High-pressure Phase Relations Research Articles

High-pressure Phase Transition of Olivine-type Mg2GeO4 to a Metastable Forsterite-III type Structure and their Equation of States

Abstract Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg2GeO4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a 3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1 (3) Å3, K0 = 124.6 (14) GPa and K0′ = 3.86 (fixed) for forsterite- and V0 = 263.5 (15) Å3, K0 = 175 (7) GPa and K0′ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs and interpreting the results of shock compression experiments.

High-pressure order-disorder transition in Mg2SiO4 : Implications for super-Earth mineralogy

(Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond pPv. In this work we have explored the high-pressure phase relations in Mg$_2$SiO$_4$ using computations based on density functional theory. We find that a partially disordered I-42d type structure would be stable in the interiors of these super-Earth planets. The discovery of a structure where two very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase.

High-pressure phase relations in the system Fe–Ni–Cu–S up to 14 GPa: implications for the stability of sulfides in the earth’s upper mantle

Base metal sulfides (Fe–Ni–Cu–S) are ubiquitous phases in mantle and subduction-related lithologies. Sulfides in the mantle often melt incongruently, which leads to the production of a Cu–Ni-rich sulfide melt and leaves a solid residue called monosulfide solid solution (mss). However, the persistence of crystalline sulfide phases like mss in the Earth’s mantle at higher temperatures and pressures deep within the Earth has long been up for debate, as the presence of both mss and sulfide melt in mantle rocks implies the fractionation of chalcophile elements during mantle melting. Recent studies have shown that the average mantle sulfide (45 wt.% Fe, 16 wt.% Ni, 1 wt.% Cu, and 38 wt.% S), is fully molten at average mantle potential temperatures (1300–1400 ^{circ }C) up to 8 GPa (ca. 240 km). However, sulfide inclusions found in diamonds show a broad compositional spectrum, ranging from Ni-poor and Fe-rich (eclogitic), to Ni-rich and Fe-poor sulfides (peridotitic), with their Cu contents being generally low. The wide compositional variety of diamond-hosted sulfide inclusions raises the possibility that results on the melting properties obtained from this average mantle sulfide compositional may not reflect that found in those inclusions. As such, further investigation of the melting properties of sulfides from a wide compositional range is necessary. Here, we present the results of an experimental study where the melting properties of typical sulfide compositions found in diamond inclusions associated with eclogites and peridotites have been determined. Experiments have been carried out between 0.1 MPa and 14 GPa, and between 920 and 1590 ^{circ }C, on box muffle furnaces, end-loaded piston cylinder, and multi-anvil apparatuses. Results show that solid mss in Fe-rich, Ni-poor sulfide inclusions associated with eclogites persist to higher pressures and temperatures compared to their less-refractory, more Ni-rich peridotitic counterparts to the depth of the mantle transition zone (410 km depth). Our results have implications for the recycling of chalcophile elements during subduction-related processes and the entrapment of sulfides in diamonds.

Enthalpy, heat capacity and thermal expansivity measurements of MgSiO3 akimotoite: Reassessment of its self-consistent thermodynamic data set

Enthalpy, isobaric heat capacity (CP) and thermal expansivity (α) of MgSiO3 akimotoite were re-determined by drop-solution calorimetry, differential scanning calorimetry and high-temperature powder X-ray diffraction measurement, respectively. From the obtained α, thermal Grüneisen parameter of MgSiO3 akimotoite was constrained to be 1.3. Using isochoric heat capacity by lattice vibrational model calculation and the Grüneisen relation equation, α and CP above maximum measurement temperature and the temperature derivative of isothermal bulk modulus were optimized by the least squares fitting to an existing P–V–T dataset which makes these parameters self-consistent. The obtained thermodynamic data set for MgSiO3 akimotoite was used to calculate the akimotoite–bridgmanite phase equilibrium boundary in MgSiO3 and high-pressure phase relations in Mg2SiO4. The result shows the stability field of the phase assemblage of MgSiO3 akimotoite + MgO below about 1400 K and around 23 GPa, suggesting the phase transition from ringwoodite to bridgmanite + ferropericlase via akimotoite + ferropericlase in cold subducted slabs.

Elasticity and high-pressure behavior of Mg2Cr2O5 and CaTi2O4-type phases of magnesiochromite and chromite

Abstract In situ high-pressure and high-temperature X-ray diffraction studies on magnesiochromite, MgCr2O4, and a natural chromite, (Mg,Fe)(Al,Cr)2O4, using a laser-heated diamond-anvil cell technique were performed at pressures to ~45 GPa. Our results on MgCr2O4 at ~15 GPa showed temperature-induced dissociation of MgCr2O4 to Cr2O3+MgO below ~1500 K and formation of modified ludwigite (mLd)-type Mg2Cr2O5+Cr2O3 above ~1500 K. Above 20 GPa, only a single phase with the CaTi2O4-type structure of MgCr2O4 was observed at 1400–2000 K. A second-order Birch-Murnaghan fit to pressure-volume data for the CaTi2O4-type phase of MgCr2O4 yields zero-pressure volume (V0) = 264.4(8) Å3 and bulk modulus (K0) = 185.4(4) GPa, and for the CaTi2O4-type structure of natural (Mg,Fe)(Al,Cr)2O4 yields V0 = 261(1) Å3 and K0 = 175.4(2) GPa. A second-order Birch-Murnaghan fit to pressure-volume data of mLd-type Mg2Cr2O5 yields V0 = 338.9(8) Å3 and K0 = 186.5(6) GPa. The obtained high-pressure phase relations of chromite spinels can be used as an indicator for shock pressure in impact rocks and meteorites. The bulk moduli of the high-pressure phases of MgCr2O4 and FeCr2O4 can help develop a thermodynamic model for Mg and Fe end-member spinels in the upper mantle and transition zone.

High-pressure and high-temperature phase relations in the systems KAlSiO4-MgAl2O4 and CaAl2O4-MgAl2O4: Stability fields of NAL phases

Phase relations in the system KAlSiO4-MgAl2O4 were determined up to 28 GPa and 1500 °C. A hexagonal aluminous (NAL) phase is stable above 16 GPa with a narrow compositional range of (1-x)KAlSiO4·xMgAl2O4, x ≈ 0.61–0.65. The stability field of NAL phase in the CaAl2O4-MgAl2O4 system was also determined up to 29 GPa and 1200 °C, resulting in a compositional range of NAL phase of (1-x)CaAl2O4·xMgAl2O4, x ≈ 0.67–0.74. Compared with the above compositional widths of NAL phases, the stability field of NAL phase in the NaAlSiO4-MgAl2O4 system is much wider, (1-x)NaAlSiO4·xMgAl2O4, x ≈ 0.47–0.70 (Ono et al., 2009). The difference may be caused by the fact that both of Na+ and Mg2+ with similar cation sizes enter 6-fold trigonal prism sites in the NAL structure. In the system KAlSiO4-MgAl2O4, KAlSiO4 kalsilite and MgAl2O4 spinel coexist below ~8 GPa, above which an assemblage of MgAl2O4 spinel, corundum, K2Mg2Si2O7-rich phase X and pyrope becomes stable. The assemblage further changes into NAL phase at 16 GPa. These results and compositional similarity suggest that composite inclusions consisting of MgAl2O4-rich spinel and kalsilite-nepheline solid solution found in diamonds in the Juina-5 kimberlite, Brazil, were formed below ~8 GPa presumably by decomposition of K-rich NAL phase trapped in diamonds in the lower mantle conditions.

High-pressure Cu–Fe–S Phase Equilibria: some Experimental and Thermodynamic Constraints on Sulfides in Subduction Zones and the Lithospheric Mantle

Abstract High-pressure phase relations for much of the Cu–Fe–S system have not previously been determined experimentally. Experimental studies have concentrated on low-pressure phase relations and cannot explain high-pressure sulfide mineral inclusion assemblages in some natural blueschists and eclogites. In particular, the coexistence of pyrite + covellite at 1·0 GPa, and pyrite + bornite at 1·9 GPa, observed in New Caledonian rocks, is precluded by tie-lines between S and bornite, and S and the intermediate solid solution (iss), in the published low-pressure experimental topologies at corresponding temperatures. In addition, the Cu content (up to ∼10 at%) of pyrrhotite in eclogite exceeds the experimentally determined maximum for Cu in solid solution with pyrrhotite at low pressures and at corresponding temperatures. We have performed six experiments in which natural chalcopyrite starting material was equilibrated at conditions ranging from 1·0 to 1·7 GPa and 500 to 650 °C. At 1 GPa chalcopyrite is replaced by iss. The iss phase undergoes a terminal breakdown reaction between 1·0 and 1·7 GPa, being replaced by a new assemblage of bornite, pyrite, and pyrrhotite. Our experimental results confirm predictions from the SUPCRT thermodynamic database (Johnson et al., 1992; Computers & Geosciences 18, 899–947) but not that of Robie & Hemingway (1995; US Geological Survey Bulletin 2131). The former database is therefore recommended for calculation of high-pressure sulfide phase relations. Chalcopyrite and its high-temperature, low-fS2 equivalent, iss are not stable at pressures corresponding to much of blueschist–eclogite-facies metamorphism. These results are also applicable to sulfide assemblages in the lithospheric mantle along both oceanic and continental geotherms; the subsolidus Cu-rich mineral in the lithosphere at depths of 30 to >65 km must be bornite–digenite solid solution (bn-ss) rather than iss as is commonly assumed.

Discovery of new-structured post-spinel MgFe2O4: Crystal structure and high-pressure phase relations

Discovery of new-structured post-spinel MgFe2O4: Crystal structure and high-pressure phase relations

Discovery of new-structured post-spinel MgFe2O4: Crystal structure and high-pressure phase relations

Discovery of new-structured post-spinel MgFe2O4: Crystal structure and high-pressure phase relations

Thermodynamic properties of ZrSiO4 zircon and reidite and of cotunnite-type ZrO2 with application to high-pressure high-temperature phase relations in ZrSiO4

Enthalpies of high-pressure phase transitons in ZrSiO4 were measured by high-temperature drop-solution calorimetry. The enthalpies for the zircon-reidite transition in ZrSiO4 and for dissociation of reidite to cotunnite-type ZrO2 + SiO2 stishovite were obtained to be 27.5 ± 3.3 and 50.3 ± 3.0 kJ/mol, respetively, at 298 K. By thermal relaxation method, low-temperature isobaric heat capacities of ZrSiO4 reidite and cotunnite-type ZrO2 were measured, giving standard entropies at 298 K of 77.29 ± 0.04 and 49.10 ± 0.04 J/mol·K, respectively. Combining the measured enthalpies and entropies with other available thermophysical data, the transition boundaries in ZrSiO4 were calculated. The calculated zircon-reidite transiton boundary is consistent with that by in situ high-pressure expreiments by Ono et al. (2004a) within the errors. The calculated dissociation boundary of reidite has a smaller slope than that by high-pressure quench expreiments by Tange and Takahashi (2004). The equilibrium zircon-reidite transition pressure is calculated to be 8 GPa at 298 K, which is by 12–15 GPa lower than the transition pressures observed by room-temperature static compression experiments. Our results suggest that zircon transforms to reidite and dissociates into the two phases at 330 and 610 km depths, respectively, along the normal mantle geotherm. Combining the calculated phase relations with kinetics estimated from the first principles calculation may provide some insights to P, T conditions which reidites found in impact ejecta layers and craters experienced.

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2‐H2O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

AbstractWe present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab‐derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950–1175°C and 2–4 GPa using a piston‐cylinder and a multi‐anvil apparatus, to test the hypothesis that volatile‐bearing mineral‐phases produced during craton formation can cause reduction in aggregate shear‐wave velocities (VS) at mid‐lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3–7.6 wt.%; Bulk 2: 2.6–5 wt.%) at 2–4 GPa, and amphibole (Bulk 1: 3–9 wt.%; Bulk 2: 2–6 wt.%) at 2–3 GPa (≤1050°C) are also present. Magnesite (Bulk 1: ∼1 wt.% and Bulk 2: ∼0.6 wt.%) is present at 2–4 GPa (<1000°C at 3 and < 1050°C at 4 GPa) and its thermal breakdown coincides with the visual appearance of trace‐melt. However, an extremely small fraction of melt is inferred at all experiments based on the knowledge of fluid‐saturated peridotite solidus and the difference between bulk H2O and total H2O stored in the hydrous phases. Calculated mineral end‐member volume‐proportions were used to calculate VS of the resulting assemblage at experimental conditions and along representative continental geotherms (surface heat flow of 40–50 mWm−2). We note that reactive crystallization of phlogopite ± amphibole by infiltration of 3–10% slab‐derived hydrous‐silicic melt can cause up to 6% reduction in VS and that the estimated reduction in VS increases with increasing melt:rock ratio. The presence of phlogopite limits amphibole‐stability, making phlogopite a more likely candidate for MLDs at >100 km depth.

High-pressure phase relation of KREEP basalts: A clue for finding the lost Hadean crust?

The phase relations, mineral chemistry and density of KREEP basalt were investigated at pressures of 12–125 GPa and temperatures up to 2810 K by a combination of large volume multi-anvil press experiments and in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. Our results showed that grossular-rich majorite garnet, liebermannite and Al-bearing stishovite are dominant in the upper-to-middle part of the upper mantle while in the lowermost transition zone a dense Ti-rich CaSiO3 perovskite exsoluted from the garnet, which becomes more pyropic with increasing pressure. At lower mantle conditions, these minerals transform into an assemblage of bridgmanite, Ca-perovskite, Al-stishovite, the new aluminium-rich (NAL) phase and the calcium-ferrite type (CF) phase. At pressures higher than 50 GPa, NAL phase completely dissolved into the CF phase, which becomes the main deposit of alkali metals in the lower mantle. The density of KREEP estimated from phase compositions obtained by energy dispersive X-ray spectroscopy (EDS) in scanning (SEM) and transmission (TEM) electron microscopes, was found substantially denser than pyrolite suggesting that the Earth primordial crust likely subducted deep into the Earth’s mantle after or slightly before the final solidification of magma ocean at 4.53 Ga. Radiogenic elements U, Th and 40K which were abundant in the final residue of magma ocean were brought down along the subduction of the primordial crust and generate heat by decay after the settlement of the primordial crust on top of the CMB, suggesting the non-homogeneous distribution of radiogenic elements in the Hadean mantle with implications for the thermal history of the Earth.

Experimental and thermodynamic investigations on the stability of Mg14Si5O24anhydrous phase B with relevance to Mg2SiO4forsterite, wadsleyite, and ringwoodite

High-pressure high-temperature phase relation experiments in Mg 14 Si 5 O 24 were performed using a 6-8 multi-anvil high-pressure apparatus in the pressure range of 12–22 GPa and temperature range of 1673–2173 K. We first found that Mg 14 Si 5 O 24 anhydrous phase B (Anh-B) dissociates to Mg 2 SiO 4 wadsleysite (Wd) and MgO periclase (Per) at about 18 GPa and 1873 K. From the results of the high-pressure experiments, the phase boundaries of 5 Mg 2 SiO 4 forsterite (Fo) + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per were determined. In addition, the isobaric heat capacity ( C P ) of Anh-B was measured by differential scanning calorimetry in the temperature range of 300–770 K and the thermal relaxation method using a Physical Property Measurement System (PPMS) in the range of 2–303 K. From the measured low-temperature C P , the standard entropy ( S ∘ 298.15 ) of Anh-B was determined to be 544.4(2) J/(mol·K). We also performed high-temperature X-ray diffraction measurements in the range 303–773 K to determine the thermal expansivity (α) of Anh-B. The obtained C P and α were theoretically extrapolated to higher temperature region using a lattice vibrational model calculation partly based on Raman spectroscopic data. Thermodynamic calculations by adopting the thermochemical and thermoelastic data for Anh-B obtained in this study and the estimated formation enthalpy for Anh-B of –13 208 kJ/mol gave phase equilibrium boundaries for 5 Fo + 4 Per = Anh-B and Anh-B = 5 Wd + 4 Per that were consistent with those determined by the present high-pressure high-temperature experiments. The results clarified that, in the Mg 14 Si 5 O 24 system, Anh-B is stable between 12 and 18 GPa at the expected temperatures of the Earth’s mantle.

High-pressure high-temperature phase relations in FeTiO3 up to 35 GPa and 1600 °C

Phase relations in FeTiO3 were precisely determined at 25–35 GPa and 600–1600 °C using multianvil high-pressure experiments with tungsten carbide anvils. Pressure generation up to about 36 GPa at 1600 °C was evaluated using Al2O3 solubility in MgSiO3 perovskite (Pv) in the system MgSiO3–Al2O3. At about 28 GPa, FeTiO3 Pv dissociates into an assemblage of calcium titanate (CT)-type Fe2TiO4 + orthorhombic-I (OI)-type TiO2 below 1200 °C. However, above 1200 °C at 28 GPa, FeTiO3 Pv decomposes into a new, denser phase assemblage of CT-type Fe2TiO4 + a new compound of FeTi2O5. The new phase FeTi2O5 was recovered as an amorphous phase at 1 atm. In situ X-ray diffraction experiments at 35.1 GPa indicated that the new phase (N-p) FeTi2O5 has orthorhombic symmetry with cell parameters a = 8.567(2) A, b = 5.753(1) A and c = 5.257(1) A. In addition, the assemblage of CT-type Fe2TiO4 + OI-type TiO2 changes to FeO wustite (Wu) + OI-type TiO2 at about 33 GPa below 1000 °C. The phase assemblages in FeTiO3 are denser in the order: FeTiO3 (Pv) → 1/2Fe2TiO4 (CT) + 1/2TiO2 (OI) → 1/3Fe2TiO4 (CT) + 1/3FeTi2O5 (N-p) → FeO (Wu) + TiO2 (OI). Our results indicate that the upper stability limit of FeTiO3 Pv is about 28 GPa at 600–1600 °C. This puts a constraint on peak shock pressure for formation of naturally discovered lithium niobate-type FeTiO3 which was interpreted to be retrograde transition product of FeTiO3 Pv on release of shock pressure.

High-pressure phase transitions in FeCr2O4 and structure analysis of new post-spinel FeCr2O4 and Fe2Cr2O5 phases with meteoritical and petrological implications

We determined phase relations in FeCr 2 O 4 at 12–28 GPa and 800–1600 °C using a multi-anvil apparatus. At 12–16 GPa, FeCr 2 O 4 spinel (chromite) first dissociates into two phases: a new Fe 2 Cr 2 O 5 phase + Cr 2 O 3 with the corundum structure. At 17–18 GPa, the two phases combine into CaFe 2 O 4 -type and CaTi 2 O 4 -type FeCr 2 O 4 below and above 1300 °C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi 2 O 4 -structured FeCr 2 O 4 ( Cmcm ), and indicated that the Fe 2 Cr 2 O 5 phase is isostructural to a modified ludwigite-type Mg 2 Al 2 O 5 ( Pbam ). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe 2 O 4 -type FeCr 2 O 4 is unquenchable and is converted into another FeCr 2 O 4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr 2 O 4 phase has a new structure related to CaFe 2 O 4 -type. The high-pressure phase relations in FeCr 2 O 4 reveal that natural FeCr 2 O 4 -rich phases of CaFe 2 O 4 - and CaTi 2 O 4 -type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 °C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12–16 GPa.

Formation and properties of rocksalt-type AlN and implications for high pressure phase relations in the system Si–Al–O–N

Pressure-dependent thermodynamic properties of the ambient and high pressure phases of aluminum nitride (w-AlN and rs-AlN) were calculated from first principles in order to determine their phase boundary in the p− T phase diagram. These predictions were checked by static HP/HT experiments, using a multianvil press and an Al/N/H precursor with low decomposition temperature as educt. The experimental data show that at temperatures between 1000 and 2000 K, the boundary line between the two phases is situated between 11 and 12 GPa, which is ∼1.3 GPa lower than the theoretical result and generally lower than previously assumed. The hardness of rs-AlN – measured for the first time – is ∼30 GPa (Knoop indenter at loads of 25–50 g), twice as hard as w-AlN. Shock wave recovery experiments on nano w-AlN allowed testing of the chemical and thermal stability of rs-AlN, and determination of its infrared absorption and 27Al NMR data. The shock wave technique will eventually enable the synthesis of larger amounts of rs-AlN, making it available for technological use. Finally, implications on the high pressure stability of phases in the Si–Al–O–N system are discussed in the light of thermoelastic properties of AlN.

High pressure phase relations of subducted volcaniclastic sediments from the west pacific and their implications for the geochemistry of Mariana arc magmas

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3–6GPa and 800–900°C with 10wt.% added H2O (in addition to ~10wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile±Fe–Ti oxide±apatite±monazite±zircon. Hydrous melt appears at temperatures exceeding 800–850°C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825–850°C) than other marine sediments (700–750°C) at 3GPa. This trend is reversed at high-pressure conditions (6GPa) where the biotite melting reaction occurs at lower temperatures (800–850°C) than the phengite melting reaction (900–1000°C).Trace element data was obtained from the 3GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.

High-pressure phase relations in the system TiO2–ZrO2 to 12 GPa: stability of αPbO2-type srilankite solid solutions of (Ti1−x , Zr x )O2 (0 ≤ x ≤ 0.6)

Phase relations in the system TiO2–ZrO2 were examined in the pressure range of 3.5–12 GPa at 1,800 °C, using multianvil apparatus. At 1,800 °C, TiO2 rutile transforms to αPbO2 structure at 10 GPa, and the αPbO2-type solid solution is stable in compositional range between TiO2 and about (Ti0.6, Zr0.4)O2 at 3.5–12 GPa. Combination of the present results with the published data at 0–3 GPa demonstrates that continuous solid solution with the αPbO2-type structure is stable between TiO2 and (Ti1−x, Zrx)O2 (x ≈ 0.6) at 0–12 GPa. This indicates that both the αPbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the αPbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the αPbO2-type solid solution dissociates into two phases, αPbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.

TiO2-ZrO2系の高圧相関係とα-PbO2型固溶体の形成

TiO2-ZrO2系の高圧相関係とα-PbO2型固溶体の形成

High-pressure and high-temperature phase transitions in FeTiO3 and a new dense FeTi3O7 structure

High-pressure and high-temperature phase relations of FeTiO 3 were investigated up to a pressure of about 74 GPa and 2600 K by synchrotron X-ray diffraction and analytical transmission electron microscopy. We conclude that FeTiO 3 ilmenite transforms into the following phase(s) with increasing pressure: FeTiO 3 (perovskite) at 20-30 GPa, Fe 2 TiO 4 (Ca 2 TiO 4 -type) + TiO 2 (OI-type) at 30-44 GPa and high temperature, FeO (wüstite) + TiO 2 (OI) at 30-44 GPa and low temperature, and wüstite + FeTi 3 O 7 (orthorhombic phase) above 44 GPa. Among these dense high-pressure polymorphs, FeTi 3 O 7 is a new compound and its structure analysis was tried using particle swarm optimization simulation. This method successfully found a new high-density FeTi 3 O 7 structure, and Rietveld refinement based on this model structure gave an excellent fit with the experimentally obtained X-ray diffraction pattern. This new high-density FeTi 3 O 7 structure consists of polyhedra for monocapped FeO 7 prisms, bicapped TiO 8 prisms, and tricapped TiO 9 prisms, which develop on the b-c plane and stack along the a axis. The dense compound assemblage found in FeTiO 3 is promising for investigating the behavior of ABX 3 compounds under ultrahigh pressures.