Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmmr11a..01a&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #MR11A-01
Other
5422 Ices, 5430 Interiors (8147), 5460 Physical Properties Of Materials, 6218 Jovian Satellites, 6223 Callisto
Scientific paper
A significant body of research exists on the structure, lattice parameters, and density of high-pressure ice polymorphs, namely Ice VI and Ice VII, as these ices may make up a considerable part of the interior of large icy satellites and select extra solar planets; though most research thus far has been constrained to the pure H2O system. Salty subsurface oceans are also believed to exist within some of these icy bodies which may have prolonged interaction with the Ice VII phase present, incorporating foreign ions or molecules into the lattice of high-pressure ices. Recent research concerning the effects that charged ions have on Ice VII has shown that the presence of these ions notably affects the structure, increasing the Ice VII molar density at any given pressure relative to pure Ice VII (Frank et al., 2006, PEPI, 155, 152-162). This study focused on the incorporation of CH3OH into Ice VII to determine if the change in density was predominantly a result of charge-induced partial ordering of the hydrogen in Ice VII (as outlined in Frank et al., 2006) or if it was controlled solely by the addition of large foreign molecules into the lattice structure. Solutions of 1.60, 5.00 and 10.0 mol% CH3OH in H2O were loaded into a diamond anvil cell. The experiments were performed at GSECARS 13-BM-D at the Advanced Photon Source at Argonne National Laboratory. The unit cell parameters were measured using monochromatic X-ray radiation, 0.3344 Å, and a MAR 345 online imaging system. Powder diffraction patterns were collected in ~1 GPa increments up to ~31, ~48, and ~35 GPa, respectively. The volume-pressure relations (at 300 K) were used to determine an equation of state (EOS) for select compositions in the CH3OH - H2O system. Diffraction data indicate that the unit cell volume of Ice VII formed from a 1.60 mol% CH3OH aqueous solution did not deviate significantly from that of Ice VII formed from pure H2O. Conversely, the volumes of Ice VII formed from 5.00 and 10.0 mol% solutions had reduced unit cell volumes relative to pure Ice VII. Zero-pressure volumes and bulk moduli were calculated with a pressure derivative fixed (4.40): 1.6 mol% CH3OH - H2O had values of 40.4±0.2 Å3 and 24.4±0.7 GPa, respectively, whereas 5.0 mol% CH3OH - H2O had values of 39.2±0.2 Å3 and 26.2±0.5 GPa, respectively. We hypothesize that charged ions will have a greater influence at lower concentrations on the properties of Ice VII than neutral species due to electrostatic interactions within the unit cell and crystallographic constraints imposed by the body-centered cubic structure of Ice VII. In conclusion, these results suggest that ice-phases formed in a solute-rich environment (or by reaction with other phases within the interior of an icy body) will most likely have a greater molar density than ice formed from pure H2O.
Aarestad B.
Frank Michael R.
Maglio S.
Prakapenka Vitali
Scott Henry P.
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