Floating, mixed-valent Fe films have been observed worldwide in wetlands, ferrous iron-rich seeps, and in seasonally reduced soils, but are usually misidentified as oil or biofilms. There has been little characterization or explanation of their formation. Along the Oregon coast such films were found on ephemeral pools where Fe(II)-rich groundwater (~100 μM Fe) has been discharged at the base of Pleistocene sand dunes. Fe(II) oxidized to Fe(III) at the air-water interface to form ~100–300 nm thick films. Analyses indicated that the films contained both Fe(III) and Fe(II) in a ratio of 3:1; Si was the other main cation; OH was the main anion and some C was also identified. The film morphology was flat under optical and electron microscopy with some attached floccules having a string-like morphology. Energy-filtered electron diffraction patterns showed three diffraction rings at 4.5, 2.6 and 1.4 Å in some places and two rings (2.6 and 1.4 Å) in others. Upon further oxidation the films became 2-line ferrihydrite. We are proposing the name ‘schwimmeisen’ for the floating, mixed-valent Fe film.

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