科研之友
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摘要
The ferric oxyhydroxide minerals akaganeite and schwertmannite are associated with acidic environments and iron alteration on Earth and may be present on Mars as well. These minerals have a tunnel structure and are crystallographically related. The extended visible region reflectance spectra of these minerals are characterized by a broad Fe3+ electronic transition centered near 0.92 mu m, a reflectance maximum near 0.73 mu m, and a shoulder near 0.59 mu m. The near-infrared (NIR) reflectance spectra of each of these minerals are dominated by broad overtones and combinations of the H2O vibration features. These occur near 1.44-1.48 and 1.98-2.07 mu m (similar to 6750-6950 and 4830-5210 cm(-1)) in akaganeite spectra, while in schwertmannite spectra they occur at 1.44-1.48 and 1.95-2.00 mu m (similar to 6750-6950 and 5005-5190 cm(-1)). Additional bands due to OH vibrational overtones are found near 1.42 mu m (similar to 7040 cm(-1)) in akaganeite and schwertmannite spectra and due to OH combination bands in akaganeite spectra at 2.46 mu m (4070 cm(-1)) with weaker components at 2.23-2.42 pm (4134-4492 cm(-1)). A strong and broad band is observed near 2.8-3.1 mu m (similar to 3300-3600 cm(-1)) in reflectance and transmittance spectra of akagandite and schwertmannite due to overlapping OH and H2O stretching vibrations. H2O bending vibrations occur near 1620 cm(-1) (similar to 6.17 mu m) in akaganeite spectra and near 1630 cm(-1) (similar to 6.13 mu m) in schwertmannite spectra with additional bands at lower frequencies due to constrained H2O molecules. OH bending vibrations occur near 650 and 850 cm(-1) (similar to 15.4 and 11.8 mu m) in akaganeite spectra and near 700 cm(-1) (similar to 14.3 mu m) in schwertmannite spectra. Sulfate vibrations are observed for schwertmannite as a v(3) triplet at 1118, 1057, and 1038 cm(-1) (similar to 8.9, 9.5, and 9.6 mu m), v(1) at 982 cm(-1) (similar to 10.2 mu m), v(4) near 690 cm(-1) (similar to 14.5 mu m), and v(2) at 608 cm(-1) (-16.5 mu m). Fe-O bonds occur near 410-470 cm(-1) (mu m) for akagandite and schwertmannite. Both minerals readily absorb H2O rmllecules from the environment and adsorb them onto the mineral surfaces and incorporate them into the tunnels. If akaganeite and schwertmannite were present on the surface of Mars they could enable transport of H2O from the near-surface to the atmosphere as the partial pressure of H2O varies diurnally.
- 出版日期2015-4
