Dion L. Heinz

The equation of state of the gold calibration standard

The compression of Au has been measured at room temperature to 70 GPa (700 kbar) using x‐ray diffraction through a diamond‐anvil cell and the ruby‐fluorescence pressure scale. Based on these data, the isothermal bulk modulus and its pressure derivative at zero pressure are K0T =167 (±11) GPa, and K′0T=5.5 (±0.8). These results are in excellent agreement with ultrasonic measurements of the elastic constants as well as an equation of state based on shock‐wave data. Hence, this study represents an independent experimental confirmation of both the ruby fluorescence pressure scale, and the predicted equation of state of the proposed Au pressure calibration standard. We derive a thermal equation of state for gold by inverting all equation‐of‐state data simultaneously. From this, we extend the gold pressure‐calibration standard to cover the range 0–200 GPa in pressure and 300–3000 K in temperature.

Static compression of iron-silicon alloys: Implications for silicon in the Earth's core

lower the density of iron, but significantly changes its compressibility neither in the bcc phase, nor at high pressures in the hcp phase. Upon comparison with the Preliminary Reference Earth Model, the calculated equations of state (EOS) of hcp-Fe85Si15, using the Mie-Gruneisen EOS, indicate that an outer core containing about 8-10 wt.% Si and inner core containing about 4 wt.% Si in iron would satisfy the seismological constraints. Addition of silicon into iron increases the bulk sound velocity of iron, consistent with silicon being a light element in the Earths core. INDEX TERMS: 1015 Geochemistry: Composition of the core; 3919 Mineral Physics: Equations of state; 3924 Mineral Physics: High-pressure behavior; 3954 Mineral Physics: X ray, neutron, and electron spectroscopy and diffraction; KEYWORDS: high pressure, light elements, iron-silicon alloy, Earths core, X-ray diffraction, equation of state

Stability of magnesiowustite in Earth's lower mantle.

Magnesiowüstite [(Mg,Fe)O] is the second most abundant mineral of Earths 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 [(Mg0.39,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.

A laser heating system that stabilizes and controls the temperature: Diamond anvil cell applications

A laser heating system is described for use with diamond anvil high pressure cells that directly senses and stabilizes visible thermal radiation emitted by hot samples. This technique stabilizes sample temperatures better than other methods and allows superior temperature control. Calibration of the system was checked by measuring the melting temperatures of five metals at ambient pressure. Assuming literature values for spectral emissivity, the calibration was found to be accurate to 3.3% (based upon one standard deviation of the percentage error from published melting temperatures). Performance of the laser heating system was verified by heating iron foil at 13 GPa. With the sample intensity unstabilized, mean temperature was 3003 K with a standard deviation of 144 K, while with it stabilized, mean temperature was 3051 K with a standard deviation of 8 K. For a given wavelength‐dependent emissivity, the difference between the actual temperature and the greybody temperature increases as the temperature in...

Melting of iron‐magnesium‐silicate perovskite

Iron-magnesium-silicate perovskite was melted in a laser-heated diamond anvil cell and monitored by thermal analysis. Two signals were identified at each pressure: a lower temperature signal and a higher temperature signal. The lower signal may correspond to the temperature where iron diffuses rapidly. The higher signal would then correspond to melting of magnesium-enriched silicate perovskite. Alternatively, the lower signal may be the solidus where (Fe.14Mg.86)SiO3 undergoes incongruent melting to an iron-magnesium-enriched solid plus a silica-enriched liquid. Then, the higher signal would be the liquidus. The slope for the melting curve (the higher signal) between 30GPa and 94GPa is slightly negative. Fitting to a straight line gives a value for the slope of −2.5 ± 0.6K/GPa. At 30GPa, the lower signal is ≈300K below the melting curve. They converge slightly at higher pressures and differ by ≈140K at 94GPa. Our melting curve is several hundred degrees (K) below previous estimates.

Equations of state in the Fe‐FeSi system at high pressures and temperatures

Earths core is an iron-rich alloy containing several weight percent of light element(s), possibly including silicon. Therefore, the high pressure-temperature equations of state of iron-silicon alloys can provide understanding of the properties of Earths core. We performed X-ray diffraction experiments using laser-heated diamond anvil cells to achieve simultaneous high pressures and temperatures, up to ~200 GPa for Fe–9 wt % Si alloy and ~145 GPa for stoichiometric FeSi. We determined equations of state of the D03, hcp + B2, and hcp phases of Fe–9Si, and the B20 and B2 phases of FeSi. We also calculated equations of state of Fe, Fe11Si, Fe5Si, Fe3Si, and FeSi using ab initio methods, finding that iron and silicon atoms have similar volumes at high pressures. By comparing our experimentally determined equations of state to the observed core density deficit, we find that the maximum amount of silicon in the outer core is ~11 wt %, while the maximum amount in the inner core is 6–8 wt %, for a purely Fe-Si-Ni core. Bulk sound speeds predicted from our equations of state also match those of the inner and outer core for similar ranges of compositions. We find a compositional contrast between the inner and outer core of 3.5–5.6 wt % silicon, depending on the seismological model used. Theoretical and experimental equations of state agree at high pressures. We find a good match to the observed density, density profile, and sound speed of the Earths core, suggesting that silicon is a viable candidate for the dominant light element.

Compression of MgS to 54 GPa

Abstract MgS (B1 or NaCl structure) was compressed in a diamond anvil cell at room temperature up to a pressure of 54 GPa. Diffraction experiments were performed using X-rays from both a rotating anode generator and synchrotron beam line. The B1 phase of MgS is stable to 54 GPa. A third order Birch-Murnaghan equation of state fitted to the data yields a bulk modulus of 78.9 ± 3.7 GPa with a pressure derivative of 3.71 ± 0.34. Universal equation of state parameters are virtually identical. The thermodynamic parameters obtained are consistent with most of the existing theoretical predictions. For comparison, a bulk modulus of 56.0 ± 1.4 GPa, with a pressure derivative of 5.3 ± 1.0, is calculated for CaS using previously published data. Finally, the pressure-volume data and elastic parameters of several alkali halides and alkaline-earth sulfides and oxides are used to evaluate the applicability of the Born model for these compounds.

A High-Pressure Test of Birch's Law

The compressional wave velocities of polycrystalline NaCl and KCl have been measured to over 17 gigapascals, with the use of Brillouin scattering and the diamond anvil cell. This pressure corresponds to 40% compression for NaCl and 60% compression for KCl (including the volume change across the B1-B2 transition). The data obey Birchs Law, which predicts that the velocity of each material is linear with density, except across the B1-B2 phase transition in KCl. This deviation from Birchs Law can be rationalized in terms of an interatomic potential model wherein the vibrational frequencies of the nearest neighbor bonds decrease when going to the eight-coordinated B2 structure from the six-coordinated B1 structure.

Multidisciplinary impact of the deep mantle phase transition in perovskite structure

A phase transition in (Mg, Fe) SiO3 (magnesium silicate-perovskite) for pressure-temperature conditions near the base of Earths mantle, first reported in May 2004, is stimulating strong multidisciplinary excitement and interactions. Experimentally and theoretically determined characteristics of this phase transition indicate that it may hold the key to understanding enigmatic seismological structures in the D” region of the lowermost mantle, with important implications for heat transport, thermal instabilities, and chemical properties of the lower mantle. All minerals undergo phase transitions with increasing depth into the Earth, reorganizing their crystal structures into denser-packed forms stable over a finite range of pressures and temperatures. The changes in material properties across such transitions often give rise to detectable contrasts in seismic velocities and density.

Pressure‐induced amorphization of covellite, CuS

CuS, or covellite (hexagonal symmetry), was compressed in a diamond anvil cell at room temperature up to a pressure of 45 GPa, and studied using x rays from both a Mo Kα source and a synchrotron. The x‐ray diffraction spectrum of CuS disappears by about 18 GPa. The presence of Cu fluorescence lines in all spectra and the reappearance of diffraction lines upon decompression confirm that CuS undergoes reversible pressure‐induced amorphization at this pressure. A third‐order Birch–Murnaghan equation of state fit to the diffraction data below 11 GPa yields a bulk modulus of 89±10 GPa with a pressure derivative of −2±2 for covellite. Further compression up to 45 GPa shows three to four diffraction lines of very low intensity, implying some high pressure ‘‘ordering’’ of the amorphous phase. The Raman spectra obtained indicate that the changes in structure are probably due to the twisting or the distortion of covalently bonded CuS4–CuS4 units in different directions.