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Ferrihydrite samples were collected from a thermal spring and a cold stream in the Landmannalaugar region of Iceland. Chemical and spectroscopic analyses have been performed on the air-dried and fine-grained fractions of these samples. The ferrihydrite from the cold stream is a pure sample, containing small amounts of Ca, P and Si. The ferrihydrite from the thermal spring is a less pure sample, containing larger amounts of amorphous Si and P with some of the Si incorporated in the ferrihydrite structure. The spectral character of these Icelandic ferrihydrites is compared with those of synthetic ferrihydrites and other iron oxide/oxyhydroxide minerals. Ferrihydrite is characterized by a broad Fe3+ excitation band near 10 900 cm−1 (c. 0.92 μm), a strong Fe-O vibrational feature near 475 cm−1 (c. 21 μm), and multiple bands due to H2O and OH. Highly pure ferrihydrite has a pair of spectral bands near 1400 and 1500 cm−1 (c. 7 μm). Natural ferrihydrites frequently exhibit an extra band near 950–1050 cm−1 (c. 10 μm) that is attributed to Si-O bonds. Hydrothermal springs may have been present at one time on Mars in association with volcanic activity. Ferrihydrite formation in such an environment may have contributed to the ferric oxide-rich surface material on Mars.

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