J. M. Blgham, U. Schwertmann, L. Carlson
Oct 1, 1990
Citations
46
Influential Citations
547
Citations
Quality indicators
Journal
Geochimica et Cosmochimica Acta
Abstract
Abstract A poorly crystallized oxyhydroxysulfate of Fe has been identified as the primary component of ochreous precipitates from sulfate-rich mine waters having pH values in the range of 2.5 to 4.0. The compound is characterized by rapid dissolution in acid (pH 3.0) ammonium oxalate, an Fe/S mole ratio ranging from 5 to 8, a high specific surface area (175–225 m 2 /g), a yellowish brown (9.0–10.0YR) color, a fibrous morphology, and a broad, 8-line X-ray diffraction profile. Analyses of synthetic analogs prepared by hydrolysis of 0.02 M FeCl 3 solutions containing up to 2000 μ/mL SO 4 show the material to have a tunnel structure akin to that of akaganeite (β-FeOOH). Sulfate occurs both as a bridging element between Fe atoms lining adjacent walls of the tunnels and as a specifically adsorbed surface component. Extraction of the tunnel SO 4 destabilizes the structure and causes the compound to transform to goethite (α-FeOOH) under ambient conditions. Mossbauer spectra taken at 4.2 K yield hyperfine fields lower than those of even the most poorly crystallized iron oxides, indicating that SO 4 inhibits magnetic ordering. A tetragonal unit cell with a 0 = 1.065 and c 0 = 0.604 nm is proposed to describe the structure. The corresponding unit cell formula is Fe 16 O 16 (OH) 12 (SO 4 ) 2 but may range to Fe 16 O 16 (OH) 10 (SO 4 ) 3 depending upon the degree of saturation of the tunnel and surface sites with SO 4 . Because of its abundance and high surface reactivity, this compound should play an important role in regulating the solubilities of both major and trace elements in surface waters impacted by acid mine drainage.