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Compressed under ambient temperature, graphite undergoes a transition at ~17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the π-bonds between graphite layers convert to σ-bonds, whereas the other half remain as π-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form σ-bonds, whereas the nonbridging carbon atoms remain unpaired with π-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

Natural olivine with 12 mol % Fe2 SiO4 and synthetic orthopyroxenes with 20% and 40% FeSiO3 were studied beyond the pressure-temperature conditions of the core-mantle boundary. All samples were found to convert entirely or partially into the CalrO3 postperovskite structure, which was recently reported for pure MgSiO3. The incorporation of Fe greatly reduces the pressure needed for the transition and establishes the new phase as the major component of the D″ layer. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D″ layer.

High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the core-mantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.

We have investigated the phase relations in the iron-rich portion of the iron-silicon (Fe-Si) alloys at high pressures and temperatures. Our study indicates that Si alloyed with Fe can stabilize the body-centered cubic (bcc) phase up to at least 84 gigapascals (compared to ∼10 gigapascals for pure Fe) and 2400 kelvin. Earth's inner core may be composed of hexagonal close-packed (hcp) Fe with up to 4 weight percent Si, but it is also conceivable that the inner core could be a mixture of a Si-rich bcc phase and a Si-poor hcp phase.

Magnesiowüstite [(Mg,Fe)O] is the second most abundant mineral of Earth's lower mantle. Understanding its stability under lower mantle conditions is crucial for interpreting the physical and chemical properties of the whole Earth. Previous studies in an externally heated diamond anvil cell suggested that magnesiowüstites decompose into two components, Fe-rich and Mg-rich magnesiowüstites at 86 GPa and 1,000 K. Here we report an in situ study of two magnesiowüstites [(Mg-0.39,Fe0.61)O and (Mg0.25, Fe0.75)O] at pressures and temperatures that overlap with mantle conditions, using a laser-heated diamond anvil cell combined with synchrotron x-ray diffraction. Our results show that addition of Mg in wüstite (FeO) can stabilize the rock-salt structure to much higher pressures and temperatures. In contrast to the previous studies, our results indicate that Mg-rich magnesiowüstite is stable in the rock-salt structure in the lower mantle. The physical and chemical properties of magnesiowüstite should change gradually and continuously in the lower mantle, suggesting that it does not make a significant contribution to seismic-wave heterogeneity of the lower mantle. Stable Mg-rich magnesiowüstite in lowermost mantle can destabilize FeO in the core-mantle boundary region and remove FeO from the outer core.

ThO2 is an important material for understanding the heat budget of Earth's mantle, as well as the stability of nuclear fuels at extreme conditions. We measured the in situ high-pressure, high-temperature phase behavior of ThO2 to ∼60 GPa and ∼2500 K. It undergoes a transition from the cubic fluorite-type structure (thorianite) to the orthorhombic α-PbCl2 cotunnite-type structure between 20 and 30 GPa at room temperature. Prior to the transition at room temperature, an increase in unit-cell volume is observed, which we interpret as anion sub-lattice disorder or pre-transformation \"melting\" (Boulfelfel et al. 2006). The thermal equation of state parameters for both thorianite [V0 = 26.379(7), K0 = 204(2), αKT = 0.0035(3)] and the high-pressure cotunnite-type phase [V0 = 24.75(6), K0 = 190(3), αKT = 0.0037(4)] are reported, holding K0' fixed at 4. The similarity of these parameters suggests that the two phases behave similarly within the deep Earth. The lattice parameter ratios for the cotunnite-type phase change significantly with pressure, suggesting a different structure is stable at higher pressure.

The high-cosmic abundance of sulfur is not reflected in the terrestrial crust, implying it is either sequestered in the Earth's interior or was volatilized during accretion. As it has widely been suggested that sulfur could be one of the contributing light elements leading to the density deficit of Earth's core, a robust thermal equation of state of iron sulfide is useful for understanding the evolution and properties of Earth's interior. We performed X-ray diffraction measurements on FeS2 achieving pressures from 15 to 80 GPa and temperatures up to 2400 K using laser-heated diamond-anvil cells. No phase transitions were observed in the pyrite structure over the pressure and temperature ranges investigated. Combining our new P-V-T data with previously published room-temperature compression and thermochemical data, we fit a Debye temperature of 624(14) K and determined a Mie-Gruneisen equation of state for pyrite having bulk modulus KT = 141.2(18) GPa, pressure derivative K'T = 5.56(24), Gruneisen parameter γ0 = 1.41, anharmonic coefficient A2 = 2.53(27) × 10-3 J/(K2·mol), and q = 2.06(27). These findings are compared to previously published equation of state parameters for pyrite from static compression, shock compression, and ab initio studies. This revised equation of state for pyrite is consistent with an outer core density deficit satisfied by 11.4(10) wt% sulfur, yet matching the bulk sound speed of PREM requires an outer core composition of 4.8(19) wt% S. This discrepancy suggests that sulfur alone cannot satisfy both seismological constraints simultaneously and cannot be the only light element within Earth's core, and so the sulfur content needed to satisfy density constraints using our FeS2 equation of state should be considered an upper bound for sulfur in the Earth's core.

The high-cosmic abundance of sulfur is not reflected in the terrestrial crust, implying it is either sequestered in the Earth’s interior or was volatilized during accretion. As it has widely been suggested that sulfur could be one of the contributing light elements leading to the density deficit of Earth’s core, a robust thermal equation of state of iron sulfide is useful for understanding the evolution and properties of Earth’s interior. We performed X-ray diffraction measurements on FeS2 achieving pressures from 15 to 80 GPa and temperatures up to 2400 K using laser-heated diamond-anvil cells. No phase transitions were observed in the pyrite structure over the pressure and temperature ranges investigated. Combining our new P-V-T data with previously published room-temperature compression and thermochemical data, we fit a Debye temperature of 624(14) K and determined a Mie-Grüneisen equation of state for pyrite having bulk modulus KT = 141.2(18) GPa, pressure derivative K'T = 5.56(24), Grüneisen parameter γ0 = 1.41, anharmonic coefficient A2 = 2.53(27) × 10–3 J/(K2·mol), and q = 2.06(27). These findings are compared to previously published equation of state parameters for pyrite from static compression, shock compression, and ab initio studies. This revised equation of state for pyrite is consistent with an outer core density deficit satisfied by 11.4(10) wt% sulfur, yet matching the bulk sound speed of PREM requires an outer core composition of 4.8(19) wt% S. Here, this discrepancy suggests that sulfur alone cannot satisfy both seismological constraints simultaneously and cannot be the only light element within Earth’s core, and so the sulfur content needed to satisfy density constraints using our FeS2 equation of state should be considered an upper bound for sulfur in the Earth’s core.

The structural transformation of the high-pressure high-temperature polymorph known as the stishovite form has been shown to actually occur at lower pressures than originally thought. This could have profound repercussions for solid Earth geophysics.

Two research teams have come up with differing conclusions on whether the Earth's upper and lower mantle is uniform in composition.