Shock metamorphism of some rock-forming minerals: Experimental results and natural observations

The products of shock metamorphism in the Jänisjärvi astrobleme in Karelia, Russia, are compared with the results of experiments in which spherical converging shock waves affected a spherical rock sample. The sample was loaded by a broad spectrum of shock pressures, which increased from ～20 GPa at the periphery of the rock sphere to > 200 GPa at its center. Experiments with rocks metamorphosed under the effect of spherical converging shock waves imitate collisions of cosmic bodies with the Earth’s surface, when transformations in rocks and minerals are induced by a single impact event. The shock-thermal decomposition of mafic minerals occurs in the same succession in nature and the experiments, with some differences between natural and experimentally produced shock-thermal aggregates likely accounted for by the smaller sizes of the experimental impact rock sample and, correspondingly, its more rapid quenching. Our shock experiments were the first to synthesize ringwoodite that was rich in Al₂O₃ and should be referred to as aluminous ringwoodite. The mineral was produced not via the martensite transition of olivine but by means of biotite replacement coupled with the migration of elements. The transformations of minerals by shock waves (amorphization and shock-thermal decomposition) were determined to be controlled mainly by the crystal structures of these minerals. The experimental products provide evidence of the migration of chemical elements within the crystal structure. The structural setting of ions in a mineral determines the onset of element migrations and the intensity of this process.     

Calculations of pressure-induced phase transitions in mantle minerals

The crystal structures and energies of SiO₂ stishovite, MgO periclase, Mg₂SiO₄ spinel, and MgSiO₃ perovskite were calculated as a function of pressure with the polarization-included electron gas (PEG) model. The calculated pressures of the spinel to perovskite phase transitions in the Mg₂SiO₄ and MgSiO₃ systems are 26.0 GPa and 27.0 GPa, respectively, compared to the experimental zero temperature extrapolations of 27.4 GPa and 27.7 GPa. The two oxide phases are found to be the most stable form in the pressure range 24.5 GPa to 31.5 GPa, compared to the experimental zero temperature extrapolation of 26.7 GPa to 28.0 GPa. The volume changes associated with the phase transitions are in good agreement with experiment. The transition pressures calculated with the PEG model, which allows the ions to distort from spherical symmetry, are in much better agreement with experiment than those calculated with the modified electron gas (MEG) model, which constrains the ions to be spherical.     

Effect of KCl on melting in the Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>–MgSiO<Subscript>3</Subscript>–H<Subscript>2</Subscript>O system at 5 GPa

To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg₂SiO₄–MgSiO₃–H₂O and Mg₂SiO₄–MgSiO₃–KCl–H₂O systems at 5 GPa. In the Mg₂SiO₄–MgSiO₃–H₂O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg₂SiO₄–MgSiO₃–KCl–H₂O systems with molar Cl/(Cl + H₂O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H₂O in the fluid in the Mg₂SiO₄–MgSiO₃–H₂O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.     

Refractive index and compressibility of Di64An36 glass over a pressure range of 0–5.0 GPa

Data on the refractive index, density, and bulk modulus variations of Di₆₄An₃₆ glass, which is used as a model basalt melt, were obtained with a polarization interference microscope and a high-pressure diamond anvil cell at ambient temperature and pressure up to 5.0 GPa. An anomalous decrease in the bulk modulus, K _t , was observed in the pressure range 0?C1.0 GPa. The values of the zero-pressure isothermal bulk modulus, K _t,0 = 22.2, and variation of the bulk modulus with pressure, ??K _t /??P = 11.35, were derived using a linear equation relating K _t and P over the pressure range with the normal behavior of the compressibility. A comparison of our results with previous data on other glasses and melts showed that the bulk moduli of silicate glasses are similar to those of corresponding melts. The values of the pressure coefficient of the bulk moduli, ??K _t /??P, for glasses derived from linear equations are 2.5 times higher than the pressure derivative of the bulk modulus, K?? _T , derived using the Birch-Murnaghan equation for corresponding melts. The difference in ??K _t /??P and K?? _T has an effect on the compressibility of glasses and melts. The compressibility of glasses up to 5.0 GPa calculated as (d ? d ₀)/d is almost two times lower than that of corresponding melts.     

Structure, thermodynamic and transport properties of liquid MgSiO3: Comparison of molecular models and laboratory results

Liquid MgSiO₃ is a model for the Earth’s magma ocean and of remnant melt present near the core-mantle boundary. Here, models for molten MgSiO₃ are computed employing empirical potential molecular dynamics (EPMD) and results are compared to published results including two EPMD studies and three first-principles molecular dynamics (FPMD) models and to laboratory data. The EPMD results derived from the Oganov (OG) potential come closest to the density of MgSiO₃ liquid at the 1-bar melting point inferred from the melting curve. At higher P, EPMD densities calculated from the OG potential and FPMD broadly match shock wave studies, with the OG potential yielding the better comparison. Matsui (M) potential results deviate from other studies above ∼50 GPa. Overall, results based on the OG potential compare best to experimental densities over the P-T range of the mantle. Isothermally, upon increasing P the mean coordination numbers () of oxygen around Si and Mg monotonically increase with pressure. Tetrahedral Si and octahedral Si monotonically increase and decrease, respectively, whereas pentahedral Si maximizes at 10-20 GPa. Tetrahedral Mg decreases monotonically as P increases whereas pentahedral, octahedral and higher coordination polyhedra each show similar behavior first increasing and then decreasing after attaining a maximum; the P of the maximum for each polyhedra type migrates to higher P as the CN increases. Free oxygen and oxygen with one nearest neighbor of either Si or Mg decreases whereas Si or Mg with two or three nearest oxygens (i.e., tricluster oxygen) increases with increasing P isothermally. The increase of tricluster oxygen is consistent with spectroscopy on MgSiO₃ glass quenched from 2000 K and 0-40 GPa and high-energy X-ray studies constraining the coordination of O around Mg and around Si at 2300 K and 1 bar. Coordination statistics from FPMD studies for O around Si and Si around O are in agreement with the EPMD results based on the M and OG potentials. Mg self-diffusivity is greater than O and Si self-diffusivities for both the M and OG potentials. All D values monotonically decrease with increasing pressure isothermally and all atoms are more diffusive in the M liquid compared to the OG liquid except at T > ∼5000 K and P > 100 GPa. Previously published EPMD diffusivities fall between values given by the M and OG potentials, at least up to 45 GPa. The M liquid is generally less viscous than the OG liquid except at P > ∼80 GPa. Activation energy and volume are around 96 kJ/mol and 1.5 cm³/mol, respectively. The FPMD viscosity results at 120 GPa and 4000 and 4500 K are essentially identical to the values from the M and OG potentials. FPMD viscosity results are similar to the OG results for P < 60 GPa; at higher P, the FPMD viscosities are higher. At 4000 K and 100 GPa the shear viscosity of liquid MgSiO₃ is ∼0.1 Pa s. More extensive laboratory results are required to better define the thermodynamic, transport and structural properties of MgSiO₃ liquids and for comparison with computational studies.     

Numerical estimates of mechanical energy transfer from the atmosphere to the Indian ocean

The field of mechanical energy transfer from the atmosphere to the ocean is computed for the first time. The numerical simulation of waves within the Indian Ocean (IO) water area for the period of 1998?C2009 is used. Mechanical energy transfer is described by two integrated parameters calculated per area unit: the speed of complete energy flux from wind to waves, I _E (x, t), and the speed of complete losses in the energy of wind waves, D _E (x, t). In order to solve this problem, the wind field W(x, t) (the NCEP/NOAA data) is used; the I _E (x, t) and D _E (x, t) fields are calculated on the basis of the WAM numerical model containing a modified source function. The results obtained allow us, first, to assess the characteristic spatial distribution of zones ??pumped?? by the wind with mechanical energy for both the wave field and the upper layer of the ocean by seasons, years, and the whole period discussed, second, to determine the extreme and average zonal values of I _E (x, t) and D _E (x, t), the degree of their shift spacing and balance B _E = (I _E + D _E ); and third, to define the characteristic time scales of variations in the wind field and wave field energies, caused by energy transfer from the wind to waves in the zones and within the Indian ocean as a whole. These results significantly specify the climatic estimates obtained earlier.     

Gamma-ray bursts as a result of the interaction of a shock from a supernova and a neutron-star companion

A supernova explosion in a close binary system in which one of the components is a compact magnetized object (neutron star or white dwarf) can form a narrow “tail” with length l _t ～10⁹ cm, width h _t ～10⁸ cm, and magnetic field B _t ～10⁶, due to the resulting shock wave flowing around the magnetosphere of the compact object. The energy released by the reconnection of magnetic field lines in this tail can accelerate electrons to relativistic speeds (γ≈10⁴), creating the conditions required for powerful synchrotron radiation at energies from hundreds of keV to several MeV, i.e., for a gamma-ray burst (GRB). The duration of this radiation will depend on the power of the shock that forms during the supernova. If the shock is not sufficiently powerful to tear off the magnetosphere tail from the compact object, the duration of the GRB will not exceed l _t /V _A ≤1 s, and the conditions necessary for an “afterglow” at softer energies will not arise. If the shock is more powerful, the tail can be torn from the magnetosphere, forming a narrow ejection, which is perceived in its relativistic motion toward the observer(Γ～10⁴) as an afterglow whose duration grows from tens of seconds at gamma-ray energies to tens of days in the optical. This may explain why afterglows are observed only in association with long GRBs (T ₉₀>10 s). Very short GRBs (T ₉₀<0.1 s) may be local, i.e., low-power, phenomena occurring in close pairs containing compact, magnetized objects, in which there is again an interaction between the magnetosphere of the compact object and a shock wave, but the shock is initiated by a flare on the companion, which is a red-dwarf cataclysmic variable, rather than by a supernova.     

Formation of a solid solution in the MgSiO3–MnSiO3 perovskite system

Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO₃–MnSiO₃ perovskite system at high pressure and high temperature. The (Mg, Mn)SiO₃ perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO₃ components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and \( \sqrt{2}\,a/c, \) monotonically decreases from MgSiO₃ to MnSiO₃ perovskite at about 50 GPa. The orthorhombic distortion in (Mg_0.5, Mn_0.5)SiO₃ perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg_0.9, Mn_0.1)SiO₃ perovskite increases with pressure. (Mg, Mn)SiO₃ perovskite incorporating less than 10 mol% of MnSiO₃ component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg_0.9, Mn_0.1)SiO₃. MnSiO₃ is the first chemical component confirmed to form a complete solid solution with MgSiO₃ perovskite at the P–T conditions present in the lower mantle.     

Enthalpies of formation at 970 K of compounds in the system MgO-Al2O3-SiO2 from high temperature solution calorimetry

The enthalpies of solution of petrologically important phases in the system MgO-Al₂O₃-SiO ₂ were measured in a melt of composition 2PbO · B₂O₃ at 970 ± 2K. The substances investigated included synthetic and natural (meteoritic) enstatite (MgSiO₃), synthetic aluminous enstatite (MgSiO_30.9Al₂O_30.1), synthetic and natural cordierite (Mg₂Al₄Si₅O₁₈), synthetic and natural sapphirine (approx. 7MgO·9Al₂O₃ · 3SiO₂), synthetic spinel (MgAl₂O₄), natural sillimanite (Al₂SiO₅), synthetic forsterite (Mg₂SiO₄), synthetic pyrope (Mg₃Al₂Si₃O₁₂), natural quartz (SiO₂), synthetic periclase (MgO) and corundum (Al₂O₃). Improvement in standardization of the calorimeter solvent made possible greater precision in this study than obtainable in former work in this laboratory on some of the same substances.The enthalpies of formation of enstatite, synthetic cordierite, forsterite and spinel are in reasonable agreement with values previously determined by solution calorimetry. The enthalpy of formation of enstatite is about 0.7 kcal less negative than the value for clinoenstatite resulting from the HF calorimetry of Torgesen and Sahama (J. Amer. Chem. Soc.70. 2156–2160, 1948), and is in accord with predictions based on analysis of published pyroxene equilibrium work. Aluminous enstatite with 10 wt.% Al₂O₃ shows an enthalpy of solution markedly lower than pure MgSiO₃: the measurements lead to an estimate of the enthalpy of formation at 970 K for MgAl₂SiO₆ (Mg-Tschermak) orthopyroxene of + 9.4 ± 1.5 kcal/mole from MgSiO₃ and Al₂O₃.Comparison of the enthalpies of formation of synthetic cordierite and anhydrous natural low-iron cordierite shows that they are energetically quite similar and that the synthetic cordierite is not likely to have large amounts of (Al, Si) tetrahedral disorder. Comparison of the enthalpies of formation of synthetic sapphirine and natural low-iron sapphirine shows, on the other hand, that the former is not a good stability model for the latter. The lower enthalpy of formation of the high-temperature synthetic sample is undoubtedly a consequence of cation disordering.The enthalpy of formation of natural sillimanite is considerably less negative than given by the tables of Robie andWaldbaum (U.S. Geol. Surv. Bull.1259 1968).The measured enthalpy of formation of synthetic pyrope is consistent with that deduced from published equilibrium diagrams in conjunction with the present measured enthalpy of formation of aluminous enstatite. Calculation of the entropy of synthetic pyrope from the present data yields surprisingly high values and suggests that synthetic pyrope is not a good stability model for natural pyrope-rich garnets. Hence, considerable doubt exists about the direct quantitative application of experimental diagrams involving pyropic garnet to discussions of the garnet stability field in the Earth's outer regions.     

A high P-T single-crystal X-ray diffraction study of thermoelasticity of MgSiO3 orthoenstatite

P-V-T data of MgSiO₃ orthoenstatite have been measured by single-crystal X-ray diffraction at simultaneous high pressures (in excess of 4.5 GPa) and temperatures (up to 1000 K). The new P-V-T data of the orthoenstatite, together with previous compression data and thermal expansion data, are described by a modified Birch-Murnaghan equation of state for diverse temperatures. The fitted thermoelastic parameters for MgSiO₃ orthoenstatite are: thermal expansion ?α/?P with values of a=2.86(29)×10^-5 K^-1 and b=0.72(16)×10^-8 K^-2; isothermal bulk modulus K _{T o} =102.8(2) GPa; pressure derivative of bulk modulus K′=?K/?P=10.2(1.2); and temperature derivative of bulk modulus K=?K/?T=-0.037(5) GPa/K. The derived thermal Grüneisen parameter is γ _th=1.05 for ambient conditions; Anderson-Grüneisen parameter is δ _{T o} =11.6, and the pressure derivative of thermal expansion is ?α/?P=-3.5×10^-6K^-1 GPa^-1. From the P-V-T data and the thermoelastic equation of state, thermal expansions at two constant pressures of 1.5 GPa and 4.0 GPa are calculated. The resulting pressure dependence of thermal expansion is Δα/ΔP=-3.2(1)× 10^-6 K^-1 GPa^-1. The significantly large values of K′, K, δ _T and ?α/?P indicate that compression/expansion of MgSiO₃ orthoenstatite is very sensitive to changes of pressure and temperature.     

Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox conditions were buffered by Ni–NiO–H₂O (ΔFMQ = ??0.21 to ??1.01), employing a double-capsule setting. Fluids, binary H₂O–CO₂ mixtures at the P, T, and fO₂ conditions investigated, were generated from graphite, oxalic acid anhydrous (H₂C₂O₄) and water. Their dissolved solute loads were analyzed through an improved version of the cryogenic technique, which takes into account the complexities associated with the presence of CO₂-bearing fluids. The experimental data show that forsterite?+?enstatite solubility in H₂O–CO₂ fluids is higher compared to pure water, both in terms of dissolved silica (mSiO₂?=?1.24 mol/kg_H2O versus mSiO₂?=?0.22 mol/kg_H2O at P?=?1 GPa, T?=?800 °C) and magnesia (mMgO?=?1.08 mol/kg_H2O versus mMgO?=?0.28 mol/kg_H2O) probably due to the formation of organic C–Mg–Si complexes. Our experimental results show that at low temperature conditions, a graphite-saturated H₂O–CO₂ fluid interacting with a simplified model mantle composition, characterized by low MgO/SiO₂ ratios, would lead to the formation of significant amounts of enstatite if solute concentrations are equal, while at higher temperatures these fluid, characterized by MgO/SiO₂ ratios comparable with that of olivine, would be less effective in metasomatizing the surrounding rocks. However, the molality of COH fluids increases with pressure and temperature, and quintuplicates with respect to the carbon-free aqueous fluids. Therefore, the amount of fluid required to metasomatize the mantle decreases in the presence of carbon at high P–T conditions. COH fluids are thus effective carriers of C, Mg and Si in the mantle wedge up to the shallowest level of the upper mantle.     

Solid solution behaviour of CaSiO<Subscript>3</Subscript> and MgSiO<Subscript>3</Subscript> perovskites

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO₃ and CaSiO₃ perovskites were calculated for pressures and temperatures of the Earth’s lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO₃ at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO₃ is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO₃ perovskite completely in MgSiO₃ perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO₃ perovskite might completely dissolve in MgSiO₃ perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO₃ is increased by other major mantle constituents such as Fe and Al.     

Nanodeformation in enstatite single crystals: Simulation of micrometeoroid impacts by femtosecond pulsed laser experiments

Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En₉₃Fs₇) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe⁰). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.     

High-pressure crystal chemistry of MgSiO3 perovskite

A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO₃ has been completed to 12.6 GPa. The compressibility of MgSiO₃ perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V ₀ at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO₆ octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO₁₂ site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO₁₂ site with pressure in MgSiO₃ perovskite follow trends observed in GdFeO₃type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO₃-type perovskites as a function of pressure.     

The effect of aluminum and water on the development of deformation fabrics of orthopyroxene

The effect of alumina and water solubility on the development of fabric in orthopyroxene in response to simple shear deformation has been investigated at a pressure of 1.5 GPa and a temperature of 1,100 °C using the D-DIA apparatus. The microstructure observations at these conditions indicate that dislocation glide is the dominant deformation mechanism. In MgSiO₃ enstatite and hydrous aluminous enstatite, partial dislocations bounding the stacking faults in [001] glide parallel to the (100) (or) the (100) [001] slip system. Electron backscattered diffraction analysis of anhydrous aluminous enstatite, however, indicates operation of the (010) [001] slip system, and microstructure analysis indicates dislocation movement involving [001] on both (100) and {210} planes. The strong covalent bonding induced by the occupation of M1 and T2 sites by Al could have restricted the glide on (100), activating slip on {210}. The resulting seismic anisotropies (~2 %) in orthopyroxene are weaker compared to olivine (~9.5 %), and reduced anisotropy can be expected if orthopyroxene coexists with olivine. Weak anisotropy observed in stable cratonic regions can be explained by the relatively high abundance of orthopyroxene in these rocks.     

Ab initio study of MgSiO3 C2/c enstatite

We have used a newly developed ab initio constant-pressure molecular dynamics with variable cell shape technique to investigate the zero temperature behaviour of high pressure clinoenstatite (MgSiO₃-C2/c) from 0 up to 30 GPa. The optimum structure at 8 GPa, as well as structural trends under pressure, compare very well with experimental data. At this pressure, we find noticeable “fluctuations” in the chain configuration which suggests the structure is on the verge of a mechanical instability. Two distinct compressive behaviours then appear: one below and another above 8 GPa. This phenomenon may be related to the observed transition to a lower symmetry P2₁/c phase which involves a reconfiguration of the silicate chains, and suggests that the C2/c structure at low pressures found here, may be an artifact of the dynamical algorithm which preserves space group in the absence of symmetry breaking fluctuations. Comparison with calculations in other magnesium silicate phases, indicates that the size and shape of the silicate units (isolated and/or linked tetrahedra and octahedra) are generally well described by the local density approximation; however, the weaker linkages provided by the O-Mg-O bonds, are not as well described. This trend suggests that, as in the recently studied case of H₂O-ice, the structural properties of more inhomogeneous systems, like enstatite, may be improved by using gradientcorrected density functionals.     

Compression and coordination changes in pyroxenoids: an EXAFS study of MgGeO3 enstatite and CaGeO3 wollastonite

The evolution of the distortion of MgGeO₃ enstatite and CaGeO₃ wollastonite with increasing pressure, has been investigated using X-ray absorption spectroscopy (XAS) in a diamond anvil cell. At room temperature and low pressure (P<7 GPa), the compressibility of the GeO₄ tetrahedron is higher in MgGeO₃ enstatite (K _[GeO4]∼135 GPa) than in CaGeO₃ wollastonite (K _[GeO4]≥ 280 GPa). The compression mechanisms of the two compounds are different: the whole mineral compressibility of Ge-enstatite appears to be very homogeneous, in contrast to that of Ge-wollastonite which exhibits an inhomogeneous tretrahedral compressibility. This result is consistent with the variation of the Debye-Waller factors of the two compounds with increasing pressure. At higher pressures, the coordination of germanium atoms in the two compounds gradually changes from fourfold to sixfold. For CaGeO₃ the coordination change starts at 7 GPa and is complete a 12 GPa, whereas it starts at about 8.5 GPa for MgGeO₃ and is not complete at 31 GPa. The progressive evolution of the measured Ge-O distances as well as the modification in the X-ray absorption near-edge structure indicate two coexisting different sites rather than a progressive site modification. The transformation is found to be partially reversible in CaGeO₃ wollastonite, whereas it is totally reversible in MgGeO₃ enstatite.     

Shock wave-induced interaction between meteoritic iron and silicates

The possibility of shock wave-induced interaction between meteoritic iron was estimated based on the results of experiments on the shock wave loading of mixtures of kamacite from the Sikhote Alin iron meteorite with quartz, albite, oligoclase, enstatite, olivine, and serpentine. The experimental samples were then examined with the application of optical microscopy, microprobe analysis, and M?ssbauer spectroscopy. As a result of shock wave load, the metal was proved to become enriched in Si, while the quartz, albite, and oligoclase melted glasses acquired bivalent Fe ions. The products of our experiments with quartz and feldspar mixtures with kamacite were determined to contain paramagnetic metallic iron, and the surroundings of iron atoms in the silicate constituent of the olivine and enstatite mixtures with kamacite become locally more heterogeneous. Our results indicate that shock waves induce redox reactions between Fe and silicates according to the scheme 2Fe⁺² + Si⁺⁴ = 2Fe⁺² + Si⁰, where Fe⁰ and Si⁰ are iron and silicon in metal and Fe⁺² and Si⁺⁴ are iron and silicon in the sillimanite matrix.