well defined and it is even used as a synonym for all non-solid phases. One reason for this loose definition is certainly the fact that fluids (especially water dominated ones) and silicate melts show converging similarites with increasing pressure. In many silicate-water systems, the water content of the melt and the silicate content of the fluid increase steadily with increasing pressures. In the silica-water system this behaviour is continued up to a second critical endpoint, where a fluid and a melt are no longer distinguishable and the term 'solidus' has to be reconsidered (Kennedy et al., 1962). Investigations in other simple systems (e.g. diopside-water, Eggler and Rosenhauer, 1978) revealed a similar behaviour, but for more complex mafic and ultramafic compositions a second critical endpoint has due to experimental limitations not yet been determined. The present experimental study started with an inspection of the system albite-water in order to evaluate the accuracy of a new technique to determine major element solubilities in fluid phases and water solubilities in melts. The system albitewater has previously been examined and a consensus with respect to phase relations, solidi and water solubilities in the melts has been achieved. Above 15 kbar the system approaches a second critical endpoint, where a distinction between fluid and melt can no longer be made (Boettcher and Wyllie, 1969; Paillat et al., 1992) and therefore a watersaturated solidus does no longer exist. Therefore it seems to be a suitable system to test our technique, which previously has only been applied to fluid/ mineral trace element partitioning (Stalder et al., 1997). In the next step this technique is being applied to the system MgO-SiO:-H20 at pressures of 60 to 100 kbar and 900 to 1100~ where the transition

Growth of MgSiO 3 orthoenstatite single crystals by the top-seeded solution growth (TSSG) method

Single crystals of orthoenstatite (MgSiO 3) have been grown by Top-Seeded Solution Growth (TSSG) method in the system MgOSiO 2Li 2CO 3MoO 3V 2O 5. The crystals are pulled along the c-axis, and are surrounded by the {1 0 0}, {0 1 0}, {2 1 0}, {1 1 1} and {2 1 1} faces. This morphology is the same as that observed in the flux-grown crystals. The tip surfaces of the crystals show that the growth speed of {1 1 1} face is faster than that of {2 1 1}. Imperfections such as inclusions and holes, are seen concentrated in the central portion, inherited from the seed interface. The faster growth along the c-axis than other crystallographic directions is responsible for the appearance of these imperfections.

Antigorite: High-pressure stability in the system MgOSiO 2H 2O (MSH)

The breakdown reactions of antigorite: (1) forming talc + forsterite + water at low pressures and (2) forming forsterite + clinoenstatite + water at high pressures were determined in reversed equilibrium experiments. Results on reaction (1) were found to be in good agreement with former experimental determinations by both Johannes [Johannes, W., 1975. Zur Synthese und thermischen Stabilität von Antigorit. Fortschr. Mineral. Beih. 53, 36.] and Evans et al. [Evans, B.W., Johannes, W., Oterdoom, H., Trommsdorff, V., 1976. Stability of crysotile and antigorite in the serpentinite multisystem. Schweiz. Mineral. Petrogr. Mitt. 56, 79–93.]. From our experiments the invariant point ( I 1), interconnecting the two reactions, can be located at about 15 kbar/650°C. This is consistent with the thermodynamic calculations using the dataset of Berman [Berman, R.G., 1988. Internally consistent thermodynamic data for minerals in the system J. Petrol. 29, 445–522.]; however, it is in contrast to recent experimental studies of Ulmer and Trommsdorff [Ulmer, P., Trommsdorff, V., 1995a. Serpentine stability to mantle depths and subduction-related magmatism. Science 268, 858-861.] who determined I 1 at 21 kbar/730°C. Our PT-conditions for I 1 could be confirmed by equilibrium experiments on reaction (10) talc + forsterite ↔ clinoenstatite + water, which is generated at I 1 as well. Up to about 25 kbar the breakdown reaction (2) is nearly pressure-independent. Towards still higher pressures the d P/d T-slope of reaction (2) bends and becomes negative. Schreinemakers analysis as well as thermodynamic calculations of the upper pressure-stability of antigorite show that the possible antigorite breakdown reaction (3) antigorite ↔ clinoenstatite + brucite + water and reaction (4) brucite + clinoenstatite ↔ forsterite + water could originate at a new invariant point I 2, provided that the reactions (2) and (11) brucite + antigorite ↔ forsterite + water intersect. Bracketing equilibrium (4) and combining these results with those on reaction (2), I 2 was located at only about 51 kbar/490°C, compared to 77 kbar/680°C according to Berman's data. However, when taking into account the dense hydrous magnesium silicate (= DHMS)-phase A, Mg 7Si 2O 8(OH) 6, the phase relations of antigorite are changed resulting (i) in the metastability of I 2 and reaction (3) and (ii) in a new invariant point I 7 at about 44 kbar and 580°C generating the new antigorite breakdown-reaction (16) antigorite ↔ phase A + clinoenstatite + water. On the basis of these new data on the stability of antigorite, earlier conclusions about dehydration depths in subducted serpentine-bearing oceanic lithosphere have to be reconsidered. The maximum pressure stability of antigorite according to reaction (16) extends between 44 and 55 kbar, that is between about 130 and 160 km depths, as opposed to about 75 kbar (220 km) following Ulmer and Trommsdorff (see above). Because many different thermal regimes are possible in subduction zones, no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time.

Subsolidus Phase Relations in the System MgO–SiO2–Cr–O and Thermodynamic Properties of Related Cr2+–Containing End–members

Abstract Subsolidus phase relations have been determined in the systems SiO2–Cr–O and MgO–SiO2–Cr–O in equilibrium with metallic Cr, at 1100 to 1500°C and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO2–Cr–O system at these conditions, i.e., only the assemblage eskolaite–Cr–metal–quartz (or tridymite) is found. In the MgO–containing system, however, extensive substitution of Cr2+ for Mg is observed in (Mg, Cr2+)2SiO4 olivine, (Mg, Cr2+)2Si2O6 pyroxene, and (Mg, Cr2+)Cr2O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at XPxCr2+ <0.2, monoclinic at higher XPxCr2+. The structure of the spinels becomes tetragonally distorted at XPxCr2+ >0.2. The experimental data have been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg‐Cr2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end–members: Cr2+–olivine (Cr2SiO4), Cr2+–pyroxene (Cr2Si2O6) and Cr2+–spinel (Cr3O4).

The low-pressure stability of phase A, Mg 7 Si 2 O 8 (OH) 6

Phase A, Mg7Si2O8(OH)6, is a dense hydrous magnesium silicate whose importance as a host of H2O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between 3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (105 Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H2O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH o f =−7126±8 kJ mol-1, S o=351 J K-1 mol-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite. The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H2O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO2–H2O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressures above approximately 6 GPa.

Thermal expansivities and compressibilities of hydrous phases in the system MgO-SiO 2 -H 2 O: talc, phase A and 10-Å phase

We have measured the thermal expansivity of talc, Mg3Si4O10(OH)2, and phase A, Mg7Si2O8(OH)6, and the compressibility of talc, phase A and 10-A phase, Mg3Si4O10(OH)2 ⋅ xH2O, using powder X-ray diffraction. The thermal expansivity of talc and phase A were measured at temperatures up to 810° C and 600° C, respectively. Volumes of both phases increase linearly with temperature, and can be described as follows: Talc: V/V0=1+2.15 (±0.05)×10-5 (T−298), V0=136.52 (±0.03) cm3 mol-1; Phase A: V/V0=1+4.86 (±0.18)×10-5 (T– 298), V0=154.42 (±0.09) cm3 mol-1. Compressibility measurements of talc, 10-A phase and phase A were made at pressures up to 6.05, 8.52 and 9.85 GPa, respectively. Values of the isothermal bulk modulus K298 and its pressure derivative K′, obtained by fitting the compressibility data to the Murnaghan equation, are as follows: Talc: K298=41.6±0.9 GPa, K′=6.5±0.4; 10-A phase: K298=32.2±5.5 GPa, K′=9.2±2.8; Phase A: K298=145±5 GPa (assuming that K′=4). Combining the new talc data with existing thermodynamic data provides a more accurate thermodynamic description of talc than previously available, enabling its high-pressure, high-temperature phase relations to be calculated. The data for 10-A phase are consistent with a positive slope for its dehydration reaction, making 10-A phase a good candidate for H2O storage in subducting slabs. The measurements of the thermal expansivity and compressibility of phase A allow its enthalpy of formation and entropy to be derived from the results of phase equilibrium experiments on phase A.

Experimental and theoretical studies of the stabilities of talc, antigorite and phase A at high pressures with applications to subduction processes

We have experimentally determined the equilibrium talc ⇄ enstatite + quartz/coesite + H 2O to 40 kbar in the system MgO SiO 2 H 2O (MSH) using both synthetic and nearly pure Mg end-member natural talc and other synthetic starting materials for the other solid phases. At 40 kbar, the equilibrium dehydration boundary lies ∼ 150°C higher than that calculated using data from the existing internally consistent thermochemical data bases. The reason for this discrepancy lies in the erroneous compressibility data of talc in the data bases. We have retrieved the compressibility of talc from the experimental phase equilibrium data, and have also calculated sereral other equilibria in the MSH system involving talc, antigorite and the dense hydrous magnesium silicate (DHMS), commonly referred to as phase A. Comparison of these equilibria with selected thermal profiles at the leading edge of young and old subducting oceanic slabs, along with the dehydration condition of basaltic amphibole and solidus of mantle peridotite, provides an explanation for the observed heights of the volcanic fronts above subducting oceanic lithosphere. Further, it is found that in cold oceanic slabs (≥ 50 Ma with subduction velocity of ≥ 10 cm/y), antigorite will transform to the DHMS phase A through a vapor conserved reaction at a depth of ∼ 200 km. Phase A will then serve as a carrier of water into the deeper mantle.

Constitution of laser melted Al2O3–MgO–SiO2ceramics

An investigation has been made of ceramics of the Al2 O3–MgO–SiO2 system produced as pellets by laser melting of powder mixtures of the component oxides, followed by relatively rapid solidification. The phase constitution and microstructural features have been investigated using X-ray diffraction, microscopy, and differential thermal analysis. Compositions were selected with the aim of determining the solidification sequences, including certain invariant reactions involving combinations of the silica, cordierite, mullite, protoenstatite, forsterite, and periclase phases. Taking account of compositional changes that occurred during melting, and the fact that glass formation interrupted the solidification sequences, the observations were generally consistent with existing phase diagram data.MST/3120

Is phase A relevant to the Earth's mantle?

Dense hydrous magnesium silicates (DHMS), such as phase A [Mg 7Si 2O 8(OH) 6], are potential hosts for water in the Earth's mantle. Previous work has delineated the stability field of phase A (PhA) coexisting with clinoenstatite (CEn) and forsterite (Fo) in the system MgOSiO 2H 2O (MSH). In this study, the effects of the presence of magnesite (Mc), diopside (Di), and pyrope (Py) on the stability of coexisting PhA + CEn + Fo have been determined. In MSH, the breakdown reaction PhA + CEn = Fo + H 2O was bracketed between 720 and 740°C at 7 GPa, between 800 and 820°C at 8 GPa, and between 860 and 880°C at 9 GPa. At 10 GPa, PhA + CEn transforms to a clinohumite (CH)-bearing assemblage between 920 and 940°C, which in turn dehydrates between 940 and 960°C to Fo + V. At higher pressures, PhA becomes unstable relative to phase E, another DHMS (Kanzaki, 1991). Addition of carbonate to MSH changes the breakdown reaction to PhA + CEn + Mc = Fo + V, where V is an H 2OCO 2 fluid. At 8 GPa, this reaction occurs between 800 and 850°C, indistinguishable from the position of the CO 2-free reaction in MSH. In the presence of diopside in the CaOMgOSiO 2H 2O system, the reaction PhA + En ss = Fo + Di ss + H 2O occurs between 700 and 730°C at 8 GPa, where En ss and Di ss are Ca-poor and Ca-rich pyroxene solid solutions, respectively. In the pyrope-containing assemblage in the MgOAl 2O 3SiO 2H 2O system, the PhA-bearing assemblage reacts to form CH between 740 and 760°C at 8 GPa. The CH-bearing assemblage dehydrates to Fo + Py + V between 800 and 830°C. At 8 GPa in the system MgOFeOSiO 2H 2O with molar Mg/(Mg + Fe) = 0.90, the reaction PhA + CEn ss = CH + V occurs at < 800°C, and the CEn ss + CH = Fo + V reaction occurs between 850 and 900°C. In no case did the presence of additional components, such as CaO, FeO, Al 2O 3, or CO 2, or the additional phases diopside, magnesite, or pyrope, stabilize the hydrous phases PhA or CH to significantly higher temperatures. Therefore, PhA will not be a stable phase in lherzolite assemblages under normal geothermal gradients in the mantle. In the interior of subducting slabs, temperatures may be sufficiently low that assemblages containing PhA may be stable to pressures greater than 10 GPa, allowing PhA to transport H 2O to the deeper mantle. It is more likely that the higher-pressure phase E will be stabilized instead.

Amorphous Regions and Thermal Stability of RF-Sputtered Films in the SiO&lt;sub&gt;2&lt;/sub&gt;-CaO System

Amorphous regions and thermal stability of sputtered films in the SiO2-CaO system were determined. The results are summarized as follows. (1) The amorphous region in the SiO2-CaO system by the sputtering method (100W, 25h) was found in the composition range CaO=0-89mol%, which is slightly larger than that of the SiO2-MgO system. (2) Crystallization temperature TC was determined from the DTA measurements of the sputtered amorphous films and melt-quenched glasses. (3) Thermal stability of the amorphous films and glasses was evaluated from the TC/TL ratio. TC/TL is highest 0.8 at CaO=0mol%. It passes maxima and minima as CaO content is increased and reaches the lowest value 0.44 at CaO=71.5mol%. Melt-quenched glasses showed similar compositional dependence of TC/TL ratio to that of sputtered films. (4) According to the empirical relationship between TC/TL and critical cooling rate for glass formation Q*, the TC/TL ratio of 0.44 corresponds to Q*=108.5°C/s which is well within the estimated quenching rate by the sputtering method.

微量のＣａＯを添加したＭｇＯ‐ＳｉＯ２系クリンカーの反応機構

The mechanism of reaction in MgO-SiO2 and MgO-SiO2-CaO systems is investigated in this paper. The following two series of experiments have been practiced; 1) Mixture of MgO powder (99.9%) and quartz grains (99.99%, 150-300 μm), and 2) Mixture of MgO powder, quartz grains and additional 0.4% of CaO were heated at the temperatures between 1400°C and 1600°C. In MgO-SiO2 system, Mg2+ ion diffused into quartz grains to form reaction layers of forsterite and thin layers of enstatite when heated below 1500°C. When the sample was heated above the eutectic temperature (1543°C) of the system, forsterite was observed on both sides of quartz and sintered MgO. These facts suggest that the liquid, which is formed in between enstatite and quartz, penetrates through forsterite into sintered MgO. As the result, SiO2 component removes from quartz and then gaps or pores are formed in quartz grains. Finally quartz grains are replaced the vacant pores. In MgO-SiO2-CaO system, forsterite was observed inside sintered MgO and an opening of gaps in quartz grains begins at a temperature as low as 1400°C. This suggests the liquid phase occures at the temperature as low as 1400°C when the third component, CaO, is added. The phases supposed to be liquid in the reaction layers were analysed EP-MA. The fact that the analysed data roughly coincide with the composition of ternary eutectic liquid will strongly support the above assumption.

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.

Electrostatic energies of mantle minerals in the MgO-SiO2 system, and their crystal-chemical properties and cation distributions.

Electrostatic energies of nine mantle minerals in the MgO–SiO2 system (forsterite, modified spinel, spinel, orthoenstatite, clinoenstatite, ilmenite, perovskite, periclase and stishovite) were calculated by the method of Bertaut (1952) with error estimations.It is noted that the structure of ortho- and clinoenstatites have atomic geometries which are skillfully arranged to gain low electrostatic energies compared with ilmenite and perovskite, although the former minerals have lower densities than the latter. This low energy results from the low values of Mg1, O3A and O3B ions in these pyroxenes but mainly from the fact that O2A and O2B oxygens, which are “undersaturated” with Pauling’s bond-strength sums, do not have so high values as O1 of modified spinel which are also “undersaturated” with them.Periclase and hence, Mg and O of the mineral exhibit an extremely high same value, showing a clear contrast with other minerals even in the high pressure. This indicates that Fe2+ ion prefers magnesiowustite (which is isostructural with periclase) in the cation distributions of Fe2+ and Mg2+ between perovskite and magnesiowustite, and spinel and magnesiowustite which might coexist in the mantle transition zone. This is qualitatively consistent with the high pressure experiments studied by high pressure workers.

Computer simulation of the phase diagram for the MgO-SiO2 system at P - T parameters of the mantle transition zone

The paper reports an attempt to study the topologies of the phase diagram for the MgO-SiO2 system at high pressure and temperature using computer simulation. Phase equilibria at MgSiO3 stoichiometry is investigated, demonstrating that the invariant point Gr+Ilm+Pv is stable at 21.6 GPa and 2270 K. A thermodynamic data base for minerals in the MgO-SiO2 system is established by supplementing the calorimetric data for low pressure phases and equations of state for low and high pressure phases with data calculated from high pressure synthesis experiments. A refined set of standard free energies of formation and phase transformations in the MgO-SiO2 system is presented. The proposed phase diagram covers a wide range of pressure (up to 25 GPa) and temperature (up to 2500 K) and forms the basis for a geochemical interpretation of the nature of seismic discontinuites in the mantle.

Coordination state of magnesium ions in rf-sputtered amorphous films in the system MgOSiO 2

Amorphous films in the system MgOSiO 2 were prepared by rf-sputtering and their density and refractive index were measured. The glass forming region was greatly enlarged and amorphous films containing 0–60 mol% MgO were obtained. The density and refractive index increased linearly with increasing MgO content. The coordination state of magnesium ions in these amorphous films was also examined by measuring the change in molar refractivity and the chemical shift of the magnesium Kα band X-ray emission spectra. Most of the magnesium ions in the amorphous films were found to be in the tetrahedrally coordinate state. This conclusion was supported by the color of the rf-sputtered amorphous films containing Co 2+ ions introduced wholly or partially in place of Mg 2+ ions.

A transferable interatomic potential for crystalline phases in the system MgO?SiO2

A central interatomic potential model is presented for compounds in the binary system MgO-SiO2. The potential, of a simple form which consists of a Coulombic term, a Born repulsive term, and a Van der Walls term for oxygen-oxygen interactions, is designed to predict the properties of magnesium silicates containing Si in octahedral and tetrahedral coordination. This is achieved by fitting simultaneously to forsterite and MgSiO3 ilmenite crystal structure data, and fixing the partial ionic charges using elastic data for forsterite. The potential is found to transfer successfully to γ-Mg2SiO4 and MgSiO3 perovskite. The potential results in local structural errors around the bridging oxygen ions in clinoenstatite and β-Mg2SiO4.