Henry P. Scott

Phase relations of Fe-Si alloy in Earth's core

Phase relations of an Fe0.85Si0.15 alloy were investigated up to 240 GPa and 3000 K using in situ X-ray diffraction in a laser-heated diamond anvil cell. An alloy of this composition as starting material is found to result in a stabilized mixture of Si-rich bcc and Si-poor hcp Fe-Si phases up to at least 150 GPa and 3000 K, whereas only hcp-Fe0.85Si0.15 is found to be stable between approximately 170 GPa and 240 GPa at high temperatures. Our extended results indicate that Fe0.85Si0.15 alloy is likely to have the hcp structure in the inner core, instead of the previously proposed mixture of hcp and bcc phases. Due to the volumetric dominance of the hcp phase in the hcp + bcc coexistence region close to the outer-core conditions, the dense closest-packed Fe-Si liquid is more relevant to understanding the properties of the outer core.

High-pressure infrared spectra of talc and lawsonite

Abstract We present high-pressure infrared spectra of two geologically important hydrous minerals: talc, Mg3Si4O10(OH)2, and lawsonite, CaAl2Si2O7(OH)2·H2O, at room temperature. For lawsonite, our data span the far infrared region from 150 to 550 cm-1 and extend to 25 GPa. We combine our new spectroscopic data with previously published high-pressure mid-infrared and Raman data to constrain the Grüneisen parameter and vibrational density of states under pressure. In the case of talc, we present high-pressure infrared data that span both the mid and far infrared from 150 to 3800 cm-1, covering lattice, silicate, and hydroxyl stretching vibrations to a maximum pressure of 30 GPa. Both phases show remarkable metastability well beyond their nominal maximum thermodynamic stability at simultaneous high-pressure and high-temperature conditions.

Moissanite (SiC) as windows and anvils for high-pressure infrared spectroscopy

The optical properties of the moissanite (single-crystal 6H-SiC) and its performance as anvil material for high-pressure infrared spectroscopic measurements have been examined. Tests were carried out to 53 GPa using moissanite anvil cells combined with synchrotron radiation and globar as infrared sources. In the region of characteristic phonon absorption of diamond (1900–2300 cm−1), the transmitted signal measured through 4 mm of moissanite single crystals is 5–10 times higher than that obtained with type-II diamond anvils. No significant change of the transmission spectra through the moissanite anvils was observed over the pressure range studied; however, we also present mid infrared absorption spectra of powdered SiC to 43 GPa.

Magnesite formation from MgO and CO2 at the pressures and temperatures of Earth's mantle

Abstract Magnesite (MgCO3) is an important phase for the carbon cycle in and out of the Earth’s mantle. Its comparably large P-T stability has been inferred for several years based on the absence of its decomposition in experiments. Here we report the first experimental evidence for synthesis of magnesite out of its oxide components (MgO and CO2) at P-T conditions relevant to the Earth’s mantle. Magnesite formation was observed in situ using synchrotron X-ray diffraction, coupled with laserheated diamond-anvil cells (DACs), at pressures and temperatures of Earth’s mantle. Despite the existence of multiple high-pressure CO2 polymorphs, the magnesite-forming reaction was observed to proceed at pressures ranging from 5 to 40 GPa and temperatures between 1400 and 1800 K. No other pressure-quenchable materials were observed to form via the MgO + CO2 = MgCO3 reaction. This work further strengthens the notion that magnesite may indeed be the primary host phase for oxidized carbon in the deep Earth

P–V equation of state for Fe2P and pressure-induced phase transition in Fe3P

As part of our ongoing investigations of elasticity and high-pressure stability in the Fe–P system, we have measured the room-temperature bulk modulus (K 0T) of Fe2P, barringerite, to 8 GPa using in situ synchrotron X-ray diffraction and diamond anvil cells. A second-order fit (i.e. dK/dP fixed at 4) to our experimental data using the Birch–Murnaghan equation of state produces a K 0T of 165±3 GPa. This value is ∼4% less than the experimental values for Fe3P. For comparison with the experimental data, we have also performed first-principle theoretical calculations on this phase. For ferromagnetic Fe2P at zero pressure, we find that the magnetic moments increase rapidly for a Hubbard U>1 eV and are significantly higher than observed experimentally. Thus, our results support previous findings that magnetism in Fe2P is largely itinerant with at most a minor component due to on-site correlation in the iron-3d shell. Additionally, we present new high-pressure diffraction data for a natural Fe3P, schreibersite, sample which conclusively demonstrate that a first-order phase transformation occurs between 15 and 20 GPa.

The high-pressure behavior of micas: Vibrational spectra of muscovite, biotite, and phlogopite to 30 GPa

Abstract The infrared spectra of natural samples of muscovite, biotite, and phlogopite are characterized to pressures of ~30 GPa, as is the Raman spectrum of muscovite to ~8 GPa. Both far-infrared and midinfrared data are collected for muscovite, and mid-infrared data for biotite and phlogopite. The response of the hydroxyl vibrations to compression differs markedly between the dioctahedral and trioctahedral micas: the hydrogen bonding in dioctahedral environments increases with pressure, as manifested by shifts to lower frequency of the hydroxyl-stretching vibrations, whereas cation-hydrogen repulsion likely produces shifts to higher frequency of the hydroxyl vibrations within trioctahedral environments. An abrupt decrease in frequency and increase in band width of the hydroxyl-stretching vibration in muscovite is observed at pressures above ~18-20 GPa, implying that the previously documented pressure-induced disordering is associated with the local environment and shifts in location of the hydroxyl unit in this material. The far-infrared vibrations of muscovite indicate that its compressional mechanism changes above 5-8 GPa, as the K-O stretching vibration with a zero-pressure frequency near 112 cm-1 shifts in its pressure dependence from 6.9 cm-1/GPa below this pressure range to 0.78 cm-1/GPa above it. Thus, it appears that the magnitude of interlayer compression is decreased above this pressure, and hence that the compression of muscovite may become less strongly anisotropic. The mid-infrared bands that are primarily produced by vibrations of the tetrahedral layer broaden under pressure in both muscovite and biotite: within biotite, a spectral region that may be associated with higher coordination of tetrahedral cations increases in amplitude above about 25 GPa. The corresponding bands in phlogopite undergo less broadening, and their behavior is fully reversible on decompression.