Nobuyoshi Miyajima

Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite. The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al2O3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al2O3 content of 25–28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al2O3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {1012} at ambient conditions. The high Al content may also induce the stabilization of Fe3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M3Al4Si1.5O12 (M=Mg, Fe, Ca, Na, K), i.e., the middle of A2O3 and AB2O4 type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P63/m or P63 with a=8.85 (2) A, c=2.78 (2) A, V=188 (1) A3. The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively.

Potential host phase of aluminum and potassium in the Earth’s lower mantle

Abstract An Al-rich phase produced by phase transformation from a natural mid-oceanic ridge basalt under lower mantle conditions was studied by X-ray diffraction and analytical transmission electron microscopy. The phase, coexisting with silicate perovskites, the Ca-ferrite-structure phase, and stishovite, has hexagonal symmetry (space group P63/m) and the composition [(M+,Ca)1(Mg,Fe)2]∑3(Al,Si)5.5-6.0O12, where M = Na+, K+. The alkali-free phase with the complex solid solution, [Ca0.79Mg0.12] ∑ 0.91[Mg]2.00 [Al4.09Si1.48] ∑ 5.57⃞0.43O12, has a unit cell with a = 8.765 (3) Å, c = 2.762 (3) Å, V = 183.7 (2) Å3, Z = 1, a formula weight = 429.31, and a calculated density = 3.88 g/cm3 at 0 GPa and 4.16 g/cm3 at 23 GPa. This Al-rich phase is considered to be same as the hexagonal phases recently reported, and thus the hexagonal phases can potentially host alkali and alkali-earth elements in the lower mantle.

Crystal structure of CaMg2Al6O12, a new Al-rich high pressure form

Abstract The crystal structure of CaMg2Al6O12, a new high-pressure phase synthesized at 21.8 GPa and 1200 °C using a multi-anvil apparatus, was solved by a newly developed structure program and refined by Rietveld analysis of the powder X-ray diffraction profile. The structure is hexagonal with cell constants a = 8.7616(2) and c = 2.7850(1) Å, and space group P63/m. The structure of this phase contains double chains of edge shared AlO6 octahedra running along the c axis. Three double chains share corners to form sixfold positions in which octahedrally coordinated Mg atoms reside. The large Ca atoms are randomly distributed at ninefold sites with half-occupancy in the hexagonal tunnel. Previously reported Al-rich silicate phases could possibly have the same structure. This structure could thus qualify as one of the possible major host phases for aluminum in the lower mantle

Amorphization of Serpentine at High Pressure and High Temperature

Pressure-induced amorphization of serpentine was observed at temperatures of 200° to 300°C and pressures of 14 to 27 gigapascals with a combination of a multianvil apparatus and synchrotron radiation. High-pressure phases then crystallized rapidly when the temperature was increased to 400°C. These results suggest that amorphization of serpentine is an unlikely mechanism for generating deep-focus earthquakes, as the temperatures of subducting slabs are significantly higher than those of the rapid crystallization regime.

Equation of state and phase transition in KAlSi3O8 hollandite at high pressure

Abstract The tetragonal hollandite structure (KAlSi3O8 hollandite) has been studied up to 32 GPa at room temperature using high-pressure in-situ X-ray diffraction techniques. A phase transformation from tetragonal I4/m phase to a new phase was found to occur at about 20 GPa. This transition is reversible on release of pressure without noticeable hysteresis and hence this new high-pressure phase is unquenchable to ambient conditions. The volume change associated with the transition is found to be small (not measurable), suggesting a second order transition. The diffraction pattern of the highpressure phase can be indexed in a monoclinic unit cell (space group I2/m), which is isostructual with BaMn8O16 hollandite. The γ angle of the monoclinic unit cell increases continuously above the transition. A Birch-Murnaghan equation of state fit to pressure-volume data obtained for KAlSi3O8 hollandite yields a bulk modulus K0 = 201.4 (7) GPa with K’0 = 4.0.

Lattice preferred orientation and stress in polycrystalline hcp-Co plastically deformed under high pressure

The results of x-ray diffraction data of a polycrystal under nonhydrostatic compression are analyzed for lattice preferred orientation and stress using lattice strain theories with an application to hcp-Co deformed up to 42.6GPa in the diamond anvil cell. We obtain a pure [001] fiber texture that develops primarily between 0 and 15GPa. We also show that for hcp metals the hypothesis of uniform stress across grains and lattice planes cannot be applied. This implies that the effective single crystal elastic moduli deduced from x-ray diffraction under Reuss or geometric averages consistently differ from those measured with other techniques, even after including effects of lattice preferred orientations. These results can be interpreted as an (hkl)-dependent effective differential stress resulting from plastic deformation.

Amorphization and decomposition of scandium molybdate at high pressure

The behavior of negative thermal-expansion material scandium molybdate Sc2(MoO4)3 is investigated at high pressure (HP) and high temperature (HT) using x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The compound exhibits unusually high compressibility (bulk modulus ~6 GPa) and undergoes amorphization at 12 GPa. On the other hand, in situ laser heating of amorphous samples inside the diamond-anvil cell is found to result in crystalline diffraction pattern and Raman spectrum different from those of the original compound. Upon release of the pressure subsequent to laser heating, the Raman spectrum and the diffraction pattern remain unchanged. Matching of several of the diffraction lines and Raman peaks in the laser-heated samples with those of MoO3 suggests a solid-state decomposition of the parent compound under HP-HT conditions into MoO3 and other compounds. Other diffraction lines are found to correspond to Sc2Mo2O9, Sc2O3, and the parent compound. Quantitative analysis of the characteristic x-ray emission from different regions of the sample during scanning electron microscopic observations is used for obtaining the compositions of the daughter compounds. The stoichiometries of two main phases are found to be close to those of MoO3 and Sc2Mo2O9. These results support the model that the pressure-induced amorphization occurred in this system because a pressure-induced decomposition was kinetically constrained.

Formation of metastable cubic-perovskite in high-pressure phase transformation of Ca(Mg, Fe, Al)Si2O6

Abstract We have carried out in-situ X-ray diffraction experiments on high-pressure transformations of a Ca- and Fe- rich pyroxene (Ca1.03Mg0.61Fe0.23Al0.14Si2O6) to investigate the stability of Ca0.5(Mg, Fe, Al)0.5SiO3 perovskite (CM-perovskite) in a multi component system at about 32 GPa and up to 1900 °C. We observed that cubic CM-perovskite was formed at about 1300 °C and decomposed into cubic Ca-perovskites and orthorhombic Mg-perovskites and stishovite at 1800 °C when using a glass starting material. In another experiment using a crystalline pyroxene starting material, two cubic perovskites; Ca-perovskite and CM-perovskite, and orthorhombic Mg-perovskite formed simultaneously during the initial stage of the transformation. However, the cubic CM-perovskite subsequently decomposed into Mg- and Ca-perovskites and stishovite at 1200 °C. These results indicate that the assembly of cubic Ca-perovskite, orthorhombic Mg-perovskite and stishovite is stable and cubic CM-perovskite is a metastable phase at around 32 GPa and temperatures over 1000 °C in this system. Chemical analyses of product phases showed that Mg, Fe, and Al were preferentially partitioned into Mg-perovskite and the compositions of Ca-perovskite were close to pure CaSiO3. The present study shows that CM-perovskite nucleates during the initial stage of Ca(Mg, Fe, Al)Si2O6 pyroxene transformation. Therefore, cold subducting slabs and impacted meteorites are the possible places in which CM-perovskite could exist. The Ca-rich glassy phase in a shocked chondrite (Tomioka and Kimura 2003) might have formed by vitrification of a metastable CM-perovskite-like phase.

High pressure and high temperature phase transformations in LiNbO3

A behavior of LiNbO3 under high pressure and temperature has been studied up to 90 GPa by means of high pressure in situ x-ray observations. Recovered samples were analyzed by transmission electron microscope (TEM). When the LiNbO3 was compressed at room temperature, a transformation occurred at about 25 GPa. The powder x-ray diffraction pattern of this “room-temperature and high-pressure” (RT–HP) phase was successfully explained by the NaIO3-type structure. No further transformation was observed at room temperature up to 90 GPa and reverse transition to starting phase occurred at about 10 GPa, thus this phase was unquenchable on release of pressure. When this RT–HP phase was heated at above 30 GPa, a phase appeared which can be recovered to ambient condition. X-ray diffraction and TEM analysis of this “high-temperature and high-pressure” (HT–HP) phase clarified that this phase has hexagonal symmetry with a most likely space group of P63. The quenched sample reverts to the starting phase on heating above ...

Stability of bismuth orthogermanate at high pressure and high temperature

The stability of cubic bismuth orthogermanate Bi4(GeO4)3 is investigated at high pressure (HP) and high temperature (HT) using x-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). At ambient temperature the compound is found to have a bulk modulus of 48 ± 2 GPa and it undergoes amorphization at 12.5 GPa, whereas in situ laser heating of amorphous sample in a diamond-anvil cell results in re-crystallization. SEM and Raman spectroscopy of the recovered sample suggest an inhomogeneous sample with regions rich and depleted in Ge, suggesting a decomposition of the parent compound. The decomposition products are identified from the x-ray diffraction. These results show that at HP–HT the compound exhibits complex decomposition paths leading to several mixed oxide phases including α-Bi2O3, Bi2GeO5, and Bi2Ge3O9. The observations of pressure-induced amorphization at ambient temperature in this compound and its decomposition into a mixture of daughter phases at HP–HT are discussed in light of a recent model of pressure-induced amorphization and decomposition.