Spectroscopic evidence for pressure-induced coordination changes in silicate glasses and melts

Infrared spectra demonstrate that at pressures above 20 gigapascals and room temperature the regular tetrahedral coordination of oxygen around both silicon and aluminum ions is severely disrupted in SiO(2), CaMgSi(2)O(6), and CaAlSi(2)O(8) composition glasses. The spectra are consistent with gradual, pressure-induced increases in the coordination numbers of silicon and aluminum. A variety of coordination environments, from sixfold to fourfold, appears to be present at pressures as high as about 40 gigapascals. This apparent change in coordination is not quenchable at room temperature: on decompression, the glasses return to tetrahedral coordination. This continuous and reversible coordination change in amorphous silicates explains the lack of observation of coordination changes in silicate glasses quenched from high pressure, the shallow melting slopes observed for mantle silicates at high pressures, and the possible presence of neutrally buoyant magmas deep within the terrestrial planets.     

Pressure-induced coordination changes in alkali-germanate melts: an in situ spectroscopic investigation

The structure of liquid Na(2)Ge(2)O(5).H(2)O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near approximately 240 wavenumbers associated with octahedral GeO(6) groups grows relative to a peak near approximately 500 wavenumbers associated with tetrahedral GeO(4) groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0% at ambient pressures to >50% at a pressure of 2.2 gigapascals. Silicate liquids plausibly undergo similar coordination changes at depth in the Earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the Earth's mantle.     

Association reaction in forsterite under shock compression

Transmission electron microscopic observation of forsterite (Mg(2)SiO(4)) shocked to peak pressures of 78 to 92 gigapascals revealed that forsterite breaks down to an assemblage of MgO plus MgSiO(3) glass. This strongly supports the interpretation that the high-pressure phase of forsterite under shock compression is due to the assemblage of MgSiO(3) perovskite plus MgO.     

(Mg,Fe)SiO3-perovskite stability under lower mantle conditions

In three different experiments up to 100 gigapascals and 3000 kelvin, (Mg,Fe)SiO3-perovskite, the major component of the lower mantle, remained stable and did not decompose to its component oxides (Mg, Fe)O and SiO2. Perovskite was formed from these oxides when heated in a diamond anvil cell at pressures up to 100 gigapascals. Both MgSiO3 crystals and glasses heated to 3000 kelvin at 75 gigapascals also formed perovskite as a single phase, as evident from Raman spectra. Moreover, fluorescence measurements on chromium-doped samples synthesized at these conditions gave no indication of the presence of MgO.     

Stability of Perovskite (MgSiO3) in the Earth's Mantle

Available thermodynamic data and seismic models favor perovskite (MgSiO3) as the stable phase in the mantle. MgSiO3 was heated at temperatures from 1900 to 3200 kelvin with a Nd-YAG laser in diamond-anvil cells to study the phase relations at pressures from 45 to 100 gigapascals. The quenched products were studied with synchrotron x-ray radiation. The results show that MgSiO3 broke down to a mixture of MgO (periclase) and SiO2 (stishovite or an unquenchable polymorph) at pressures from 58 to 85 gigapascals. These results imply that perovskite may not be stable in the lower mantle and that it might be necessary to reconsider the compositional and density models of the mantle.     

Comparative Compressibilities of Silicate Spinels: Anomalous Behavior of (Mg,Fe)2SiO4

Compressibilities of five silicate spinels, including gamma-Mg(2)SiO(4), gamma-Fe(2)SiO(4), Ni(2)SiO(4) and two ferromagnesian compositions, were determined on crystals positioned in the same high-pressure mount. Subjection of all crystals simultaneously to the same pressure revealed differences in compressibility that resulted from compositional differences. Ferromagnesian silicate spinels showed an anomalous 13 percent increase in bulk modulus with increasing iron content, from Mg(2)SiO(4) (184 gigapascals) to Fe(2)SiO(4) (207 gigapascals). This result suggests that ferrous iron and magnesium, which behave similarly under crustal conditions, are chemically more distinct at high pressures characteristic of the transition zone and lower mantle.     

The Phase Boundary Between agr- and beta-Mg2SiO4 Determined by in Situ X-ray Observation

The stability of Mg(2)SiO(4), a major constituent in the Earth's mantle, has been investigated experimentally by in situ observation with synchrotron radiation. A cubic-type high-pressure apparatus equipped with sintered diamond anvils has been used over pressures of 11 to 15 gigapascals and temperatures of 800 degrees to 1600 degrees C. The phase stability of alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) was determined by taking account of the kinetic behavior of transition. The phase boundary between alpha-Mg(2)SiO(4) and beta-Mg(2)SiO(4) is approximated by the linear expression P = (9.3 +/- 0.1) + (0.0036 +/- 0.0002)T where P is pressure in gigapascals and T is temperature in degrees Celsius.     

Shock-produced olivine glass: first observation

Transmission electron microscope (TEM) observations of an experimentally shock-deformed single crystal of natural peridot, (Mg(0.88)Fe(0.12))(2)SiO(4), recovered from peak pressures of about 56 x 10(9) pascals revealed the presence of amorphous zones located within crystalline regions with a high density of tangled dislocations. This is the first reported observation of olivine glass. The shocked sample exhibits a wide variation in the degree of shock deformation on a small scale, and the glass appears to be intimately associated with the highest density of dislocations. This study suggests that olivine glass may be formed as a result of shock at pressures above about 50 to 55 x 10(9) pascals and that further TEM observations of naturally shocked olivines may demonstrate the presence of glass.     

Electronic transitions in perovskite: possible nonconvecting layers in the lower mantle

We measured the spin state of iron in magnesium silicate perovskite (Mg(0.9),Fe(0.1))SiO(3) at high pressure and found two electronic transitions occurring at 70 gigapascals and at 120 gigapascals, corresponding to partial and full electron pairing in iron, respectively. The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D" layer above the core-mantle boundary. The resulting increase in radiative thermal conductivity suggests the existence of nonconvecting layers in the lowermost mantle.     

Stability and structure of MgSiO3 perovskite to 2300-kilometer depth in Earth's mantle

Unexplained features have been observed seismically near the middle (approximately 1700-kilometer depth) and bottom of the Earth's lower mantle, and these could have important implications for the dynamics and evolution of the planet. (Mg,Fe)SiO3 perovskite is expected to be the dominant mineral in the deep mantle, but experimental results are discrepant regarding its stability and structure. Here we report in situ x-ray diffraction observations of (Mg,Fe)SiO3 perovskite at conditions (50 to 106 gigapascals, 1600 to 2400 kelvin) close to a mantle geotherm from three different starting materials, (Mg0.9Fe0.1)SiO enstatite, MgSiO3 glass, and an MgO+SiO2 mixture. Our results confirm the stability of (Mg,Fe)SiO3 perovskite to at least 2300-kilometer depth in the mantle. However, diffraction patterns above 83 gigapascals and 1700 kelvin (1900-kilometer depth) cannot presently rule out a possible transformation from Pbnm perovskite to one of three other possible perovskite structures with space group P2(1)/m, Pmmn, or P4(2)/nmc.     

Structural and bonding changes in cesium iodide at high pressures

Cesium iodide, a simple ionic salt at low pressures, undergoes a second-order transformation at 40 gigapascals (400 kilobars) from the cubic B2 (cesium chloride-type) structure to the body-centered tetragonal structure. Also, the energy gap between valence and conduction bands decreases from 6.4 electron volts at zero pressure to about 1.7 electron volts at 60 gigapascals, transforming cesium iodide from a highly ionic compound to a semiconductor. The structural transition increases the rate at which the band gap closes, and an extrapolation suggests that cesium iodide becomes metallic near (or somewhat above) 100 gigapascals. Similar changes in bonding character are likely to occur in other alkali halides at pressures above 100 gigapascals.     

Strength of diamond

The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.     

Silicon coordination and speciation changes in a silicate liquid at high pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with (29)Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na(2)Si(2)O(5) glass quenched from 8 gigapascals and 1500 degrees C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.     

Pressure-induced solid carbonates from molecular CO2 by computer simulation

A combination of ab initio molecular dynamic simulations and fully relaxed total energy calculations is used to predict that molecular CO2 should transform to nonmolecular carbonate phases based on CO4 tetrahedra at pressures in the range of 35 to 60 gigapascals. The simulation suggests a variety of competing phases, with a more facile transformation of the molecular phase at high temperatures. Thermodynamically, the most stable carbonate phase at high pressure is predicted to be isostructural to SiO2 alpha-quartz (low quartz). A class of carbonates, involving special arrangements of CO4 tetrahedra, is found to be more stable than all the other silica-like polymorphs.     

Melting of (Mg,Fe)2SiO4 at the Core-Mantle Boundary of the Earth

The lower mantle of the Earth is believed to be largely composed of (Mg,Fe)O (magnesiowustite) and (Mg,Fe)SiO3 (perovskite). Radiative temperatures of single-crystal olivine [(Mg0.9,Fe0.1)2SiO4] decreased abruptly from 7040 +/- 315 to 4300 +/- 270 kelvin upon shock compression above 80 gigapascals. The data indicate that an upper bound to the solidus of the magnesiowustite and perovskite assemblage at 4300 +/- 270 kelvin is 130 +/- 3 gigapascals. These conditions correspond to those for partial melting at the base of the mantle, as has been suggested occurs within the ultralow-velocity zone beneath the central Pacific.     

Sound velocities in olivine at Earth mantle pressures

The independent elastic constants of an upper mantle mineral, San Carlos olivine [(Mg(1.8)Fe(0.2))SiO(4)], were measured from 0 to 12.5 gigapascals. Evidence is offered in support of the proposition that the explicit temperature dependence of the bulk modulus is small over the range of temperatures and pressures thought to prevail above the 400-kilometer discontinuity, and thus the data can be extrapolated to estimate the properties of olivine under mantle conditions at a depth of 400 kilometers. In the absence of high-temperature data at high pressures, estimates are made of the properties of olivine under mantle conditions to a depth of 400 kilometers. In contrast with low-pressure laboratory data, the predicted covariance of shear and compressional velocities as a function of temperature nearly matches the seismically estimated value for the lower mantle.     

Compression of Ice to 210 Gigapascals: Infrared Evidence for a Symmetric Hydrogen-Bonded Phase

Protonated and deuterated ices (H2O and D2O) compressed to a maximum pressure of 210 gigapascals at 85 to 300 kelvin exhibit a phase transition at 60 gigapascals in H2O ice (70 gigapascals in D2O ice) on the basis of their infrared reflectance spectra determined with synchrotron radiation. The transition is characterized by soft-mode behavior of the nu3 O-H or O-D stretch below the transition, followed by a hardening (positive pressure shift) above it. This behavior is interpreted as the transformation of ice phase VII to a structure with symmetric hydrogen bonds. The spectroscopic features of the phase persisted to the maximum pressures (210 gigapascals) of the measurements, although changes in vibrational mode coupling were observed at 150 to 160 gigapascals.     

The postspinel phase boundary in Mg2SiO4 determined by in situ X-ray diffraction

The phase boundary between spinel (gamma phase) and MgSiO3 perovskite + MgO periclase in Mg2SiO4 was determined by in situ x-ray measurements by a combination of the synchrotron radiation source (SPring-8) and a large multianvil high-pressure apparatus. The boundary was determined at temperatures between 1400 degrees to 1800 degreesC, demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyses. The boundary was located at 21.1 (+/-0.2) gigapascals, at 1600 degreesC, which is approximately 2 gigapascals lower than earlier estimates based on other high-pressure studies.     

Synthesis and Equation of State of (Mg,Fe) SiO3 Perovskite to Over 100 Gigapascals

Silicate perovskite of composition (Mg(0.88)Fe(0.12)) SiO(3) has been synthesized in a laser-heated diamond-anvil cell to a pressure of 127 gigapascals at temperatures exceeding 2000 K. The perovskite phase was identified and its unit-cell dimensions measured by in situ x-ray diffraction at elevated pressure and room temperature. An analysis of these data yields the first high-precision equation of state for this mineral, with values of the zero-pressure isothermal bulk modulus and its pressure derivative being K(0T) = 266 +/- 6 gigapascals and K'(0T) = 3.9 +/- 0.4. In addition, the orthorhombic distortion of the silicate-perovskite structure away from ideal cubic symmetry remains constant with pressure: the lattice parameter ratios are b/a = 1.032 +/- 0.002 and c/a = 1.444 +/- 0.006. These results, which prove that silicate perovskite is stable to ultrahigh pressures, demonstrate that perovskite can exist throughout the pressure range of the lower mantle and that it is therefore likely to be the most abundant mineral in Earth.     

Long-range topological order in metallic glass

Glass lacks the long-range periodic order that characterizes a crystal. In the Ce(75)Al(25) metallic glass (MG), however, we discovered a long-range topological order corresponding to a single crystal of indefinite length. Structural examinations confirm that the MG is truly amorphous, isotropic, and unstrained, yet under 25 gigapascals hydrostatic pressures, every segment of a centimeter-length MG ribbon devitrifies independently into a face-centered cubic (fcc) crystal with the identical orientation. By using molecular dynamics simulations and synchrotron x-ray techniques, we elucidate that the mismatch between the large Ce and small Al atoms frustrates the crystallization and causes amorphization, but a long-range fcc topological order still exists. Pressure induces electronic transition in Ce, which eliminates the mismatch and manifests the topological order by the formation of a single crystal.