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Silicon carbon oxide minerals at extreme pressure and temperature conditions

Final Report Summary - SICOMIN (Silicon carbon oxide minerals at extreme pressure and temperature conditions.)

This project proposed to explore systematically the chemistry of CO2 at different thermodynamic conditions (pressure, temperature and composition). Particular emphasis was placed on the system carbon dioxide:silica, where recent experiments have evidenced the formation of a Si0.4C0.6O2 solid solution after heating a CO2-filled microporous silica polymorph, silicalite, in excess of 3500 K while compressed at approx. 16 GPa. Remarkably, this [CO4]-based solid solution is recovered at ambient conditions. This novel phase adopts a densely packed -cristobalite structure with carbon and silicon in fourfold coordination to oxygen atoms at pressures where silica normally adopts a sixfold coordinated rutile-type stishovite structure. These findings reveal a unique non-molecular oxide chemistry at high pressures of great interest for materials science (potential formation of materials with novel interesting physical properties), as well as Earth and planetary sciences (oxidized carbon phases). No further experimental results on silicon carbon oxides were in the literature.
Concurrently, the project sought to provide the Fellow with the training of important technical professional skills that complement previous applicant qualifications and new conceptual approaches to enrich his background. Among the projected skills to be implemented were (i) the design and use of mineral heating setups, (ii) the study of thermal conductivity in potential deep Earth materials and (iii) the geophysical background of the Fellow. Finally, the interdiciplinary SiCOMin project intended to facilitate the Fellow transition to the high-quality European scientific workforce.
In the 11 month duration of the project (early termination was requested), important work on the project has been carried out and several manuscripts summarizing the obtained results have been sent for publication.
We have explored the existence of novel carbonate compounds by reacting CO2 and several highly porous silica zeolites (ITQ29, ZSM-5, etc...) at P-T conditions of 5-50 GPa and 300-3000 K, in laser heated DACs. Other different promising starting materials such as amorphous silicon carbon oxides were also used. The structure of the solids was investigated in situ by micro x-ray diffraction (XRD) at cutting-edge synchrotron facilities: GSECARS (Advanced Photon Source) and 12.2.2 (Advanced Light Source) beamlines. Two successful beamtime applications with 33 shifts awarded up to now allowed us to structurally characterize our samples while compressed and laser-heated.The Fellow is still involved in the data analysis of the some of the experimental runs. Initial results indicate that CO2 and SiO2 are more stable under extreme conditions than expected from literature, but some of the experimental configurations present new phase/s. On the other hand, chemical reacticity of CO2 with supposed stable noble metals is verified. The recovered samples have been characterized by Raman and electron microscopies. Moreover, ab initio total-energy calculations on the CO2-filled silicalite system and some of the chemical reaction products are performed to complement our results.
The compressibility and high-pressure high-temperature phase boundaries of carbon dioxide and silica were accurately determined. The lattice parameters and unit-cell volumes of phase I, II, IV and V were obtained from the aforementioned XRD experiments. Moreover, neutron diffraction data of the CO2-IV phase from the Spallation Neutron Source using a new pressure cell and an externally resistive heating system allowed studying its compression and thermal expansion. Finally, we also studied the evolution of the structure of alpha-quartz subjected to compression outside its normal stability field by means of infrared spectroscopy. The change in polarity of the IR active modes is related to the tilting of the [SiO4] tetrahedra induced by high pressure. In summary, new high-density and high-temperature data of the binary CO2:SiO2 system are now available.
On the other hand, as part of the training, the Fellow performed a series of XRD experiments in the laser-heated diamond anvil cell on Fe5Si3 (potential constituent of the Earth’s core) up to 1 Mbar and 4000 K. These measurements yield a new thermoelastic equation of state for Fe5Si3. A lower bound on the melting behavior up to 58 GPa was also determined. This information helps constrain compositionally-sensitive models describing the density, compressibility, and dynamics of Earth’s core. We also measure the temperature-versus-laser power what provides information about the heat flow environment in the diamond anvil cell. A comparison of pure iron vs Fe5Si3 results yields a measure of how the presence of Si influences the thermal conductivity of iron at high pressures and temperatures.
The Fellow has participated in several international conferences and workshops where he has presented some of the initial results of the project (Deep Carbon Observatory Meeting, American Geophysical Union Meeting, AIRAPT-EHPRG Meeting, CDAC Meeting, IVICFA Workshop). Four manuscripts have been either submitted or are in last-stage preparation.
In October 2015 the Fellow was awarded with the Ramon y Cajal Senior Grant for promotion of talent and employability of the Spanish Ministry for Science and Innovation. He will join this 5 years, tenure-track position at University of Valencia in February 2016. The guiding thread of his research will be the study of “Mineral and metal polymorphism at inner Earth Conditions”, with particular emphasis on the silicate-carbonate chemistry. Note that the existence of a novel silicate-carbonate family will open up new directions in Earth sciences and solid state-chemistry, and it could pave the way for designing strategies for the important environmental challenge of long-term CO2 sequestration.