Characterization of natural and synthetic floating, mixed valent Fe-film (Schwimmeisen)

GEOC 180

Zeitel N. Gray, GrayZ@pdx.edu, Georg H. Grathoff, GrathoffG@pdx.edu, and Robert B. Perkins. Geology Department, Portland State University, 1721 SW Broadway, 17 Cramer Hall, Portland, OR 97207
Schwimmeisen has been observed in wetlands, iron-rich seeps, and seasonally reduced soils. It forms when Fe(II)-rich groundwater oxidizes to Fe(III) at the air-water interface. The natural films are 100 to 300 nm thick and contain both Fe(III) and Fe(II) as well as Si. Energy-filtered electron diffraction patterns (EFED) show three diffraction rings at ~4.5, 2.6, and 1.4 .

`We synthesized films using Cornell and Schwertmann's (2000) recipe for Green Rust with and without the addition of humic acid and with surface oxidation using introduced air or photo-oxidation with 7% UVB light over periods of 8 to 600 hours. The few films that formed without organic acids were crystalline under TEM. The films that formed using 0.2% humic acid solutions appeared poorly crystalline as did the natural samples. Many of the synthesized films show similar diffraction rings and contain a similar ratio of Fe(II):Fe(III)as the natural films.