Lead isotopes and age of Hawaiian lherzolite nodules

The reaction between enstatite (En_95.3Fs_4.7) and CaCO₃ has been studied at pressures between 23 and 77 kbars and at temperatures between 800° and 1400°C. At 1000°C enstatite and CaCO₃ react to form dolomite and diopside solid solutions at pressures below approximately 45 kbars and magnesite and diopside solid solutions at higher pressures. The curve for the reaction dolomite_ss + enstatite_ss ? magnesite_ss + diopside_ss lies between 40 to 45 kbars at 1000°C and between 45 and 50 kbars at 1200°C. It is very close to the graphite-diamond transition curve. These experimental results indicate that calcite (or aragonite) is unstable in the presence of enstatite, and that dolomite and magnesite are the stable carbonates at high pressures. The forsterite + aragonite assemblage is, however, stable to at least 80 kbars at 800°C. It is suggested that in the upper mantle where enstatite is present, dolomite is stable to depths of about 150 km and magnesite is stable at greater depths in the continental regions, assuming that the partial pressure of CO₂ is equal or close to the total pressure. It is also suggested that carbonate inclusions in pyroxene can be used as an indicator of the depth of their equilibration; dolomite inclusions in enstatite would be formed at depths shallower than 150 km and magnesite inclusions in diopside at greater depths. Eclogite and peridotite inclusions in kimberlite may be classified on this basis.     

Phase relations in the system diopside-jadeite at high pressures and high temperatures

Phase behaviour in the system diopside-jadeite (CaMgSi₂O₆NaAlSi₂O₆) have been investigated in the pressure region 100–300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. The omphacite solid solution extends from 30 to at least 200 kbar for the entire system. Omphacites, ranging in composition from pure diopside to more than 40 mole % jadeite, transform to diopside (II) at pressures greater than 230 kbar. Diopside (II), which probably possesses a perovskite-type structure, cannot be preserved when experiments are quenched to ambient conditions. Jadeite-rich omphacites were found to decompose into an assemblage of NaAlSiO₄(CaFe₂O₄-type structure) + stishovite + diopside (II) (?) at pressures greater than about 260 kbar. These results suggest that an eclogitic model mantle would not display the 400-km seismic discontinuity. Moreover, sodium in the transition zone and lower mantle would most likely be accommodated in phases of omphacite and diopside (II).     

Melting of hydrous upper mantle and possible generation of andesitic magma: An approach from synthetic systems

Phase equilibria in a portion of the system forsterite-plagioclase (An₅₀Ab₅₀ by weight)-silica-H₂O have been determined at 15 kbar pressure under H₂O-saturated conditions. The composition of the liquid pertinent to the piercing point forsterite + enstatite solid solution + amphibole + liquid + vapor is similar to that of calc-alkaline andesite. The electron microprobe analysis of the glass coexisting with the above three crystalline phases is very close to that of the piercing point determined by phase assemblage observations; however, the glass near (< 8 μm) forsterite crystals is significantly depleted in the normative forsterite component. With the addition of 10 wt.% KAlSi₃O₈, the composition of this piercing point becomes even closer to the compositions of calc-alkaline andesites. It is also shown that the liquid coexisting with forsterite and enstatite solid solution remains silica-rich (60–62 wt.%) over a wide (～ 100°C) temperature range. The present experimental studies support the view that liquids similar in composition to calc-alkaline andesites can be generated by direct partial melting of hydrous upper mantle at least at or near 15 kbar.     

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.     

Pyroxenes in the system Mg2Si2O6-CaMgSi2O6 at high pressure

The enstatite-diopside solvus in the system Mg₂Si₂O₆-CaMgSi₂O₆ has been experimentally determined within the pressure range 5–40 kbars and the temperature range 900–1500°C. Experiments involving reversal of the phase boundaries by unmixing from glass starting material and by reaction of pure clinoenstatite and diopside showed difficulty in achieving equilibration due to persistence of metastable, subcalcic clinopyroxene and to the sluggishness of reaction rate. The experimental data showed that the temperature dependence of the diopside limb is less than previously accepted. At 1500°C and 30 kbars subcalcic diopside found by Davis and Boyd (1966) is shown to be metastable with respect to enstatite and more calcic diopside of composition En_42.3Di_57.7. The solvus widens with increasing pressure between 5 and 40 kbars at 1200°C, but at 900°C the pressure effect on the solvus is very small. The stability relationships of the four pyroxenes, protoenstatite, enstatite, iron-free pigeonite and diopside are summarized, based on data from the literature and the present study.     

The mineralogy of an eclogitic earth mantle

Pyroxene (omphacitic) and garnet (pyrope-rich) are the two major mineral components of an eclogite. No high-pressure phase transformation has been observed in omphacite and pyrope in the pressure range between 30 and 200 kbar and at 1000°C. The phase behaviour of the DSDP3-18 glass (basaltic and eclogitic composition) has been investigated in the pressure range between 100 and 280 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Both omphacite and garnet were observed in the range 100 to 150 kbar and garnet is the only phase observed in the 180-kbar run. However, it was inferred from other evidence that garnet also coexists with diopside (II) in the 180-kbar run. Diopside (II) is an unquenchable phase which is impossible to preserve on release of pressure. Glasses were the only products quenched from runs carried out at pressures greater than 210 kbar. These glasses were also interpreted as diopside(II). The phase behaviour of this complex eclogite composition at pressures below 200 kbar generally resembles that of a simple enstatite-pyrope system; pyroxene progressively dissolves in garnet with increasing pressure. The P-T conditions for the pyroxene ? garnet transition and the accompanying density (or velocity) change in the eclogitic composition are not consistent with those of the 400-km discontinuity in the Earth's mantle. Thus, an eclogitic mantle composition would not undergo a phase transformation which would be capable of accounting for the major seismic discontinuity observed in the vicinity of 400 km.     

Elastic properties of polycrystalline diopside (CaMgSi2O6)

A polycrystalline specimen of clinopyroxene diopside has been hot-pressed at P = 15 kbar and T = 850°C in a piston-cylinder apparatus. Compressional (ν_P) and shear (ν_S) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The velocities at 7.5 kbar are ν_P = 8.06 km/sec and ν_S = 4.77 km/sec. These data are consistent with velocity-density trends for orthopyroxenes due to the compensating effects of the monoclinic structure (positive) and Ca content (negative). With the addition of the new data for diopside, it is possible to calculate directly the velocities of various upper-mantle mineral assemblages.     

Synthesis of a perovskite-type polymorph of CaSiO3

Synthetic crystalline (wollastonite) and glass forms of CaSiO₃ have been compressed to loading pressures above 160 kbar and heated to about 1500° C by a laser in a diamond-anvil cell. After cooling, an X-ray diffraction study carried out whilst the sample was maintained at high pressure revealed that it had transformed to a cubic perovskite-type 3olymorph with a = 3.485 ± 0.008A?. After release of pressure, however, the sample showed a mixture of glass plus a few weak lines corresponding to ε-CaSiO₃ which is thus interpreted as a retrogressive transition product. The density of the perovskite polymorph of CaSiO₃ is about 9.2% greater than that of an isochemical mixture of CaO + SiO₂ (stishovite) at about 160 kbar.     

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.     

Phase relations in the system LiFMgF2 at elevated pressures: Implications for the proposed mixed-oxide zone of the earth's mantle

Solvi and liquidi for various LiFMgF₂ mixtures have been determined at pressures up to 40 kbar by differential-thermal-analysis in a piston-cylinder high-pressure device. The melting curves of pure LiF and MgF₂ were also studied and the initial slopes (dT_m/dP)_{P = 0} were found to be 11.2 and 8.3°C/kbar, respectively. The eutectic composition (LiF)_0.64(MgF₂)_0.36 is independent of pressure to 35 kbar and the eutectic temperature rises approximately 6.3°C per kbar. Initial slopes of 11°C/kbar and 35°C/kbar are inferred for the melting curves of MgO and SiO₂ (stishovite) respectively, on the basis of data for their structural analogue compounds. The observed solid solution of LiF in MgF₂ and other evidence suggest the possibility of solid solution in the system (Mg,Fe)OSiO₂ (stishovite) under mantle conditions which may have important consequences for the elastic properties of a “mixed-oxide” zone of the earth's mantle.     

Experimental investigation on forsterite-grossularite incompatibility in presence of excess water

A mixture containing equal amounts of forsterite and grossularite by weight (Fo₅₀Gr₅₀) has been studied at temperatures between 750 and 1400°C under pressures ranging from 6 to 25 kbar in presence of excess water. The assemblages noted under low pressure (<8 kbar) are as follows: Diopside_ss+forsterite_ss+monticellite_ss+vapor and Diopside_ss+forsterite_ss+monticellite_ss+liquid+vapor. (ss denotes solid solution) Under intermediate pressures between 8 and 24 kbar following assemblages were noted in the order of increasing temperature: Diopside_ss+forsterite_ss+spinel+vapor, Diopside_ss+forsterite_ss+spinel+liquid+vapor, Diopside_ss+forsterite_ss+liquid+vapor, and Forsterite_ss+liquid+vapor. At pressures above 24 kbar the assemblages are as follows: Diopside_ss+forsterite_ss+garnet+vapor, Diopside_ss+forsterite_ss+garnet+liquid+vapor, Diopside_ss+forsterite_ss+liquid+vapor, and Forsterite_ss+liquid+vapor. Electron microprobe analyses of diopside and forsterite crystallized at 1050°C and 23 kbar, show that the former contains 6 to 6.5 wt % of Al₂O₃ as solid solution whereas the latter incorporates 1.3 wt % of monticellite in solid solution. The monticellite content of forsterite increases at low pressures at a given temperature to about 6 wt % at 1050°C and 6 kbar. The study indicates that forsteritic olivine does not coexist with pure grossularite in the studied temperature and pressure ranges, although the former is in equilibrium with pyrope-rich garnet, containing 23 mole % grossularite. The study supports the conclusion ofWerner andLuth (1973) that the solubility of monticellite in forsterite decreases with increasing pressure at a given temperature. The results of the investigation are also in agreement with the findings ofKushiro andYoder (1966), who noted that spinel peridotites found in folded belts and in alkalic basalts are produced under intermediate pressures, whereas garnet peridotite xenoliths found in kimberlite and in orogenic belts are formed at high pressures.     

Elasticity of the ilmenite - perovskite phase transformation in CdTiO3

Ultrasonic data for the velocities of the ilmenite and perovskite polymorphs of CdTiO₃ have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 25 kbar. This transition is characterized by the following velocity (ν)-density (?) relationships: (1) the changes in compressional (ν_p) and bulk sound (ν_?) velocities are comparable in percentage magnitude to the density jump, while the shear (ν_s) velocity jump is three times greater than that for ?; (2) (ν_p/ν_s) decreases across the transition from the low- to high-pressure phase; and (3) low slopes (linear or logarithmic) on ν-? diagrams. The (ν_p/ν_s) behaviour for the ilmenite-perovskite transformation is unusual for the transitions studied in our laboratory. The observed relationships (1) and (2) are typical of the elasticity behaviour across phase transformations which involve increases in cation-anion co-ordination and in nearest-neighbour interatomic distances, such as those exhibited by CdTiO₃ in transforming from the ilmenite to the perovskite phase. Elasticity systematics for isostructural sequences are used to estimate the bulk moduli of the perovskite polymorphs of CaSiO₃ (2.7 Mbar) and MgSiO₃ (2.8 Mbar).     

High-pressure phase transformations in baddeleyite and zircon,with geophysical implications

Phase transformations in baddeleyite (ZrO₂) and zircon (ZrSiO₄) have been investigated in the pressure range between 100 and 300 kbar at about 1000°C in a diamond-anvil press coupled with laser heating. Baddeleyite has been found to transform to an orthorhombic cotunnite-type structure at pressures greater than 100 kbar, and is the first oxide known to adopt this structure. The lattice parameters of the cotunnite-type ZrO₂ at room temperature and atmospheric pressure area = 3.328 ± 0.001 ,b = 5.565 ± 0.002 , andc = 6.503 ± 0.003A? withZ = 4 , and its volume is 14.3% smaller than baddeleyite and 7.6% smaller than the fluorite-type ZrO₂. It is suggested that all the polymorphic structures of ZrO₂ are possible high-pressure models for the post-rutile phase of SiO₂. The polyhedral coordination in these model structures varies from 7 to “9”, compared with 6 for stishovite. If SiO₂ were to adopt any of these structures in the deep mantle, Birch's hypothesis of a mixed-oxide lower mantle may still be viable, but the primary coordination of silicon would be greater than 6. Zircon has been found to transform to a scheelite-type structure at about 120 kbar as noted earlier. The scheelite-type ZrSiO₄ was found to decompose further into a mixture of ZrO₂ (cotunnite-type) plus SiO₂ (stishovite) in the pressure range 200–250 kbar. As implied by the transitions in zircon, the large cations of U and Th in the earth's deep mantle are most likely to occur in dioxides with structures such as the cotunnite-type, rather than to occur in silicates.     

Disproportionation of kyanite to corundum plus stishovite at high pressure and temperature

Natural kyanite (Al₂SiO₅) has been found to disproportionate into a mixture of its component oxides, corundum and stishovite, at a loading pressure of about 160 kbar and temperature between 1000–1400°C in a diamond-anvil press. The exact transition pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The phase boundary, however, has been estimated to be P(kbar) = (138 ～ 174) + 0.011 T (°C) on the basis of the available thermodynamic data. The shock-wave Hugoniot data above 650 kbar for andalusite (Al₂SiO₅) and sillimanite (Al₂SiO₅) as starting materials are consistent with the present results.     

High-pressure phase transformations in the system CaSiO3-Al2O3

Phase assemblages for five selected compositions in the system CaSiO₃-Al₂O₃ have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al₂O₃. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl₂O₄-SiO₂. Garnet solid solutions are stable between about 10 and 25 mole% Al₂O₃. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO₃ composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.     

Experimental investigation of phase transformations of olivine and enstatite at the lower part of the mantle transition zone: Implications for structure of the 660 km seismic discontinuity

High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China.     

High-pressure inclusions in tholeiitic basalt and the range of lherzolite-bearing magmas in the Tasmanian volcanic province

Spinel-lherzolite xenoliths have been found in olivine tholeiite near Andover in the Tasmanian Tertiary volcanic province. They show a high-pressure mineralogy of predominant olivine (Mg₉₀), with aluminous enstatite (Mg₉₀) and lesser aluminous diopside and chrome-bearing spinel, and resemble lherzolite xenoliths commonly found in undersaturated lavas. Such xenoliths are unusual in tholeiitic basalts and the occurrence directly attests to a mantle origin for at least some tholeiitic magmas.The lherzolites are accompanied by doleritic and pyroxenitic xenoliths and by olivine, orthopyroxene, clinopyroxene and plagioclase xenocrysts. If near-liquidus phases are represented amongst the xenocrysts, then the magnesian number of the host basalt and its xenocryst assemblage provisionally suggest a magma derived by more than 15–20% partial melting of mantle peridotite, before commencing xenocryst crystallisation at pressures between 8–13 kbar.With this new record, lherzolite-bearing lavas in Tasmania now cover an extremely wide compositional range, extending from highly undersaturated olivine melilitite to olivine tholeiite. They also include a considerable number of fractionated alkaline rocks that are only sparsely reported in the literature as lherzolite hosts. This latter group contains representatives of a previously suggested but unestablished alkaline fractionation series based on olivine nephelinite, viz. calcic olivine nephelinite → sodic olivine nephelinite → potassi-sodic olivine nephelinite → mafic nepheline benmoreite → mafic phonolite.Lherzolite and megacryst-bearing lavas are relatively more abundant in peripheral parts to the main basalt sequences in Tasmania. This suggests that they developed in fringing zones of less intense mantle melting which enhanced stagnation and fractionation of magmas within the mantle before eruption. Calculated crustal thicknesses under these areas suggest that the magmas were generated at pressures exceeding 6–11 kbar, with the Andover tholeiitic magma exceeding 9 kbar.     

Synthesis of a new high-pressure phase of manganese dioxide

Both amorphous and crystalline (rutile) forms of MnO₂ have been subjected to loading pressures of 220 and 250 kbar and heated by a laser to approximately 1000°C in a diamond-anvil press. In all runs, X-ray diffraction study of the quenched sample reveals a mixture of pyrolusite (the rutile form of MnO₂) plus an unknown phase. This phase has been tentatively indexed on the basis of a large cubic cell with lattice parameter a₀ = 9.868 ± 0.011Å. Shock-wave data for MnO₂ up to 1300 kbar indicate that any phase transformation must involve only a small volume change. If the high-pressure phase is the cubic phase of this paper, then the latter has 36 formula units per unit cell, implying a zero-pressure volume change of 3.2% from the rutile to cubic phase. The cubic phase may provide an alternative model for the high-pressure phase of oxides having the rutile structure.     

High-pressure phase transformations of fluorite-type dioxides

Phase transformations in six fluorite-type dioxides (“TbO₂”, PbO₂, “PrO₂”, CeO₂, UO₂ and ThO₂ in the order of increasing cation size, where the quotation marks indicate non-stoichiometric materials) have been investigated in the diamond-anvil press coupled with laser heating. Together with earlier work, the results show that the post-fluorite phase transformations of these dioxides fall into two groups. The smaller cation group (HfO₂, ZrO₂ and “TbO₂”) transforms to a cotunnite or a distorted cotunnite-type structure at pressures in the vicinity of 100 kbar and at about 1000°C. The larger cation group (from PbO₂ to ThO₂) is believed to transform to a different type of orthorhombic modification at high pressures. It is plausible that this high-pressure phase may possess a Ni₂Si-related structure, as was observed in ThO₂ and “PrO₂” at pressures greater than 150 and 200 kbar, respectively. The dioxides with Ni₂Si-related structure are the most closely packed dioxides known to exist. It is suggested that all the polymorphic structures of ZrO₂ (baddeleyite, tetragonal fluorite, cubic fluorite and cotunnite) plus the new phase found in the large cation group dioxides are possible high-pressure models for the post-rutile and/or post-Fe₂N phases of SiO₂. The polyhedral coordination in these model structures varies from 7 to 10, compared with 6 for the rutile-and Fe₂N-type SiO₂. The fact that the larger cation group adopts a very closely packed structure at high pressures substantiates the earlier hypothesis that the radioactive elements U and Th in the earth's deep mantle are most likely to occur as dioxide rather than as silicate.     

High-pressure decompositions in cobalt and nickel silicates

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.