NOTE. Clay Minerals (1991) 26,

Transcription

1 Clay Minerals (1991) 26, IDENTIFICATION NOTE OF GREEN RUST IN AN OCHRE SLUDGE The bluish-green compounds designated green rusts are layer-structured hydroxides, isostructural with pyroaurite, but containing ferrous and ferric ions in the brucitic sheet (Allman, 1968; Taylor, 1973; Brindley & Bish, 1976). Synthetic preparations of the compounds react readily with oxygen from the atmosphere, causing a change of both structure and colour. Based on the greenish colours often encountered in wet soils and sediments and the rapidity with which this colour changes on exposure to oxygen, Taylor & McKenzie (1980) argued that green rust may form in natural environments. From studies of the oxidation products of synthetic green rust, Taylor (1980) suggested that the green rusts may be important precursors for different iron oxides in soils, depending on environmental conditions. However, the presence of green rust in a natural environment needs to be demonstrated to substantiate its importance. The high susceptibilty towards oxidation is believed to be the major obstacle for the identification of green rust, and various procedures have been designed and demonstrated to stabilize synthetically prepared green rust at room temperature (Taylor, 1982; Hansen, 1989). Keeping the green rust at temperatures much below room temperature also inhibits the oxidation and permits M6ssbauer spectra to be obtained at these temperatures (Murad & Taylor, 1984, 1986; Cuttler et al., 1990). In this note we report the MOssbauer spectra of the greenish eoloured core of an ochre aggregate (before and after partial oxidation) indicating the presence of green rust formed in a natural soil-like environment. Samples At the waterworks at Thorsbro 20 km SW of Copenhagen, the ochre sludge, formed by oxidation of ferrous ions in the groundwater, is long-term deposited in freely-drained, open basins m 2 in size. In addition to iron oxide, the freshly deposited ochre also contains calcite. The ph of the sludge is Organic matter is added to the sludge in the form of leaves from deciduous trees growing close to the basins, and from willow-herb vegetation growing on the ochre. No accumulation of organic matter was observed, indicating a rapid turn-over of organic matter. At intervals the sludge is transferred to the basin as a suspension, and the consistency of the sludge varies a 10t depending on the water content. The top layer of the sludge in one of the basins consists of rather firm aggregates with diameters between 5 and 20 cm. On breaking some of these aggregates a greenish coloured core surrounded by a reddish brown rind is observed. The relative extent of the core and rind material and the sharp transition between them are clearly shown in the sectioned ochre aggregate in Fig. 1. The green colour of the core gradually vanished over a period of hours. An absorber for the M6ssbauer investigation of the greenish core was prepared by quickly cutting a sample of the core and pressing it into a Plexiglas absorber holder, which was quickly closed by a Plexiglas lid. An absorber of partially oxidized core material was prepared by grinding a sample of the green core and exposing it to ambient air overnight. After this treatment the sample was placed in a Plexiglas absorber holder. Both samples were quickly frozen by dropping them into liquid nitrogen. In the following, the two samples are referred to as the green core and the oxidized core The Mineralogical Society

2 578 Note I 1 5crn FIG. 1. Photograph of a sectioned ochre aggregate obtained after 30 s exposure to ambient air. M~ssbauer spectra were obtained using a conventional constant acceleration spectrometer using an o~-fe foil at room temperature for calibration and as a reference for isomer shifts. Results and discussion The MOssbauer spectra of the green core and the oxidized core, obtained at 80 and 12 K are shown in Fig. 2. Both spectra obtained at 80 K exhibit a magnetically split sextet with asymmetrically broadened lines, and the central parts of the spectra are dominated by a paramagnetic doublet due to ferric ions. Lowering the temperature to 12 K caused an increase in the relative area of the magnetically split component. However, also at this temperature the absorption lines are asymmetrically broadened. The hyperfine parameters of the sextet, calculated from the spectra of the green core and the oxidized core at 12 K (Table 1), indicate that goethite is the dominant iron oxide in both samples. The hyperfine fields are slightly reduced as compared to well-crystallized pure goethite. This feature, as well as the asymmetric broadening of the lines, is most likely caused by poor crystallinity (Murad, 1988). The temperature dependence of the spectra indicate that a major part of the goethite crystallites is very small and exhibits superparamagnetic relation at 80 K (M0rup et al., 1980) and therefore contributes to the central ferric doublet. In addition to the sextet, the spectrum of the green core obtained at 12 K also exhibits weak absorption lines at -0-2 and 2-6 mms 1 indicating the presence of paramagnetic ferrous ions. In the spectra obtained at 80 K, the absorption line at mms -1 is more intense and the low-velocity line of this ferrous component coincides with the ferric doublet contributing to the asymmetry of this doublet. The increased relative area of the Fe(II) doublet at 80 K suggests that part of the Fe(II) component is magnetically split at 12 K. The spectrum of the oxidized core at 80 K exhibits only a weak absorption line at -2-6 mms 1 and the ferric doublet is almost symmetric. In order to obtain a better spectral resolution of the central part of the spectra at 80 K, these spectra were also recorded in a smaller velocity range (Fig. 3). A

3 Note 579 i t i i J Green core 9 :" "..','~*"~ ~ 80K : 9 i Oxidized I core -r r 9,o, 9 og t ~ :: i~. ~2 : :12K "f, /'~. t~. ~.~ -::. ~ "I, 9. 12K 9 l ~ : 9 9 o9 ~149 9 ~ 9 9 ~ 9 *9.o # g Velocity (mm s -1) Velocity (rnm s q) Fl~. 2. M6ssbaner spectra of the green core and the oxidized core obtained at 80 and 12 K at source velocities up to _+12 mms -1. TABLE 1. M6ssbauer parameters for the sextet at 12 K. For comparison the parameters for well-crystallized goethite are also given. Bhf (T) e(mms 1) 6(mms-1) FI,6 (mms-t) Green core Oxidized core Well-crystallized goethite a Bhf = magnetic hyperfine field, e = quadrupole shift, di = isomer shift, FI,6 = full width at half height of lines 1 and 6. a Mcirup et al. (1983). comparison of the two spectra shows that following oxidation the absorption line due to ferrous iron at -2.6 mms 1 decreases in intensity, and is broadened and shifted slightly towards smaller velocities. Moreover, the resolution between the absorption line at ~1.0 rams -1 and the fourth line of the sextet at 1.7 rams -1 is reduced. This indicates that the ferric ions, formed by oxidation of the Fe(II) ions, contribute to a paramagnetic component with the high-velocity line between 1.0 mms a and 1.7 mms 1. If green rust is formed in the carbonate-containing ochre sludge, it is expected to be a carbonate form, and therefore a comparison with synthetic hydroxycarbonate green rusts is relevant. Murad & Taylor (1984, 1986) have studied the M6ssbauer spectra of synthetic

4 580 Note l 1 1 I I G reen core "~ e9 9. ".g edjb lid Oxidized core 9 e9.r i, ] Velocity (mm s -1) Fro. 3. M6ssbauer spectra of the green core and oxidized core obtained at 80 K at source velocities up to rams 1. The line at 2.64 rams 1 is a guide for the change in line position. hydroxycarbonate green rusts and their oxidation products at 120 K and have demonstrated how readily they react with oxygen. The changes in the M6ssbauer spectra upon oxidation were studied for two samples of different composition which showed different behaviour. One sample changed in such a way that the spectrum developed new paramagnetic ferric and ferrous components, whereas the other also developed a magnetically split sextet. These components are due to partially oxidized green rust and iron oxides. For both samples the oxidation resulted in a decrease in the intensity of the ferrous doublets (the spectra were decomposed into two ferric and two ferrous components) and a broadening of the high-velocity line of the ferric components. One of the samples showed a significant shift of the high-velocity line for the ferrous doublet towards smaller velocities. Furthermore, for both samples an increase in the magnetically split part of the low-temperature (liquid helium) spectra following oxidation was reported. It was also found that part of the Fe(II) component became magnetically split at this temperature. The changes in the spectra with temperature and upon partial oxidation are qualitatively identical to those seen in the spectra of the present samples. At 80 K the superparamagnetic ferric doublet overlaps the

5 Note 581 contributions of the paramagnetic ferric and ferrous components of the green core making it difficult to obtain reliable M0ssbauer parameters of the individual components. However, the position of the well-resolved ferrous line at mms -I may be used to compare with results obtained from studies of synthetic samples. For the sample form the green core, this line is positioned at 2.64 mms- t (at 80 K), which may be compared to line positions of mms t (at 120 K) for the most intense ferrous doublet as calculated from the data of Murad & Taylor (1984, 1986). Although these line positions are in good agreement, the line-width of the ferrous component in the green core sample is much higher than that of the most intense component in the synthetic samples (0.47 mms 1 and 0-28 mms -1, respectively). Inasmuch as the sample from the green core is a natural sample, its crystal chemistry may well be more complex (including partial oxidation in its natural state) leading to broadening of the lines. Thus the results indicate that the ferrous component found in the green core is due to green rust. The observation that the Fe(II) component becomes partially magnetically split at 12 K is also in accordance with this conclusion. An X-ray powder diffractogram of the oxidized core revealed only calcite and iron oxides (goethite is dominant, but small amounts of ferrihydrite can not be excluded), indicating that the mineralogy is relatively simple. Samples from other natural environments containing green rust, may, for example, also contain ferrous layer-silicates which may have components with absorption lines at -2.6 mms 1 and which oxidize relatively easily. The presence of such components makes identification of green rust very difficult. However, in contrast to the ferric oxides, which are the final oxidation products of green rusts, most ferric components in layer-silicates are expected to remain paramagnetic at temperatures above 12 K. Another important factor facilitating detection of green rust in the present sample is its relatively high content. From the 80 K spectrum of the green core it is estimated that ferrous iron amounts to -6% of the total iron, and assuming a ferrous to ferric ratio of 1-5 in the green rust, it is estimated that - 10% of the Fe in the sample is present in the green rust. The morphology of the green core, the sharp colour transition, and the absence of ferrous iron in the freshly precipitated ochre indicate that anaerobic transformation of organic matter plays a major role in the reduction of ferric compounds necessary for the formation of green rust. In conclusion, the results presented indicate that a green rust compound had formed by natural processes in the ochre sludge. Laboratory of Applied Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark. Received 11 March 1991; revised 8 April C. BENDER KOCH S. MORUP REFERENCES ALLMANN R. (1968) The crystal structure of pyroaurite. Acta Cryst. B24, BRINDLEY G.W. & BISH D.L. (1976) Green rust: a pyroaurite type structure. Nature 263, 353. CU~LER A,H., MAN V., CRAr~SnAW T.E. & LONOWORTH G. (1990). A M6ssbauer study of green rust precipitates: I. Preparation from sulphate solutions. Clay Miner. 25, l. HANSeN H.C.B. (1989) Composition, stabilization, and light absorption of Fe(II)Fe(III) bydroxycarbonate ("green rust"). Clay Miner. 24,

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