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In-situ single-crystal and powder X-ray diffraction (XRD) experiments were performed on diaspore at high temperatures. The powder XRD experiments showed that the dehydration reaction from diaspore to corundum occurs between 703 and 733 K. The in-situ single-crystal XRD measurements of diaspore could successfully determine the cell parameters, fractional atomic coordinates and anisotropic displacement parameters at high temperatures, i.e., from 295 to 698 K. Temperature variations in the cell parameters indicate that thermal expansion of the a -axis is a little higher than those of the b -axis and the c -axis. However, the axial thermal expansivity is not as anisotropic as was previously suggested. The results of structure refinements indicate that such lattice expansion behavior is the result of thermal expansion of the tunnels through O2–H···O1 hydrogen-bond separation in the diaspore structure. To the best of our knowledge, this is the first time that the thermal expansion of diaspore has been investigated at an atomic level by in-situ single-crystal XRD experiments at high temperatures.

We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO 3 (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO 3 (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO 2  + perovskite via reactions with SiO 2 . Magnesite is not the only host phase for subducted carbon—solid CO 2 also carries carbon in the lower mantle. Furthermore, CO 2 itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.

High‐pressure and high‐temperature experiments of albitic plagioclase up to 41 GPa and 270 °C were carried out using an externally heated diamond anvil cell. Raman spectroscopy and transmission electron microscopy of the recovered samples revealed that the amorphization of albite was complete at ∼37 GPa and room temperature. The amorphization pressure at 170 °C was nearly the same as that at room temperature. In contrast, the pressure largely decreased to ∼31 GPa at 270 °C. In comparison with the amorphization pressure of albite in laboratory shock experiments, that in the present static compression experiments is significantly lower (>10 GPa) even at room temperature. This suggests that shorter pressure duration results in a lower degree of amorphization of plagioclase. The formation of maskelynite in shocked meteorites does not necessarily require the very high shock pressure (30–90 GPa) that was previously estimated on the basis of shock recovery experiments.

The silicate perovskite phase relation between CaSiO 3 and MnSiO 3 was investigated at 35–52 GPa and at 1,800 K using laser-heated diamond anvil cells combined with angle-dispersive synchrotron X-ray diffraction and energy-dispersive X-ray spectroscopic chemical analyses with scanning or transmission electron microscopy. We found that MnSiO 3 can be incorporated into CaSiO 3 perovskite up to 55, and 20 mol % of CaSiO 3 is soluble in MnSiO 3 perovskite. The range of 55–80 mol % of MnSiO 3 in the CaSiO 3 –MnSiO 3 perovskite system could be immiscible. We also observed that the two perovskite structured phases of the Mn-bearing CaSiO 3 and the Ca-bearing MnSiO 3 coexisted at these conditions. The Mn-bearing CaSiO 3 perovskite has non-cubic symmetry and the Ca-bearing MnSiO 3 perovskite has an orthorhombic structure with space group Pbnm . All the perovskite structured phases in the CaSiO 3 –MnSiO 3 system convert to the amorphous phase during pressure release. MnSiO 3 is the first chemical component confirmed to show such a superior solid solubility in CaSiO 3 perovskite.

Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO 3 –MnSiO 3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO 3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO 3 components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a / b and 2 a / c , monotonically decreases from MgSiO 3 to MnSiO 3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg 0.5 , Mn 0.5 )SiO 3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg 0.9 , Mn 0.1 )SiO 3 perovskite increases with pressure. (Mg, Mn)SiO 3 perovskite incorporating less than 10 mol% of MnSiO 3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg 0.9 , Mn 0.1 )SiO 3 . MnSiO 3 is the first chemical component confirmed to form a complete solid solution with MgSiO 3 perovskite at the P – T conditions present in the lower mantle.

The pressure responses of portlandite and the isotope effect on the phase transition were investigated at room temperature from single-crystal Raman and IR spectra and from powder X-ray diffraction using diamond anvil cells under quasi-hydrostatic conditions in a helium pressure-transmitting medium. Phase transformation and subsequent peak broadening (partial amorphization) observed from the Raman and IR spectra of Ca(OH) 2 occurred at lower pressures than those of Ca(OD) 2 . In contrast, no isotope effect was found on the volume and axial compressions observed from powder X-ray diffraction patterns. X-ray diffraction lines attributable to the high-pressure phase remained up to 28.5 GPa, suggesting no total amorphization in a helium pressure medium within the examined pressure region. These results suggest that the H–D isotope effect is engendered in the local environment surrounding H(D) atoms. Moreover, the ratio of sample-to-methanol–ethanol pressure medium (i.e., packing density) in the sample chamber had a significant effect on the increase in the half widths of the diffraction lines, even at pressures below the hydrostatic limit of the pressure medium.

At ambient pressure, the hydrogen bond in materials such as ice, hydrates, and hydrous minerals that compose the Earth and icy planets generally takes an asymmetric O-H···O configuration. Pressure significantly affects this configuration, and it is predicted to become symmetric, such that the hydrogen is centered between the two oxygen atoms at high pressure. Changes of physical properties of minerals relevant to this symmetrization have been found; however, the atomic configuration around this symmetrization has remained elusive so far. Here we observed the pressure response of the hydrogen bonds in the aluminous hydrous minerals δ-AlOOH and δ-AlOOD by means of a neutron diffraction experiment. We find that the transition from P 2 1 nm to Pnnm at 9.0 GPa, accompanied by a change in the axial ratios of δ-AlOOH, corresponds to the disorder of hydrogen bond between two equivalent sites across the center of the O···O line. Symmetrization of the hydrogen bond is observed at 18.1 GPa, which is considerably higher than the disorder pressure. Moreover, there is a significant isotope effect on hydrogen bond geometry and transition pressure. This study indicates that disorder of the hydrogen bond as a precursor of symmetrization may also play an important role in determining the physical properties of minerals such as bulk modulus and seismic wave velocities in the Earth’s mantle.

Then as now for El Niño Coarse resolution palaeoclimate proxy evidence has suggested that the Pliocene warm period (PWP) between 3 million and 5 million years ago was characterized by permanent El Niño conditions in which the equatorial Pacific was uniformly warm, instead of having the modern-day 'cold tongue' extending westward from South America. New high-resolution climate proxy data from fossil corals raise doubts over this assertion. Well-preserved PWP-era fossil corals with clear skeletal annual bands, discovered in the Philippines, show that ocean conditions in the western Pacific during the PWP were characterized by El Niño variations that are similar to those we see today. Coarse-resolution palaeoclimate proxy evidence has suggested that the Pliocene warm period (∼3–5 million years ago) was characterized by permanent El Niño conditions in which the equatorial Pacific was uniformly warm, instead of having the modern-day 'cold tongue' extending westward from South America. This study uses high-resolution climate proxy information from fossil corals to challenge this assertion and shows that ocean conditions in the western Pacific during the Pliocene warm period were characterized by El Niño variations similar to modern-day variations. The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3–5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean 1 . This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change 2 , ENSO variability—and related global climate teleconnections—could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial 1 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . Here we show that permanent El Niño conditions did not exist during the PWP. Our spectral analysis of the δ 18 O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ 18 O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ 18 O events in the fossil corals closely resemble the decreases in δ 18 O seen in the live coral during El Niño events. Prior research advocating a permanent El Niño state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.

First-principles calculations of Mg 2+ -containing aragonite surfaces are important because Mg 2+ can affect the growth of calcium carbonate polymorphs. New calculations that incorporate Mg 2+ substitution for Ca 2+ in the aragonite {001} and {110} surfaces clarify the stability of Mg 2+ near the aragonite surface and the structure of the Mg 2+ -containing aragonite surface. The results suggest that the Mg 2+ substitution energy for Ca 2+ at surface sites is lower than that in the bulk structure and that Mg 2+ can be easily incorporated into the surface sites; however, when Mg 2+ is substituted for Ca 2+ in sites deeper than the second Ca 2+ layer, the substitution energy approaches the value of the bulk structure. Furthermore, Mg 2+ at the aragonite surface has a significant effect on the surface structure. In particular, CO 3 groups rotate to achieve six-coordinate geometry when Mg 2+ is substituted for Ca 2+ in the top layer of the {001} surface or even in the deeper layers of the {110} surface. The rotation may relax the atomic structure around Mg 2+ and reduces the substitution energy. The structural rearrangements observed in this study of the aragonite surface induced by Mg 2+ likely change the stability of aragonite and affect the polymorph selection of CaCO 3 .