Molding Electron Density at High Pressures: Chemical Bonding and Valence at Planetary-Interior Conditions

Details Molding Electron Density at High Pressures: Chemical Bonding and Valence at Planetary-Interior Conditions Molding Electron Density at High Pressures: Chemical Bonding and Valence at Planetary-Interior Conditions

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.s21e0370p&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #S21E-0370

Other

3924 High-Pressure Behavior, 3929 Nmr, Mossbauer Spectroscopy, And Other Magnetic Techniques

Scientific paper

Recognizing that terrestrial planets consist of a metallic core surrounded by an oxidized shell, we examine the oxidation state of minerals at the high pressures characteristic of planetary interiors. Ion valences are often considered fixed within a crystal of given composition, such that the stability of a mineral phase is determined through equilibration with surrounding gas species (e. g., as quantified by oxygen fugacity). One effect of pressure is to alter the nature of chemical bonding, however, so we consider whether it is possible to change the valences of ions within a crystal merely through compression. 57Fe Mössbauer spectroscopy shows that the electron density in magnetite is shifted from octahedral (B) to tetrahedral (A) sites with increasing pressure. This order - disorder transition, from inverse ([Fe3+]A[Fe2.5+Fe 2.5+]BO4) to normal spinel ([Fe 2+]A[Fe3+Fe 3+]BO4), occurs at 10 GPa at 300 K with no movement of ions. Iron in the octahedral and tetrahedral sites are respectively oxidized and reduced, without any chemical reaction with the surrounding medium. The samples were studied in a gasketed diamond-anvil cell over the pressure - temperature range 0-40 GPa and 80-410 K, and with Ar as a pressure medium. Oxidation and reduction need not be coupled, as demonstrated by experiments on Fe2+(OH)2, the iron analog of brucite. Mössbauer spectroscopy documents that the iron is oxidized upon compression, with 10-70 percent conversion of Fe 2+ to Fe 3+ as pressure is increased above 10-40 GPa at 300 K. In this process, an electron is liberated into the unit cell. X-ray diffraction at 300 K confirms that there is no structural change over this pressure range, but temperature-dependent measurements show a drop of 2.5 orders of magnitude in electrical resistance at 50 GPa with the sample going to a narrow-gap semiconducting state. Our experiments documenting pressure-induced internal oxidation demonstrate that such external variables as oxygen fugacity do not constrain ion valences within minerals. Instead, we propose that the chemical potential of the bonding electrons represents a more useful variable.

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