(1991) 26, 577-582 IDENTIFICATION OF GREEN RUST IN AN

Clay Minerals (1991) 26, 577-582
NOTE
IDENTIFICATION
OF G R E E N R U S T I N A N O C H R E S L U D G E
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 --250 m 2 in size. In addition to iron oxide, the freshly deposited ochre also contains
calcite. The pH of the sludge is 8.35. 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.
9 1991 The MineralogicalSociety
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 t e m p e r a t u r e 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 b r o a d e n e d 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 b r o a d e n e d . 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 c o m p a r e d 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 t e m p e r a t u r e d e p e n d e n c e 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 --2-6 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 F e ( I I ) c o m p o n e n t 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 o r d e r 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
579
Note
i
t
i
9
i
J
i
Green
core
:"
..','~*"~
"
:
r
Oxidized
~
core
-r
80K
9
I
,o
9
,
9
og
t~
"f,
::
i~.
~2
/'~.
t~. ~
.~ - : : .
~ "I,
: :12K
:
12K
~
9l
9~
9
9
.o
~149
9 o9
9
.
9~
9
*9
#
-12
-8
-
0
4
19
-12
Velocity (mm s -1)
-
g
-
0
4
8
12
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 _+12mms-1.
TABLE1. M6ssbauer parameters for the sextet at 12 K. For comparison the
parameters for well-crystallized goethite are also given.
Green core
Oxidized core
Well-crystallized
goethite a
Bhf (T)
e(mms 1)
6(mms-1)
FI,6(mms-t)
49.8
49.6
-0.13
-0.11
0-49
0-49
-0.8
-0.9
50.6
-0.12
0-48
-0.25
B h f = 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).
c o m p a r i s o n of the two spectra shows that following o x i d a t i o n the a b s o r p t i o n line due to
ferrous iron at - 2 . 6 m m s 1 decreases in intensity, a n d is b r o a d e n e d a n d shifted slightly
towards smaller velocities. M o r e o v e r , the r e s o l u t i o n b e t w e e n the a b s o r p t i o n line at
~ 1 . 0 rams -1 a n d the f o u r t h line of the sextet at 1.7 rams -1 is reduced. This indicates that the
ferric ions, f o r m e d b y oxidation of the F e ( I I ) ions, c o n t r i b u t e to a p a r a m a g n e t i c c o m p o n e n t
with the high-velocity line b e t w e e n 1.0 m m s a a n d 1.7 m m s 1.
If green rust is f o r m e d in the c a r b o n a t e - c o n t a i n i n g ochre sludge, it is expected to be a
c a r b o n a t e form, a n d therefore a c o m p a r i s o n with synthetic h y d r o x y c a r b o n a t e green rusts is
relevant. M u r a d & T a y l o r (1984, 1986) have studied the M 6 s s b a u e r spectra of synthetic
580
Note
l
G reen
1
1
core
I
I
"~
e9
9 . ".g
edJb
lid
Oxidized
core
9
e9
i
-6.0
-4.0
-2.0
,
.r
]
0.0
2.0
4.0
6.0
Velocity (mm s -1)
Fro. 3. M 6 s s b a u e r spectra of the green core a n d oxidized core o b t a i n e d 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
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 --2-6 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 m m s - t (at 80 K), which may be compared to line positions of 2.552.60 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 1991.
C. BENDER KOCH
S. MORUP
REFERENCES
ALLMANNR. (1968) The crystal structure of pyroaurite. Acta Cryst. B24, 972-977.
BRINDLEYG.W. & BISHD.L. (1976) Green rust: a pyroaurite type structure. Nature 263, 353.
CU~LERA,H., MANV., CRAr~SnAWT.E. & LONOWORTHG. (1990). A M6ssbauer study of green rust precipitates: I.
Preparation from sulphate solutions. Clay Miner. 25, 289-30l.
HANSeNH.C.B. (1989)Composition, stabilization, and light absorption of Fe(II)Fe(III) bydroxycarbonate("green
rust"). Clay Miner. 24, 663-669.
496
C. Breen
BREaN C., DEAN~ A+T. & FLYNN J.J. (1987) The acidity of trivatent cation-exchanged montmorillonite+
Temperature programmed desorption and infrared studies of pyridine and n-butylamine. Clay Miner. 22,
169-178.
BREEN C. (1991) Thermogravimetric study of the desorption of cyclohexylamine and pyridine from an acid-treated
Wyoming bentonite. Clay Miner. 26, 473M86.
CALVE'rR. & PROSTR. (1971) Cation migration into empty octahedral sites and surface properties of clays, Clays
Clay Miner. 19, 175-186.
DEANE A.T, (1987) Physicochemical studies of clay-organic interactions. PhD thesis, NIHE, Dublin, Eire.
GASSER R.P.H. (1987) An Introduction to Chemisorption and Catalysis by Metals, pp. 66-71. Oxford Science
Publications, Clarendon Press, Oxford.
HOFMANNV. & KLEMENR. (1950) Verlust der Austaush fahigkeit yon Lithiumionen und Bentonit durch Erhitzung.
Z. Anorg. Allg. Chem. 262, 95-99.
KOPPELMANM.H. & DILLARDJ.G. (1977) A study of the adsorption of Ni(II) and Cu(ll) by clay minerals. Clays
Clay Miner. 25, 457M62.
THOMAS J.M. (1982) Sheet silicate intercalates: New agents for unusual chemical conversions. Pp. 56-97 in:
Intercalation Chemistry (M.S. Whittingham & A.J. Jacobson, editors). Academic Press, New York.
TRONCON! F~. & FORZNrT! P. (1985) Experimental criteria for diffusional limitations during temperature
programmed desorption from porous catalysts. J. Catal. 93, 197-200.
VELGHEF., SCHOONHEYDrR.A. & UYTTERHOEVENJ.B. (1977) The co-ordination of hydrated Cu(II)- and Ni(II)-ions
on montmorillonite surface. Clays Clay Miner. 25, 375-380.
WARDJ.W. (1968) Spectroscopic study of the surface of zeolite Y: the adsorption of pyridine. J. Coll. lnter. Sci. 28,
269-277.