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Occurrence and distribution of iron-bearing minerals in micron and submicron sized sediment fractions

The presence and distribution of iron phases in different micron- and submicron-sized mineral fractions in surface marine sediment from the Neretva Channel (Adriatic Sea) was investigated by Mössbauer spectroscopy in combination with X-ray diffraction and FE-SEM. Prior to fractionation, modified Kaiser’s method was used to remove the organic matter from sediment surfaces. Different size fractions were separated by gravitational settling and confirmed by DLS. The sediment fractions were composed of calcite, quartz, feldspar, and a significant amount of clay minerals, mainly illite, chlorite and smectite. There were considerable differences in the presence of these minerals in different fractions. The share of clay minerals was notably higher in smaller fractions. The Mössbauer spectrums of all samples taken at room temperature consist of two quadropole doublets without traces of magnetic hyperfine structure. The obtained values for ferrous ion (IS: 1.1, QS: 2.7) indicate the presence of iron in clay minerals and/or oxides and oxyhydroxides. According to previous investigations, these results suggest that clay minerals, presumably chlorites, could be the host of ferrous ion. The obtained values for ferric ion (IS: 0.37, QS: 0.6) could be ascribed to clay minerals or ferric oxides and oxyhydroxides associated with them. The Xray diffraction revealed no presence of these solids. However, the absence of their XRD peaks does not indicate that there are not present, since according to this analysis, poorly crystalline phases of these solids cannot be detected. Furthermore, the room temperature Mössbauer spectrum is not sufficient to eliminate the presence of associated nanosized iron oxides and oxyhydroxides, and further analyses are required. Since samples contain various clay minerals that could be possible hosts of iron, the final signal may be a result of more than one component. A gradual decrease of ferrous iron from 34 % in the original sample to 13 % in the smallest fraction was observed, while the Mössbauer parameters remained the same. The observed decrease was continuous throughout size fractions, suggesting little or no effect of the organic matter removal on the ratio of different iron phases present in the sample.

Marine sediment ; iron phases ; FE-SEM ; XRD ; Mossbauer spectroscopy

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