Przemyslaw Dera

Siderite at lower mantle conditions and the effects of the pressure-induced spin-pairing transition

evidenced by a sharp volume collapse of about 10%. The initially colorless crystals assume an intense green color after the transition, which progressively turns to red above 60 GPa. We present clear evidence for the instability of an intermediate spin state in siderite at ambient temperature. At the transition pressure, domains of high and low spin siderite coexist. The unit cell volume difference between magnesite and siderite is significantly decreased by the spin transition, enhancing the solubility between the two calcite-type minerals. A siderite component in magnesite at lower mantle pressure would significantly increase its density and slightly increase the carbonate bulk modulus. Citation: Lavina, B., P. Dera, R. T. Downs, V. Prakapenka, M. Rivers, S. Sutton, and M. Nicol (2009), Siderite at lower mantle conditions and the effects of the pressure-induced spinpairing transition, Geophys. Res. Lett., 36, L23306, doi:10.1029/ 2009GL039652.

Magneto‐elastic coupling in compressed Fe7C3 supports carbon in Earth's inner core

[1]xa0The nature of light element(s) in the core holds key to our understanding of Earths history of accretion and differentiation, but the core composition remains poorly constrained. Carbon has been proposed to be a major constituent of the inner core, with broad implications for the global carbon cycle, the budget of volatiles in the Earth and origin of carbon-based life in the Solar System. However, existing estimates of the inner cores carbon content remain highly controversial because of poor constraints on the behavior of compressed iron carbides. Here we investigated the structure, elasticity, and magnetism of Eckstrom-Adcock carbide Fe7C3up to core pressures, using synchrotron-based single-crystal X-ray diffraction and Mossbauer spectroscopy techniques. We detected two discontinuities in the compression curve up to 167 gigapascals (GPa), the first of which corresponds to a magnetic collapse between 5.5 and 7.5xa0GPa and is attributed to a ferromagnetic to paramagnetic transition. At the second discontinuity near 53xa0GPa, Fe7C3softens and exhibits Invar behavior, presumably caused by a high-spin to low-spin transition. Considering the magneto-elastic coupling effects, an Fe7C3-dominant composition can match the density of the inner core, making the core potentially the largest reservoir of carbon in Earth.

Compressibility and thermal expansion of hydrous ringwoodite with 2.5(3) wt% H2O

Abstract Ringwoodite (γ-Mg2SiO4) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H2O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg2+ = 2H+), but also possibly as (Si4+ = 4H+ and Mg2+ + 2H+ = Si4+). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H2O, measured by secondary ion mass spectrometry (SIMS), and determined its compressibility at 300 K by single-crystal and powder X-ray diffraction (XRD), as well as its thermal expansion behavior between 140 and 740 K at room pressure. A third-order Birch-Murnaghan equation of state (BM3 EOS) fits values of the isothermal bulk modulus KT0 = 159(7) GPa and (dKT/dP)P=0 = K′ = 6.7(7) for single-crystal XRD; KT0 = 161(4) GPa and K′ = 5.4(6) for powder XRD, with KT0 = 160(2) GPa and K′ = 6.2(3) for the combined data sets. At room pressure, hydrous ringwoodite breaks down by an irreversible unit-cell expansion above 586 K, which may be related to dehydration and changes in the disorder mechanisms. Single-crystal intensity data were collected at various temperatures up to 736 K, and show that the cell volume V(cell) has a mean thermal expansion coefficient αV0 of 40(4) ×10−6/K (143-736 K), and 29(2) ×10−6/K (143-586 K before irreversible expansion). V(Mg) have α0 values of 41(3) ×10−6/K (143-736 K), and V(Si) has α0 values of 20(3) ×10−6/K (143-586 K) and 132(4) ×10−6K (586-736 K). Based on the experimental data and previous work from 29Si NMR, we propose that during the irreversible expansion, a small amount of H+ cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si4+ cations move to the normally vacant octahedral site at (½, ½, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg2+ = 2H+ dominates up to about 2 wt% H2O, where a discontinuity in the volume vs. H2O content trend suggests that other hydration mechanisms become important at very high H2O contents.

A new high-pressure phase transition in natural Fe-bearing orthoenstatite

Abstract Single-crystal X-ray structure refinements have been carried out on natural Fe-bearing orthoenstatite (OEN) at pressures up to 14.53 GPa. We report a new high-pressure phase transition from OEN to a monoclinic phase (HPCEN2) with space group P21/c, with a density change of ~1.9(3)%. The HPCEN2 phase is crystallographically different from low-pressure clinoenstatite (LPCEN), which also has P21/c symmetry. Upon release of pressure HPCEN2 reverts to OEN, and the transition pressure is bracketed between 9.96 and 14.26 GPa at room temperature. We find no evidence for a C2/c phase at high pressure. The lattice constants for the new phase at 14.26 GPa are a = 17.87(2), b = 8.526(9), c = 4.9485(10) Å, β = 92.88(4)° [ρ = 3.658(9) g/cm3]. Refinement of the new structure indicates rotation of tetrahedral chain as the key characteristic of this transition. This experiment points to the possibility of OEN and HPCEN2 as the stable phases in Earth’s upper mantle.

High-pressure polymorphism of Fe2P and its implications for meteorites and Earth's core

[1]xa0Minerals with composition (Fe,Ni)2P, are rare, though important accessory phases in iron and chondritic meteorites. The occurrence of these minerals in meteorites is believed to originate either from the equilibrium condensation of protoplanetary materials in solar nebulae or from the later accretion and condensation processes in the cores of parent bodies. Fe-Ni phosphides are considered a possible candidate for a minor phase present in the Earths core, and at least partially responsible for the observed density deficit with respect to pure iron. We report results of high-pressure high-temperature X-ray diffraction experiments with synthetic barringerite (Fe2P) up to 40 GPa and 1400 K. A new phase transition to the Co2Si-type structure has been found at 8.0 GPa, upon heating. The high-pressure phase can be metastably quenched to ambient conditions at room temperature, and then, if heated again, transforms back to barringerite, providing an important constraint on the thermodynamic history of meteorite.

Crystal structure of hydrous wadsleyite with 2.8% H2O and compressibility to 60 GPa

Abstract Hydrous wadsleyite (β-Mg2SiO4) with 2.8 wt% water content has been synthesized at 15 GPa and 1250 °C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) Å, β = 90.14(1)°, and V = 540.7(1) Å3, and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17.5 GPa), while a third-order Birch-Murnaghan equation of state (EoS) [V0 = 542.7(8) Å3, KT0 = 137(5) GPa, K′ = 4.6(3)] still fits the data well. In comparison, the second-order fit gives V0 = 542.7(8) Å3, KT = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite KT0 and water content CH2O is: KT0 = 171(1)-12(1) CH2O (up to 2.8 wt% water). The axial-compressibility βc is larger than both βa and βb, consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis.

Hydrogen bond symmetrization and equation of state of phase D

We have synthesized phase D at 24 GPa and at temperatures of 1250-1100 C in a multianvil press under conditions of high silica activity. The compressibility of this high-silica-activity phase D (Mg{sub 1.0}Si{sub 1.7}H{sub 3.0}O{sub 6}) has been measured up to 55.8 GPa at ambient temperature by powder X-ray diffraction. The volume (V) decreases smoothly with increasing pressure up to 40 GPa, consistent with the results reported in earlier studies. However, a kink is observed in the trend of V versus pressure above {approx}40 GPa, reflecting a change in the compression behavior. The data to 30 GPa fit well to a third-order Birch-Murnaghan equation of state (EoS), yielding V{sub o} = 85.1 {+-} 0.2 {angstrom}{sup 3}; K{sub o} = 167.9 {+-} 8.6 GPa; and K{prime}{sub o} = 4.3 {+-} 0.5, similar to results for Fe-Al-free phase D reported by Frost and Fei (1999). However, these parameters are larger than those reported for Fe-Al-bearing phase D and for Fe-Al-free phase D. The abnormal volume change in this study may be attributed to the reported hydrogen bond symmetrization in phase D. Fitting a third-order Birch-Murnaghan EoS to the data below 30 GPa yields a bulk modulus K{sub o} = 173 (2) GPamorexa0» for the hydrogen-off-centered (HOC) phase and K{sub o} = 212 (15) GPa for the data above 40 GPa for the hydrogen-centered (HC) phase, assuming K{prime}{sub o} is 4. The calculated bulk modulus K{sub o} of the HC phase is 18% larger than the bulk modulus K{sub o} of the HOC phase.«xa0less

β‐diopside, a new ultrahigh‐pressure polymorph of CaMgSi2O6with six‐coordinated silicon

Minerals containing silicon in four-fold coordination (IVSi4+) are common in crustal rocks, while those involving six-coordinated silicon (VISi4+) dominate the Earths lower mantle and determine its properties. Here we show a new type of phase transition determined by single-crystal high pressure X-ray diffraction experiments in a diamond anvil cell (DAC) using natural diopside (CaMgSi2O6), the archetypic member of clinopyroxene family, and one of the most abundant minerals of the Earths upper mantle. Above 50 GPa at ambient temperature diopside transforms to a previously unknown post-clinopyroxene phase,β-diopside, with half of the tetrahedralIVSi4+ layers converted to octahedral VISi4+coordination. This phase is most probably a metastable state that is kinetically accessible at room temperature and the transformation is fully reversible on decompression. This new type of phase transition provides important clues to the exact mechanisms of breakdown of clinopyroxene in the Earths mantle and may be expected to take place in other pyroxenes at pressures higher than previously explored.

Structure and behavior of the barringerite Ni end-member, Ni2P, at deep Earth conditions and implications for natural Fe-Ni phosphides in planetary cores

[1]xa0High-pressure and high-temperature behavior of synthetic Ni2P has been studied in a laser-heated diamond anvil cell up to 50 GPa and 2200 K. Incongruent melting associated with formation of pyrite-type NiP2 and amorphous Ni-P alloy was found at an intermediate pressure range, between 6.5 and 40 GPa. Above 40 GPa, Ni2P melts congruently. At room conditions, Ni2P has hexagonal C22-type structure, and without heating it remains in this structure to at least 50 GPa. With a bulk modulus K0 = 201(8) GPa and K = 4.2(6), Ni2P is noticeably less compressible than hcp Fe, as well as all previously described iron phosphides, and its presence in the Earth core would favorably lower the core density. In contrast to Fe2P, the c/a ratio in Ni2P decreases on compression because of the lack of ferromagnetic interaction along the c direction. Lack of the C22→C23 transition in Ni2P rules out a stabilizing effect of Ni on the orthorhombic phase of natural (Fe1−xNix)2P allabogdanite.

Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSiO3 akimotoite

AbstractnMgSiO3 akimotoite is stable relative to majorite-garnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg–akimotoite were synthesized under similar conditions: Z674 (containing about 550xa0ppmxa0wt H2O) was synthesized at 22xa0GPa and 1,500xa0°C and SH1101 (nominally anhydrous) was synthesized at 22xa0GPa and 1,250xa0°C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153–839xa0K) of a1xa0=xa020(3)xa0×xa010−9xa0K−2 and a0xa0=xa017(2)xa0×xa010−6xa0K−1, with an average of α0xa0=xa027.1(6)xa0×xa010−6xa0K−1. Compressibility at ambient temperature of Z674 was measured up to 34.6xa0GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch–Murnaghan equation of state (BM2 EoS) fitting yields: V0xa0=xa0263.7(2)xa0Å3, KT0xa0=xa0217(3)xa0GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: αaxa0=xa08.2(3) and αcxa0=xa010.68(9) (10−6xa0K−1); βaxa0=xa011.4(3) and βcxa0=xa015.9(3) (10−4xa0GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15–300xa0K and 0–28xa0GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between −1.02xa0×xa010−5 and +1.7xa0×xa010−5xa0K−1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances.