Pressure- and temperature-dependent Raman spectra of Ca2Fe2O5 oxygen defect perovskite.

The Raman spectra of Ca2Fe2O5 were investigated up to 21.8 GPa at room temperature and up to 1073 K at ambient pressure, respectively. A phase transition begins around 13.6 GPa and it is reversible after decompression. No temperature-induced phase transition was observed due to the quality of Raman spectra at temperatures above 773 K. The effects of pressure and temperature on the Raman vibration were quantitatively analyzed. All the observed Raman active vibrations of Ca2Fe2O5 show positive linear pressure dependences and negative temperature dependences with different slopes. Combined with previous experimental results, the isothermal and isobaric mode Grüneisen parameters of Ca2Fe2O5 were estimated, and the intrinsic anharmonicity was discussed.

Raman and X-ray diffraction study of pressure-induced phase transition in synthetic Mg2TiO4.

Synthetic Mg2TiO4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The Raman measurements showed that cubic Mg2TiO4 spinel transforms to a high pressure tetragonal (I41/amd, No.141) phase at 14.7 GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3 GPa. In the X-ray diffraction experiment, transformation of the cubic Mg2TiO4 to the tetragonal structure was complete by 29.2 GPa, ~5 GPa higher than the transition pressure obtained by Raman measurements, owing to slow kinetics. The obtained isothermal bulk modulus of Mg2TiO4 spinel is KT0 = 148(3) GPa when KT0' = 6.6, or KT0 = 166(1) GPa when KT0' is fixed at 4. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V0 = 270(2) Å3 when KT0' is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γth) of cubic and tetragonal Mg2TiO4 phases are 1.01 and 0.63. Based on the radii ratio of spinel cations, a simple model is proposed to predict post-spinel structures.

A Spectral Comparison of Jarosites Using Techniques Relevant to the Robotic Exploration of Biosignatures on Mars.

The acidic sulfate-rich waters of the Meridiani Planum region were potentially a habitable environment for iron-oxidizing bacteria on ancient Mars. If life existed in this ancient martian environment, jarosite minerals precipitating in these waters may record evidence of this biological activity. Since the Meridiani jarosite is thermodynamically stable at the martian surface, any biosignatures preserved in the jarosites may be readily available for analysis in the current surface sediments during the ongoing robotic exploration of Mars. However, thermal decomposition experiments indicate that organic compound detection of sediments containing jarosite may be challenging when using pyrolysis experiments; the instrument commonly used to assess organic matter in martian samples. So, here, we assess if the biogenicity of the Meridiani-type jarosites can be determined using complimentary spectroscopic techniques also utilized during the robotic exploration of Mars, including the upcoming ExoMars2020 rover mission. An abiotic jarosite, synthesized following established protocols, and a biological jarosite counterpart, derived from a microbial enrichment culture of Rio Tinto river sediments, were used to compare four spectroscopy techniques employed in the robotic exploration of Mars (Raman spectroscopy, mid-infrared (IR) spectroscopy, visible near-infrared reflectance (VNIR) spectroscopy and Mössbauer spectroscopy) to determine if the complimentary information obtained using these instruments can help elucidate the biological influence of Meridiani-type jarosites. Raman spectral differences might be due to the presence of unreacted reagents in the synthetic spectra and not biological contributions. Reflectance (IR/VNIR) spectra might exhibit minor organic absorption contributions, but are observed in both sample spectra, and do not represent a biosignature. Mössbauer spectra show minor differences in fit parameters that are related to crystal morphology and are unrelated to the biological (i.e., organic) component of the system. Results of this study suggest that the identification of biosignatures in Meridiani-type jarosites using the in situ robotic exploration on Mars may be possible but will be challenging. Our work provides additional insight into extraterrestrial biosignature detection and data interpretation for Mars exploration and indicates that sample return missions are likely required to unequivocally resolve the possible biogenicity of the Meridiani sediments or other jarosite-containing sediments.

Electronic structure of carbon dioxide under pressure and insights into the molecular-to-nonmolecular transition.

Knowledge of the high-pressure behavior of carbon dioxide (CO2), an important planetary material found in Venus, Earth, and Mars, is vital to the study of the evolution and dynamics of the planetary interiors as well as to the fundamental understanding of the C-O bonding and interaction between the molecules. Recent studies have revealed a number of crystalline polymorphs (CO2-I to -VII) and an amorphous phase under high pressure-temperature conditions. Nevertheless, the reported phase stability field and transition pressures at room temperature are poorly defined, especially for the amorphous phase. Here we shed light on the successive pressure-induced local structural changes and the molecular-to-nonmolecular transition of CO2 at room temperature by performing an in situ study of the local electronic structure using X-ray Raman scattering, aided by first-principle exciton calculations. We show that the transition from CO2-I to CO2-III was initiated at around 7.4 GPa, and completed at about 17 GPa. The present study also shows that at ~37 GPa, molecular CO2 starts to polymerize to an extended structure with fourfold coordinated carbon and minor CO3 and CO-like species. The observed pressure is more than 10 GPa below previously reported. The disappearance of the minority species at 63(± 3) GPa suggests that a previously unknown phase transition within the nonmolecular phase of CO2 has occurred.

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO3 orthopyroxene.

Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CaIrO(3)-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO(3). Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) A(3) when K'(0) = 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to approximately 400-550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CaIrO(3)-type phases at 80-106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.

Strength and elasticity of SiO2 across the stishovite-CaCl2-type structural phase boundary.

Radial x-ray diffraction experiments were conducted under nonhydrostatic compression on SiO2 to 60 GPa in a diamond anvil cell. This ratio of differential stress to shear modulus t/G is 0.019(3)-0.037(5) at P=15-60 GPa. The ratio for octahedrally coordinated stishovite is lower by a factor of about 2 than observed in four-coordinated silicates. Using a theoretical model for the shear modulus, the differential stress of stishovite is found to be 4.5(1.5) GPa below 40 GPa and to decrease sharply as the stishovite-CaCl2-type phase transition boundary is approached. Inversion of measured lattice strains provides direct experimental evidence for softening of C11-C12.