Iron compounds in lake sediments

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Iron Compounds in Lake Sediments J. M. D. COEY Groupe des Transitions de Phase, C . N . R . S . ,B.P. 166,38 042. Grenoble, France

D. W. SCHINDLER Freshwater Institute, University of Manitoba, Winnipeg, Manitoba R3T2N6

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F . WEBER Centre de Skdimentologie et Gkochimie de la Surface, 1, rue Blessig, 67. Strasbourg, France Received February 12,1974 Revision accepted for publication June 28,1974

Dried surface sediment from various Canadian lakes was characterized by X-ray diffraction and Mossbauer spectroscopy. At most two quadrupole doublets were found at room temperature, one ferrous, the other largely ferric. Their intensity ratio is related to the crystallinity of the sediments. It is also correlated with oxygen content, and inversely correlated with ferrous iron content of the overlying water. The major iron phase in the sediments is X-ray amorphous ferric hydroxide. Chlorite is the major ferrous mineral. Des sediments superficiels sCchCs provenant de divers lacs canadiens ont CtB caractCrisCs par diffraction des rayons X et par spectroscopie Mossbauer. Deux doublets quadrupolaires, au maximum, apparaissent B la temperature ambiante, I'un ferreux, I'autre essentiellement ferrique. Leur rapport d'intensite est lie au degre de cristallinite des sediments. Le rapport augmente d'ailleurs avec la teneur en oxygene et diminue avec celle du fer bivalent dans I'eau du lac. Le fer des sediments est contenu principalement dans un hydroxyde ferrique, amorphe aux rayons X, et le fer bivalent en majeure partie dans la chlorite.

Introduction Little is known about compounds containing iron in lakes, due mainly to the fact that some of them are formed from solution, and are so poorly crystallized that they cannot readily be identified by conventional techniques. Because of the lack of knowledge of iron mineralogy in lakes, it is nearly impossible to obtain a clear picture of the chemical transformations affecting several important nutrients (Mortimer 1971). The Mossbauer effect offers the prospect of obtaining information about the oxidation state and chemical composiion of iron in lake sediments, iron being the only common element with a suitable isotope. The spectrum is determined by the hyperfine interactions of 57Fe, which depend on the immediate nuclear environments, and particularly on the electronic configuration, bonding and anion coordination of the iron cations. It should be possible to study iron phases specifically, and to characterize particles with dimensions less than 100 A. The amount of iron in the inorganic fraction of lake sediments is usually similar to, or greater than the local crustal abundance (about 5 wtz), and is sufficient for a good Mijssbauer spectrum. The organic fraction is composed of light elements

which do not much scatter the 14.4 keV y-rays involved. There has been considerable investigation of iron in well-crystallized silicate minerals by Mossbauer spectroscopy (Bancroft et al. 1967; Maddock 1972), and the technique has also been successfully applied to the analysis of iron in unknown phases (Bancroft et al. 1968). However, known layer silicates or clay minerals and amorphous phases have .received rather less attention, though there have been some studies of well-characterized samples (Weaver et al. 1967; Taylor et al. 1968). The Mossbauer effect offers particular advantages in this area and is a valuable supplement to the standard methods of determinative mineralogy. Here we present the first study of iron in lake sediments using Mossbauer spectroscopy. We find that, in conjunction with X-ray diffraction, the main ironbearing mineral phases can be identified, and that there are some very suggestive correlations between the valence of iron in the sediments and the chemical state of the overlying water.

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Can. J. Earth Sci., 11,1489-1493 (1974)

Results We have studied sediments from a number of Canadian lakes, all but two of them in the

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TABLE 1 . Results of X-ray diffraction and Mossbauer analyses of lake sediments Quartz

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Char L. L. 161 L. 122 L. 240 L. 304 L. 120

32% 34 23 24 14 9

Plagioclase 9% 28 12 10

Chlorite

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