Clinopyroxene Solid Solutions Research Articles

High‐P granulites of the Songshugou area (Qinling Orogen, east‐central China): Petrography, phase relations, and U/Pb zircon geochronology

AbstractHigh‐pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging‐wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution.

Study of silica-undersaturated magmas through the Kalsilite- Nepheline-Diopside-Silica system at 4.0 GPa and dry conditions

This study experimentally investigates the Kalsilite-Nepheline-Diopside-Silica system at high pressure and temperature, with emphasis on silica-undersaturated volume (leucite-nepheline-diopside — Lct-Nph-Di; and kalsilite-nepheline-diopside — Kls + Nph + Di — planes), at 4.0 GPa (~120 km deep), temperatures up to 1,400ºC and dry conditions, to better understand the influence of K2O, Na2O, and CaO in alkali-rich silica-undersaturated magma genesis. In the Lct-Nph-Di plane, we determined the stability fields for kalsilite (Klsss), nepheline (Nphss) and clinopyroxene (Cpxss) solid solutions, wollastonite (Wo) and sanidine (Sa); and three piercing points: (i) pseudo-eutectic Kls + Nph + Di + liquid (Lct62Nph29Di9) at 1,000ºC; (ii) Kls + Sa + (Di + Wo) + liquid (Lct75Nph22Di2) at 1,200ºC; and (iii) pseudo-eutectic Kls + Di + Wo + liquid (Lct74Nph17Di9) at 1,000ºC. Kalsilite stability field represents a thermal barrier between ultrapotassic/potassic vs. sodic compositions. In the plane Kls-Nph-Di, we determined the stability fields for Klsss, Nphss and Cpxss and two aluminous phases in smaller proportions: spinel (Spl) and corundum (Crn). This plane has a piercing point in Kls + Nph + Di(± Spl) + liquid (Kls47Nph43Di10) at 1,100ºC. Our data showed that pressure extends K dissolution in Nph (up to 39 mol%) and Na in Kls (up to 27 mol%), and that these solid solutions, if present, determinate how much enriched in K and Na an alkaline magma will be in an alkaline-enriched metasomatic mantle. Additionally, we noted positive correlation between K2O and SiO2 concentration in experimental melts, negative correlation between CaO and SiO2, and no evident correlation between Na2O and SiO2.

Linear structural evolution induced tunable photoluminescence in clinopyroxene solid-solution phosphors

Clinopyroxenes along the Jervisite (NaScSi2O6) – Diopside (CaMgSi2O6) join have been studied, and a solid-solution of the type (Na1−xCax)(Sc1−xMgx)Si2O6 has been identified in the full range of 0 ≤ x ≤ 1. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show smooth variation of structural parameters with composition. The linear structural evolution of iso-structural (Na1−xCax)(Sc1−xMgx)Si2O6 solid-solutions obeying Vegard's rule has also been examined and verified by high resolution transmission electron microscopy (HRTEM). The continuous solid-solutions show the same structural type, therefore the photoluminescence spectra of Eu2+ doped samples possess the superposition of spectral features from blue-emitting component (CaMgSi2O6:Eu2+) and yellow-emitting one (NaScSi2O6:Eu2+). This indicates that the spectroscopic properties of (Na1−xCax)(Sc1−xMgx)Si2O6 clinopyroxene solid-solutions are in direct relations with structural parameters, and it is helpful for designing color-tunable photoluminescence with predetermined parameters.

Phase relations of potassium-bearing clinopyroxene in the system CaMgSi2O6-KAlSi2O6 at 7�GPa

The pseudo-binary system CaMgSi2O6-KAlSi2O6, modeling the potassium-bearing clinopyroxene (KCpx) solid solution, has been studied at 7 GPa and 1,100–1,650 °C. The KCpx is a liquidus phase of the system up to 60 mol% of KAlSi2O6. At higher content of KAlSi2O6 in the system, grossular-rich garnet becomes a liquidus phase. Above 75 mol% of KAlSi2O6 in the system, KCpx is unstable at the solidus as well, and garnet coexists with kalsilite, Si-wadeite and kyanite. No coexistence of KCpx with kyanite was observed. Above the solidus, KAlSi2O6 content of the KCpx coexisting with melt increases with decreasing temperature. Near the solidus of the system (about 1,250 °C) KCpx contains up to 5.6 wt% of K2O, i.e. about 22–26 mol% of KAlSi2O6. Such high concentration of potassium in KCpx is presumably the maximal content of KAlSi2O6 in the Fe-free clinopyroxene at 7 GPa. In addition to the major substitution MgM1CаМ2⇔AlМ1KМ2, the KCpx solid solution contains Ca-Eskola and only minor Ca-Tschermack components. Our experimental results indicate that the natural assemblage KCpx+grossular-rich garnet might be a product of crystallization of the ultra-potassic SiO2-rich alumino-silicate mantle melts (>200 km).

Equilibria among titanite, hedenbergite, fayalite, quartz, ilmenite, and magnetite; experiments and internally consistent thermodynamic data for titanite

For iron-rich compositions, reactions among titanite, clinopyroxene, olivine, quartz, magnetite, and ilmenite can constrain the conditions under which common rocks form. Phase equilibrium experiments in the system CaO-FeO-Fe 2 O 3 -TiO 2 -SiO 2 constrain two of these reactions: clinopyroxene + ilmenite = titanite + olivine (1) clinopyroxene + ilmenite = titanite + magnetite + quartz (2) in the range 600-1100 ℃ and <1 bar -3.8 kbar, and under controlled f O₂ conditions. The experimental results indicate that the assemblage clinopyroxene-ilmenite represents reduc­ing conditions relative to titanite-magnetite-quartz. However, the latter is not necessarily indicative of oxygen fugacity conditions higher than that of the FMQ buffer. The data also strongly suggest that the equilibrium positions of reaction I and 2 reflect changes in pres­sure. temperature. f O₂ ,. a SiO₂ ,. μFe 2+ Ti 4+ Fe 3+ -2 and μCaFe -1 in the coexisting Fe-Ti oxides and silicates. Standard state thermodynamic data and solution models for Ca-Fe olivine and clinopyroxene solid solutions. Fe-Ti ilmenite and magnetite solid solutions, and quartz (QUILF database) allow refinement of standard state thennochemical data for end-member titanite (A2/a space group): ΔH 0 = -2607.41 kJ/mol S 0 = 106.00 J/(mol · K) ΔG 0 = -2458.95 kJ/mol and require modification of published heat capacity equations to: The derived data for titanite are internally consistent with an enthalpy of formation of -2610.13 ± 2.90 (2 std. error) kJ/mol, established by drop solution calorimetry, and are specific to the QUILF database. However, analysis of the experimental results using dif­ferent internally consistent databases suggest that the revision proposed in this study of the currently accepted entropy value of 129.20 J/(mol · K) is largely independent of the databases used. The new entropy is required by the enthalpy of formation for titanite adopted here, which may also require revision of the heat capacity.

Melting study of an alkali basalt JB-1 up to 12.5 GPa: behavior of potassium in the deep mantle

In order to study the role of alkali elements in global mantle circulation, we studied melting phase relations of an alkali basalt JB-1 up to 12.5 GPa under dry conditions. Compatibility of Na and K in clinopyroxene solid solution changes dramatically in the studied pressure range. The liquidus clinopyroxene/meltpartition coefficient for Na2O increases from 0.08 at 1 atm to 2.0 at 10 GPa and that for K2O increases discontinuously at around 7 GPa. Subsolidus clinopyroxene of JB-1 contains over 1 wt.% K2O at pressures above 7 GPa, whereas it contains less than 0.1% at pressures below 5 GPa. Accordingly, subsolidus assemblage of the alkali basalt contains small amount of potassium feldspar up to 6 GPa but contains only clinopyroxene, garnet and coesite at pressures above 7 GPa. There is a huge temperature increase in the JB-1 solidus at pressures between 6 and 7.5 GPa where the subsolidus mineral assemblage changes. The geometry of the solidus curve indicates that partial melting of the alkali basaltic materials must occur at 6–7 GPa in adiabatically ascending mantle flow with broad range of potential mantle temperatures (PMT≥1200°C). Origin of the occurrence of ultrapotassic magmas in continental alkali basalt provinces and that for mildly potassium rich magmas in ocean island provinces are discussed in the light of present experiments.

Graphical analysis of pigeonite-augite liquidus equilibria

A combination of experimental and topological constraints indicates that critical equilibria play an important role in determining the pyroxene-liquid phase relations along the Mg2Si206-CaMgSi206 (enstatite-diopside) join and in naturallherzolites. Topological constraints do not permit the three-pyroxene equilibrium, orthopyroxene + diopside = pigeonite, to intersect either of the two melting equilibria, pigeonite + diopside = liquid or pigeonite = orthopyroxene + liquid in the enstatite-diopside join. Thus, with increasing pressure, pigeonite cannot abruptly replace diopside on the solidus of simplified Iherzolites in this join. A supercritical low-Ca clinopyroxene, however, can replace diopside with increasing pressure along the solidus in the following manner: As the temperature of the solidus sweeps up the orthopyroxene + diopside two-phase field, the solidus crosses that region of the two-phase field where the diopside-rich limb of the two-phase field is inflected sharply toward enstatite by the proximal top of the metastable pigeonite + diopside solvus; once the solidus is at a higher temperature than the top of the metastable solvus, the diopside coexisting with orthopyroxene becomes supercritical and its Ca content drops rapidly but continuously. In the natural system and its synthetic analogs pigeonite may replace augite on the lherzolite solidus (orthopyroxene + augite + olivine + plagioclase + liquid) at constant pressure and decreasing Mg' [MgO/(MgO + FeO)] as the solidus liquid, which becomes less aluminous, intersects the liquid coexisting with orthopyroxene + pigeonite + augite, which becomes more aluminous with decreasing Mg'. However, with Mg' and the normative feldspar composition of the liquid nearly constant, the solidus liquid becomes sufficiently more aluminous with increasing pressure to prevent the three-pyroxene equilibrium from overtaking and intersecting the lherzolite solidus. Thus, in the natural system as in the enstatite-diopside join, pigeonite does not replace augite abruptly. Rather, the replacement of augite by low-Ca clinopyroxene with increasing pressure is relatively gradual and is influenced by the presence of a pseudoternary pigeonite + augite solvus. Experimental evidence suggests that the top of the solvus emerges from the local solidus at 13-15 kbar and forms a critical end point to the pigeonite + augite + olivine liquidus boundary. The emergence of the critical end point distorts both the remaining liquidus boundaries and the clinopyroxene solid-solution surface, such that the CaO content of the clinopyroxene (now supercritical) coexisting with orthopyroxene decreases more rapidly with further increases in pressure. The critical end point migrates to the orthopyroxenewollastonite join in just a few kilobars and completely eliminates the pigeonite + augite + olivine liquidus boundary curve by -18 kbar.

Mixing behavior in quaternary garnet solid solution and an extended Ellis and Green garnet-clinopyroxene geothermometer

Applications of the different formulations of garnet-clinopyroxene geothermometers to crustal granulites and particularly to metamorphosed manganese formations show a wide scatter of estimates. This is primarily related to the uncertainty associated with the formulation of the activity-composition relationship in garnet solid solution besides that in the clinopyroxene solid solution. The largest uncertainty appears to have resulted from the adopted Mg-Mn interaction parameter in garnet which is yet to be experimentally determined. A fresh statistical regression of eleven pairs of garnet and clinopyroxene equilibrated at identical P-T conditions from a manganese formation in India yields W Mg-Mn Gt (=WMn-Mg) value of 1600±500 cal/mols. An extension of the Ellis and Green (1979) geothermometer, known to work well in normal granulites, incorporating terms associated with the activity coefficients of the exchangeable cations, now yields consistent temperature estimation from normal granulites as well as from metamorphosed manganese formations. The geothermometer, however, will not work well for garnet having XFe≥0.65 and clinopyroxenes containing appreciable Na, Al and/or Fe3+.

Pyroxene-melt equilibria: an updated model

An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using new and existing experimental data in order to refine calculations of liquid lines of descent, which simulate the effect of igneous differentiation processes. We combine the Davidson and Lindsley (1985) model for activities of components in clinopyroxene and orthopyroxene solid solutions, aip, where i represents a quadrilateral endmember, with the Nielsen and Drake (1979) expressions for component activities in the melt, aiL(two-lattice melt model). The chemical potential differences for pyroxene-melt equilibria are expressed in the form: $$\Delta \mu _{\iota } = 0 = In \left( {{{a_i^p } \mathord{\left/{\vphantom {{a_i^p } {a_i^L }}} \right.\kern-\nulldelimiterspace} {a_i^L }}} \right) + A_i + {{B_i } \mathord{\left/{\vphantom {{B_i } T}} \right.\kern-\nulldelimiterspace} T}$$

Crystal structure of Di50CaTs50 synthetic clinopyroxene (CaMgo0.50AlSil1.5O6). Crystal chemistry along the Di-CaTs join

The crystal structure of the Di50CaTs50 clinopyroxene Ca(Mg0.5Al0.5) (Al0,5Si1.5)O6, synthesized at T=1 100°C andP=14 kb, was refined in the C 2/c space group. Cell parameters are:a=9.655,b=8.771,c= 5.278 A, β=106.22°. The finalR value is: 2.1%. Comparison with the Di and CaTs structures shows that the variation of the M 2-0, M 1-0 and T-0 bond lengths is continuous and almost linear in the clinopyroxene solid solutions along the Di-CaTs join. The modifications caused in M 2 and M 1 polyhedra by shifting in tetrahedral chains along z are significant. The different values of shifts with composition account for the anomalous trend for the β angle in the DiCaTs solid solution.

Melting of a dry peridotite KLB‐1 up to 14 GPa: Implications on the Origin of peridotitic upper mantle

Melting phase relations of a fertile lherzolite KLB‐1 have been studied in the pressure range from 1 atm to 14 GPa (140 kbar). Olivine is the liquidus phase at all pressures studied. The second mineral to crystallize changes with increasing pressure; chromian spinel (1 atm), Ca‐poor orthopyroxene (up to 3 GPa), pigeonitic clinopyroxene (up to 7 GPa), pyrope‐rich garnet (above 7 GPa). The melting temperature interval of the peridotite is more than 600°C wide at 1 atm but narrows to about 150°C at 14 GPa. The partial melts along the peridotite solidus become increasingly more MgO‐rich as pressure increases throughout the pressure range studied. At 5–7 GPa, the partial melts formed within 50°C of the solidus contain more than 30 wt % MgO and are very similar to Al‐undepleted‐type peridotitic komatiite which is common in Archean volcanic terrains. Due to the increase of enstatite component in clinopyroxene solid solution at high pressure and temperature, the orthopyroxene liquidus field narrows as pressure increases and disappears at 3.5 GPa. Harzburgites which are common in the basal peridotite in ophiolite suites may have been produced as residues by partial melting at relatively shallower depths (< 100 km). Lherzolites showing protogranular textures might have been produced as single‐pyroxene peridotite residue by partial melting at relatively greater depths (> 100 km). A diapiric model is consistent with the genesis of komatiite magma by partial melting of mantle peridotite at 150–200 km depth. Based on the following observations, (1) convergence of the liquidus and solidus of the peridotite at pressures > 14 GPa, (2) the near solidus partial melt composition very close to the bulk rock at 14 GPa, and (3) change in liquidus mineral from olivine to majorite garnet at pressures between 16 and 20 GPa in preliminary experiments, it is proposed that the upper mantle peridotite was generated originally as a magma (or magmas) by partial melting of the primitive earth at 400–500 km depth.

Clinopyroxene solid solutions and water in magmas: results in the system phonolitic tephrite-H2O

AbstractClinopyroxene solid solutions crystallized at Ptot = 2 kbar in the natural system phonolitic tephrite-H2O (lava type from Etna eruption 1971) indicate a linear increase of ln Xcpxcats with H2Oliq %. After coupling this with the In Xcpxcats pressure dependence found through experiments, a simple relation has been set up for clinopyroxene- and plagioclase-bearing magmas. This relation allows us to calculate the amount of water dissolved in the melt, at any known pressure in the crustal range.

Multiply exsolved clinopyroxene megacrysts from the Frank Smith Mine, Cape Province, South Africa

Two clinopyroxene megacrysts from the Frank Smith kimberlite show multiple exsolution of garnet, orthopyroxene and oxide phases. In one of the megacrysts the lamellar texture has partly recrystallized to a granular one. The chemical compositions of the exsolved phases are similar to those of discrete nodules and of granular pyroxenites found in the same kimberlite. Conditions of formation of the original homogeneous clinopyroxene solid solution are estimated to be ∼1500°C and 37 kb, whereas the re-equilibration conditions are closer to 900°C and 35 kb. The megacrysts may have belonged to the Karroo age magnetism which predated kimberlite activity and have been brought to the surface as accidental xenocrysts by the Frank Smith kimberlite.

Ca?Sr substitution in clinopyroxenes along the join CaMgSi2O6?SrMgSi2O6

In order to define the limits of expansion of the M2 polyhedron in theC2/c clinopyroxenes of formulaXM2MgM1 [Si2O6] as the mean ionic radius in the M2 site increases, the join CaMgSi2O6−SrMgSi2O6 (Di−SrPx) has been investigated atP=1 atm and between 1090°C and 1350°C. The extent of the clinopyroxene solid solutions is limited to the compositional range Di100−Di70SrPx30. Within this range the unit-cell parameters of the clinopyroxenes show a linear variation with the increase of Sr content. The comparison of the variations caused in the unit-cell dimensions by the increase of the mean ionic radius in the M2 site (Di−SrPx series) with those caused by the decrease of the mean ionic radius in M2 (Di−En series) displays a different trend ofb in the two series. This different trend ofb suggests a different mechanism of the structure deformation in the two solid solution series. The narrow extent of the Di−SrPx solid solutions atT=1200°C shows that the increase of the mean ionic radius in the M2 site is restricted to the range 1.12–1.16 A.

Clinopyroxene with Si < AlIV in the join CaFeAISiO6-CaTiAl2O6

SummaryUnusual clinopyroxenes were synthesized in the study of the join CaFeAlSiO6-CaTiAl2O6 in air at I atm. Clinopyroxene solid solution, hibonite solid solution, X-phase, perovskite, and corundum are present, and at subsolidus temperatures the clinopyroxene singlephase field extends up to about 19 wt% CaTiAl2O6. The CaTiAl2O6 component in the clinopyroxene however increases beyond the clinopyroxene single-phase field and attains 23 wt%. These clinopyroxene solid solutions are extremely poor in SiO2 and extremely rich in Al2O3, Fe2O3, and TiO2, and more than half of tetrahedral sites are occupied by AlIV, contravening the aluminium avoidance principle.

Garnet and clinopyroxene solid solutions, and geothermometry based on Fe-Mg distribution coefficient

The available thermodynamic mixing data of aluminosilicate garnet and clinopyroxene have been critically reviewed, and integrated with the thermochemical and selected experimental data to express the Fe-Mg distribution coefficient ( K D ) between these phases as function of pressure, temperature and composition. The predicted compositional dependence of K D agrees with the available experimental and observational data. Owing to the lack of adequate data on the mixing properties of Jadeite with diopside and hedenbergite, the geothermometric application of the model has to be currently restricted to the Na-poor bulk compositions. The temperature of a variety of rocks that have equilibrated under a wide range of P-T conditions have been estimated, and found to agree, on the average within 25°C, with other reliable temperature estimates of these rocks. The latter are, however, often in sharp disagreement with the temperatures determined on the basis of Råheim and Green's (1974, Contrib. Mineral. Petrol. 48, 179–203) experimental calibration of K D as function of temperature and pressure on tholeiitic bulk composition.

Pyroxene thermometry in simple and complex systems

Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO3-MgSiO3-Al2O3. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.

Study on the compositional dependence of the apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene solid solution

Compositional dependence of apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene from Mt. Higasiakaisi is studied by means of a multicomponent regular solution model. It is shown that garnet and clinopyroxene solid solutions are positively non-ideal, and the non-ideal parameters according to the symmetric regular solution model are 2.58 kcal and 2.39 kcal, respectively, assuming the equilibration temperature of the mass to be 550° C.

ORDER-DISORDER OF FERRIC IRON AND ALUMINUM IN THE SYSTEM CaMgSi2O6-CaFeALSiO6 AT HIGH PRESSURE

The order-disorder of ferric iron and aluminum in CaMgSi2O6 95mol%-CaFeALSiO6 5mol% clinopyroxene solid solution crystallized at atmospheric pressure and high pressure have been determined by Mossbauer spectroscopy. Ordering of ferric iron and aluminum between octahedral site and tetrahedral site in clinopyroxene strcture proceeds with increasing pressure. The order-disorder of ferric iron and aluminum in pyroxenes may provide a potential means of determining the temperature and pressure of formation of these minerals.

The system MgO-FeO-SiO 2 at high pressures and temperatures — phase equilibria and elastic properties

Advances, during the Upper Mantle Project, in investigations on phase equilibria and elastic properties of the mantle minerals in the MgO-FeO-SiO 2 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 Mg 2SiO 4-Fe 2SiO 4 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 Mg 2SiO 4, 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 Co 2SiO 4 and Mn 2GeO 4. The appearance of the modified spinel phase in the Mg 2SiO 4-Fe 2SiO 4 system was confirmed in further investigations at the author's laboratory. The isothermal section of the phase diagram for the Mg 2SiO 4-Fe 2SiO 4 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 Fe 2SiO 4 to (Mg 0.9Fe 0.1) 2SiO 4. At 1,000°C, however, all the attempts to synthesize a true spinel phase of (Mg 0.9Fe 0.1) 2SiO 4 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 SiO 4 is highly temperature dependent. A remarkable expansion of the β(Mg, Fe) 2SiO 4 region is expected at the higher temperatures. Based on the cell parameters of βMg 2SiO 4 and the extrapolated value for γMg 2SiO 4 (true spinel), the density increase associated with forsterite- βMg 2 SiO 4 transformation, and forsterite — γMg 2 SiO 4 transformation was calculated to be 7.9% and 10.8% respectively. Experimental data on the phase equilibria of the MgSiO 3-FeSiO 3 system are presented. A highpressure disproportionation of clino-pyroxene solid solutions into stishovite plus spinel solid solutions was found. It was established that the disproportionation curve for clino-ferrosilite (FeSiO 3) was well represented by the boundary curve for the coesite-stishovite transformation. A preliminary phase diagram for the MgSiO 3-FeSiO 3 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 SiO 2 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, Fe 1,000O, was synthesized at high-pressures above 40 kbar at 775°C by a reaction between wüstite, Fe 0.950O, and metallic iron. The cell dimension of Fe 1,000O was determined to be 4.323 ± 0.001 A ̊ . Compressional- and shear-wave velocities of the synthetic (Mg, Fe) 2 SiO 4 olivine, Fe 2SiO 4 spinel, (Mg, Fe)SiO 3 orthopyroxene, eoesite, stishovite and Fe 0.98O 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 SiO 4 olivine, (Mg, Fe)SiO 3 orthopyroxene and (Mg, Fe)O magnesiowüstite are approximately parallel to each other. The Compressional- and shear-wave velocities of the true spinel phase of Mg 2 SiO 4 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 .