Clay Minerals (1990) 25, 289-301
A M O S S B A U E R S T U D Y OF G R E E N R U S T
P R E C I P I T A T E S : I. P R E P A R A T I O N S F R O M S U L P H A T E
SOLUTIONS
A. H. CUTTLER,
V. MAN,
T. E. CRANSHAW*
AND G . L O N G W O R T H *
Polytechnic South West, Plymouth, Devon PL4 8AA, and *AEA Technology, Harwell, Didcot, Oxfordshire,
OXI10RA, UK
(Received 7 August 1989; revised 28 December 1989)
A B S T R A C T : The preparation of green rusts from sulphate solutions and representative
M6ssbauer spectra are described. As the samples oxidized readily, attention focused on the
M6ssbauer parameters at liquid nitrogen and helium temperatures. The spectra recorded at 77 K
could be fitted satisfactorily with one ferrous iron quadrupole doublet with a separation of
2.93 mms 1 and one ferric iron quadrupole doublet with a separation of 0.45 rams -1. In some
spectra a ferric iron magnetic hyperfine of strength 49.2 T was also apparent. At 4.2 K, the ferrous
iron exhibited a hyperfine splitting with a field of 12.4 T whilst the ferric iron exhibited a hyperfine
splitting with a field of strength 50-4 T. The ratio of ferrous to ferric ions was 2.25 + 0.25 at 77 K and
at 4.2 K, and -1.6 with a large variation at room temperature. The liquid helium spectra did not
always give a good chi-squared fit, the main reason being attributed to relaxation. The line-width of
the ferrous iron site at 77 K is slightly larger than that for iron metal and could be explained by a
variation in the number of near Fe3+ neighbours at different Fee+ sites, consistent with the
assumption that the ferrous iron site is in the hydroxide sheet. The effect of different numbers of Fe2+
and Fe3+ neighbours probably contributed to the increase in line-widths at 4.2 K compared with
those at 77 K. The ferrous iron doublet is marginally different to those of chloride and hydroxycarbonate green rusts and the aluminium analogues.
G r e e n rusts are u n s t a b l e c o m p o u n d s c o n t a i n i n g a m i x t u r e o f ferrous and ferric iron and
w e r e first d e s c r i b e d by K e l l e r (1948) w h o p r o d u c e d a c h l o r i d e and a s u l p h a t e g r e e n rust.
T h e ratio of ferrous to ferric iron was f o u n d to lie b e t w e e n 0.8 and 4-0. A study by B e r n a l et
al. (1959) of iron o x y h y d r o x i d e s , which i n c l u d e d g r e e n rusts, identified two f o r m s of t h e
s u l p h a t e species as well as the c h l o r i d e f o r m . T h e s e g r e e n rusts w e r e p r e p a r e d " b y the
partial o x i d a t i o n of ferrous iron s o l u t i o n s " b u t the m e t h o d was n o t d e s c r i b e d in sufficient
detail to e n a b l e r e p e a t studies to b e m a d e .
M i s a w a et al. (1973, 1974) m a d e a study by ultra-violet s p e c t r o s c o p y a n d c h e m i c a l
analysis of the aerial o x i d a t i o n of n e u t r a l and slightly alkaline ferrous s u l p h a t e solutions.
A n i n t e r m e d i a t e g r e e n c o m p l e x was o b s e r v e d which was s h o w n to b e a p r e c u r s o r of g r e e n
rust II and o n that basis the ferrous to ferric ratio was e s t i m a t e d to b e a b o u t unity.
G a n c e d o et al. (1976) studied the c o r r o s i o n p r o d u c t s of iron in a q u e o u s solutions o f
a m m o n i u m nitrate and f o u n d the r o o m t e m p e r a t u r e M 6 s s b a u e r q u a d r u p o l e d o u b l e t s to
h a v e p a r a m e t e r s similar to t h o s e o f synthetic g r e e n rusts I and II, n a m e l y , 2.0 a n d
2-37 m m s -1, respectively. It was n o t e d that the ferrous to ferric iron ratios w e r e 1.88 and
0.63, respectively. M c G i l l et al. (1976) in a study of the c o r r o s i o n of cast i r o n in c a r b o n a t e
solutions r e p o r t e d the f o r m a t i o n of a g r e e n rust c o m p o u n d which g a v e similar lines to g r e e n
rust I b u t which was structurally different. B r i n d l e y & Bish (1976) p o i n t e d o u t that this was
9 1990 The Mineralogical Society
290
A.H.
Cuttler et al.
likely to be a member of the pyroaurite group (which is based on a mixture of divalent and
trivalent ions), giving rise to a carbonate-hydroxide. Taylor (1973) also proposed that the
green rusts belonged to this class which Allmann (1968) had analysed in detail.
Taylor & MacKenzie (1980) made a study of the analogous ferrous iron-aluminium
compounds. In this study they used oxygen-free conditions and controlled the ratio of
ferrous iron to aluminium enabling quantitative evaluations to be made of the reaction and
its products. In these studies by Taylor & Mackenzie (1980), the sulphate, chloride and
carbonate compounds were prepared and stated to be "relatively stable". Titration data and
X-ray diffraction (XRD) data were given but no M6ssbauer studies were made. This
method appeared suitable, and was adapted, for a pure iron solution (Taylor, 1980). A later
study by Murad & Taylor (1984) of the FeZ+Fe 3+ and FeZ+A13+ forms included M6ssbauer
spectroscopy at room temperature and at 120 K, and two ferrous quadrupole doublets were
observed in both compounds with two ferric doublets in the ferric species but only one ferric
doublet in the aluminium species. The ferrous iron quadrupole doublets were 2.89 and
2-23 mms -1 for the ferrous-ferric species, and 2-89 and 2.48 mms -1 for the ferrous ironaluminium species. A structure for the green rusts was proposed with tentative assignments
of the ferrous doublets to specific sites.
A recent paper by Olowe et al. (1989) described the preparation by aerial oxidation of a
ferrous sulphate-sodium hydroxide mixture with an initial pH of 7.8, the reaction being
stopped when the pH fell to 7.2 (after about 2 h). By measuring the M6ssbauer parameters
at Various temperatures between 77 K and 460 K, they concluded that the spectra could be
fitted using three ferrous doublets and two ferric doublets. The outer two ferrous doublets
with quadrupole splittings of 2.96 mms 1 and 2.90 at 77 K were not resolved. The third
ferrous doublet with line separation 2.63 mms -1 was partially resolved only for the higher
velocity line. The ferric doublets were not resolved either from each other or from the lowvelocity components of the ferrous lines. The outer ferrous doublets were interpreted as
arising from the hydroxide layer and the inner ferrous doublet to "a sulphate environment".
The ferric component has been interpreted as arising from amorphous ferric hydroxide.
The present work is part of a comprehensive study of green rusts synthesized in aqueous
chloride, sulphate, nitrate and carbonate solutions using both ferric and aluminium
hydroxides to precipitate ferrous iron from solution. Most of this work, described by Man
(1987), covered infra-red spectroscopy, XRD, M~ssbauer studies, and surface area and
titration measurements. The similar methods of preparation of the sulphate and chloride
precipitates are described in detail, and the M6ssbauer spectra of the sulphate green rusts
are presented and their significance discussed.
METHOD
The method of preparation for the sulphate green rusts was essentially the same as that of
Taylor & Mackenzie (1980) but with the substitution of ferric iron for aluminium, i.e. the
method required the preparation of separate solutions of ferrous and ferric iron in oxygenfree conditions, taking each above pH 7 before mixing. The pH was maintained by the
addition of an alkaline solution of either sodium or potassium hydroxide. The ratio of
ferrous to ferric iron was at least 5 : 1 to ensure that not all ferrous iron would be adsorbed
from the solution according to the range of values given by Keller (1948). Ferrous sulphate
was used as the source of divalent iron and either ferric nitrate or ferric chloride as the
M6ssbauer study o f green rust
291
trivalent iron. Identical parameters were obtained for the products from the different ferric
salts.
The ferric iron was prepared in a conical flask as a 0.08 M solution of 50 ml volume under
oxygen-free conditions. The solution was stirred magnetically and nitrogen or argon
bubbled at a rate of 200 ml per min. The top of the flask was sealed with parafilm and the
temperature monitored. In most experiments the temperature was in the range 22 to 27~
The alkalinity of the solution was increased gradually to p H 7 when the ferric iron had
"gelled" and was dark red in colour. The alkali solutions were 0.1 or 0.01 i sodium or
potassium hydroxide and were introduced through the parafilm using a burette. These
concentrations were lower than used by Taylor & Mackenzie (1980) but were chosen to
reduce the precipitation of ferrous hydroxide in the second and third stages of the
experiment. After the solution had reached p H 7, the burette was removed and the flask
resealed, flushing with nitrogen (argon) to prevent the solution from settling.
The second stage consisted of the preparation, under oxygen-free conditions, of a 200 ml
0.1 M solution of ferrous sulphate in a flask sealed with parafilm and flushed with nitrogen or
argon. In some cases mixing was entirely by nitrogen (argon) flushing, and in a few,
magnetic stirring was also included. No attempt was made to remove any trace
contamination of the nitrogen gas by oxygen. With a flow rate of 200 ml per min the total
consumption of gas was between 24 1 and 48 1 which at the p.p.m, level gave an oxygen
content of the order of 100/~g. It was calculated that this quantity would not give rise to
appreciable oxidation of the ferrous iron and this was supported by evidence from similar
experiments with the analogous ferrous iron-aluminium solutions. The ferrous iron solution
was taken to p H 7 by the addition of alkali as with the ferric iron. In some experiments the
pH was raised rapidly, in others slowly, to permit the solution to come to equilibrium.
When p H 7 was reached, the ferric solution was added causing the p H to fall slightly. The
pH was adjusted to 7-7-2 by the addition of alkali and maintained at the chosen level until
no more alkali was consumed (usually after two or more hours). The concentrations of
ferrous iron or ferric iron were occasionally reduced but never such that the ratio of ferrous
to ferric iron fell below five. Cuttler et al. (1984) have reported such experiments in which
the surface area and the ferrous : ferric ratio of the end product were the parameters most
affected.
When the consumption of alkali had ceased, the solution was transferred under nitrogen
gas to a separating flask which was then stoppered. The precipitate was allowed to settle,
samples were withdrawn into centrifuge tubes, stoppered, and spun to concentrate the
precipitate. A sample was then prepared for M6ssbauer spectroscopy by placing a small
quantity of the "paste" between two polyethylene discs mounted in a perspex holder which
was wrapped with sellotape in both directions to provide a seal against oxidation. This
sample was then placed immediately in liquid nitrogen to reduce the possibility of further
oxidation. At the same time, samples were taken for X R D analysis to confirm the formation
of green rusts. Some samples were stored in liquid nitrogen for analysis at a later time and
for transport to A E A Technology, Harwetl for the liquid helium measurements, as an
earlier sample not treated in this way underwent some degradation on transport.
Most MOssbauer spectra were recorded on an Inotech 5200 multichannel analyser using a
velocity generator similar to that described by Clark et al. (1967). The measurements at
liquid helium temperature and a few at 77 K and 293 K were recorded at A E A Technology,
Harwell. The spectra were folded in all instances using 256 channels at Polytechnic South
West, and 512 channels at A E A Technology, Harwell. The source was 57Co in a rhodium
292
A.H.
Cuttler et al.
matrix and isomer shifts have been quoted relative to iron metal at room temperature. The
linearity and stability of both systems was better than one part in three hundred.
DATA ANALYSIS
The M6ssbauer spectra were analysed on a Prime 850 computer using an interactive
graphics programme. The error graph and z-squared values were used as the indicators for
the fits. Spectra recorded at 77 K and 293 K presented no problems in analysis as the
quadrupole doublets did not significantly overlap each other or the magnetic hyperfine
component for which the parameters could be obtained easily. However, the 4.2 K
spectrum contained overlapping ferrous and ferric spectra, both showing magnetic
hyperfine splitting, and in addition there appeared to be some relaxation of the ferrous
hyperfine field. In view of these uncertainties and difficulties encountered in obtaining
satisfactory x-squared values, the analyses were performed to extract approximate
parameters paying attention to the fitting of the main lines. The problem of line
identification was alleviated by the resemblance of the ferrous iron spectrum to that of
ferrous hydroxide (Miyamoto et al., 1967, or Greenwood & Gibb, 1971). Initial estimates of
the ferrous iron parameters were made following the procedure outlined by van Dongen
Torman et al. (1975). More exact solutions were obtained b3~ stripping the ferric iron
components using distribution functions followed by interactive fitting of the ferrous iron
hyperfine spectrum using calculated line positions from solutions of the energy Hamiltonian
close to the estimated parameters. The intensities were determined from the paper by
Kundig (1967). The only restriction was on the electric quadrupole component which was
kept at the same value as at 77 K since this value emerged from the initial estimates. To give
a better fit it was necessary to add a ferrous iron quadrupole doublet. Whilst the z-squared
values were acceptable in two of the low-temperature spectra, in the other two they
remained high. Examination of the error graphs indicated no particularly outstanding
features, only a general decrease in transmission. This decrease and the ferrous iron doublet
were attributed to relaxation processes, and more exact refinements were not considered
appropriate in view of the problems associated with the overlap of the spectra.
RESULTS AND DISCUSSION
The M6ssbauer spectra at 293 K, 77 K and 4-2 K of a sample prepared at A E A Technology,
Harwell are shown in Fig. 1, and the averaged parameters from a number of samples are
given in Table 1. Figs. l(a) and l(b) show the presence of a magnetic hyperfine component
in the spectrum at 4-2 K and at 77 K. This magnetic hyperfine component showed
considerable relaxation at 293 K (Fig. lc). The interpretation of this latter spectrum is a
problem due to the oxidation of the sample during measurement, as inferred from a change
both in the colour and in the ferrous:ferric iron ratio. Thequadrupole doublet appeared to
be slightly smaller in magn.itude than that at 77 K but greater than the value of 2.3 mms -1
reported by Gancedo et al. (1976). The spreads in the ferric iron magnetic hyperfine
components at 77 K and at 4.2 K are similar, the lower temperature giving a slightly greater
average value which appears to be similar to the value for goethite (Forsythe et al., 1968)
but larger than that of the initial ferric gel.
The ferrous : ferric iron ratios of the paramagnetic components of a large number of green
rusts lay between 2-0 and 2.5 and in most samples close to the average of 2-25. The
MOssbauer study of green rust
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FxG. 1. MOssbauer spectra of sulphate green rust prepared at AEA Technology, Harwell; recorded
(a) at 4-2 K, (b) at 77 K, and (c) at 293 K,
294
A.H.
Cuttler et al.
TABLE1. Average Mrssbauer parameters for sulphate green rusts recorded at 293 K, 77 K and 4.2 K.
Sample/Temperature
Iron
species
Isomer
shift
mms 1
Quadrupole
splitting Half-width Magnetic
rams 1
mms-1
field T
Dried/293 K
Fe2+
Fe3+
1.16 + 0.10 2.63 + 0-07 0.42+ 0.07
0.35 • 0.05 0.50• 0-15 0.40• 0.10
---
Stored frozen precipitate/77 K
Fe2+
Fe2+
Fe3+
Fe3+
----
Stored frozen precipitate/4.2K
Fe2+
1.26 • 0.05 2.93 • 0.05
1.36 +_0-05 3-30+_0.15
0.45 • 0.05 0.46 • 0.05
0.47 • 0-05 -0.23 •
0.06
1.29 • 0.05 -2.94 •
0.05
1-35 _+0.10 3.20• 0.10
0-47 _+0.10 -0.20 •
0.10
Fe2+
Fe3+
0.32 • 0.06
0.32 • 0.06
0.32 • 0.05
Intensity
2.25 • 0.25
-0.1
1.00
Distribution 49-2• 0.5
Variable
0.60 • 0-10
0.70 • 0.10
Variable
Variable
12.4• 0-3
Distribution 50.4+ 0-5
1.00
measured ratios at room temperature were about 1.6, showing oxidation of the samples
during recording. The ratio of ferrous : ferric intensities at 4-2 K was the same as at 77 K,
confirming that the spectrum had been satisfactorily fitted.
Although there was a slight variation in the ferrous:ferric iron ratio from sample to
sample, there did not appear to be great variation in the quadrupole splittings or the
magnetic hyperfine fields. The similarity to goethite of the magnetic hyperfine field and the
associated quadrupole m o m e n t do not imply that the ferric iron has the same structure as
goethite; the similarity is a consequence of the fact that the layers containing the iron ions in
goethite, other oxyhydroxides and the green rusts have closely related structures with iron
in octahedral coordination. The proposed green rust structure of A l l m a n n (1968) is of
sheets similar to Fe(OH)2 but with a mixture of ferrous and ferric ions, the excess charge
being compensated by S O ] - between the layers. The coupling of the magnetic fields is
through the oxygen or O H bonds and would be expected to be similar to that in ferric iron
oxyhydroxides. The value of the ferrous : ferric ratio was within the range 0-8-4.0 : 1 for
green rust 1 determined by Keller (1948), (also Bernal et al., 1959) but is lower than the
ratio of 3 : 1 obtained by Taylor & Mackenzie (1980) for the analogous ferrous ironaluminium green rusts. It is believed that the lower ratio for the pure iron species resulted
from a different mechanism of formation. This belief arose from the m e a s u r e m e n t of the
alkali consumption which was less than that for the corresponding Fe-A1 species and has
been supported by the verification of charge exchange processes in the formation of the
pure iron green rusts.
It is considered that this structure, containing mixed valence iron ions, is the only one
consistent with the observed ferrous : ferric ratios. The ferrous iron lines at 77 K were fitted
with a single doublet with a line-width only slightly greater than that of metallic iron. The
value of 2.93 mms -1 is in good agreement with the outer doublets recorded by Olowe et al.
(1989) although they noted that these lines were not resolved. A n alternative explanation,
that of a distribution of doublets arising from different n u m b e r s of near Fe 3+ neighbours,
might be a more plausible interpretation of the nature of the ferrous ion sites in the
MOssbauer study o f green rust
295
hydroxide sheet. Such a distribution would appear to have only a small influence on the
crystal field at the nucleus under consideration. There is no evidence for the third ferrous
doublet observed by Olowe et al. (1989). The variation in the nature of the near neighbours
might also affect the magnetic properties of the iron ions. This could account for the
increase in the widths of the ferrous and the ferric absorption lines at 4-2 K compared with
those at 77 K although, for the ferrous component, there appears to be some relaxation of
the magnetic field, which might mask the effect of non-equivalent sites.
The intensity of the magnetic hyperfine component relative to the ferrous iron
component at 77 K varied from sample to sample. Fig. 2(a) and 2(b) shows two samples,
prepared at Polytechnic South West, in which the magnetic hyperfine component is almost
non-existent or very small compared with the spectrum in Fig. l(b). The spectrum in
Fig. 2(c) is from a sample washed in de-oxygenated water and vacuum-dried, and shows a
decrease in the transmission of the paramagnetic ferric iron component relative to that of
the ferrous iron component. It would appear that the oxidation of the ferrous iron has
contributed also to an increase in the magnetically ordered ferric component. The
variability in the intensity of this magnetic component indicates that it is not essential for the
green rust structure; indeed green rusts prepared at concentrations <0.05 M do not show
such a component. It is believed that the ferric magnetic hyperfine spectrum arises from
partial ordering of the ferric gel before charge exchange processes have redistributed all the
adsorbed ferrous iron throughout the material. The narrow line-widths imply either that
there is a single site for the Fe 2+ ions, or that there is a distribution of sites with differing
numbers of Fe 2+ and Fe 3+ neighbours. If this is the case, then the effects of the near
neighbours must be small. This interpretation would have implications for the interpretation of other iron compounds as well as the interpretation of the spectra of hydroxycarbonate green rusts by Murad & Taylor (1984). Unpublished studies on similar
compounds by the authors indicated a mixed hydroxide/carbonate system. The quadrupole
splittings of sulphate green rusts at 77 K are consistently larger than those of green rusts
prepared using solutions of ferrous chloride or ferrous carbonate. The difference is small
(0.10 mms 1 or less) and might be used, with care, to distinguish between such materials.
There is some evidence for a frozen ferrous sulphate solution. Occasionally, there is a
slight asymmetry on the high-energy side of the ferrous iron line at 2.4 m m s - 1 as can be seen
in Fig. 2(a) and 2(b) although these are exceptions and account for less than 5% of the
ferrous iron. The quadrupole splitting for frozen solutions is 3.2 mms-1 and the isomer shift
is 1.36 mms -1 (Deszi et al., 1965). The presence of this doublet is probably due to a short
spin and the use of the unwashed precipitate to reduce oxidation.
Fig. 3 shows three M6ssbauer spectra at 4.2 K. The spectrum in Fig. 3(a) was prepared at
A E A Technology, Harwell and gave a magnetic hyperfine field for the ferrous iron of
12.2 T. The spectra in Fig. 3(b) and 3(c) correspond to those in Fig. 2(a) and 2(b) and gave a
magnetic hyperfine field for the ferrous iron of 12-5 T. The spectrum of the sample prepared
at A E A Technology was recorded immediately after preparation, whereas all other samples
were prepared and stored at 77 K for a week or more before the M6ssbauer spectra were
measured. It is thought that this difference is not significant as it is within experimental error
although it is just possible that it resulted from ageing processes in the sample. Table 2 lists
the relevant data concerning the magnetic field, the quadrupole field, the angles and the
asymmetry parameter. These confirm that the ferrous iron is in an environment similar to
that of ferrous hydroxide, Fe(OH)2 (Miyamoto et al., 1967; Greenwood & Gibb, 1971) and
sheet-silicates (Coey, 1988). The addition of a ferrous doublet with a splitting of 3.2 mms 1
A . H . Cuttler e t al.
296
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FIG. 2. M6ssbauer spectra of sulphate green rusts recorded at 77 K, with initial ferrous-ferric iron
ratios of (a) 40 : 1, (b) 20 : 1 and (c) a sample of dried material from (b).
MOssbauer study
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FIG. 3. MOssbauerspectra of sulphate green rusts recorded at 4-2 K, with initial ferrous-ferric iron
ratios of (a) 4:1, (b) 20 : I and (c) 40 : 1,
--
A . H . Cuttler et al.
298
TABLE2. M6ssbauer parameters of the ferrous component at 4.2 K for a fresh sample, and for samples stored at
77 K.
Sample
Freshly prepared
Stored frozen precipitate
Isomer
shift
mms-1
Quadrupole
splitting
mms-1
Halfwidth
mms-1
1.28
1.35
1.29
1.38
-2.94
3.20
-2.94
3.40
0.60
0-70
0-54
0.60
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field
parameter
T
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12-5
0.20
90
90
Intensity
%
86
14
75
25
* Not determined.
and an isomer shift of 1.25 mms -1 was necessary to fit most spectra. Whilst these values
were similar to those of frozen sulphate solutions (Deszi et al., 1965), this did not a p p e a r to
be the explanation as in most samples no such doublet was observed at 77 K. This doublet
has been interpreted as arising from a fast relaxation process associated with the ferrous
iron magnetic hyperfine field. This relaxation would give values similar to the averages of
the first four and the last four lines in Table 3 and has been shown in Fig. 3 with the other
components. In some spectra where the value of x-squared r e m a i n e d high, it a p p e a r e d that
there was a broad, featureless, residual spectrum which was attributed to magnetic
relaxation for which no m o d e l was available.
Comparison of the magnetic hyperfine fields and line-widths for the green rust spectra in
Fig. 3 with that of ferric gel, Fig. 4, leads to the conclusion that the FeZ+-Fe 3+ interaction
orders the ferric ion sites resulting in a larger field and n a r r o w e r line-widths. It is believed
that the ordering process results from charge transfer following adsorption of ferrous ions
on to the ferric gel. There appears to be no evidence for the ferrous iron site in the
hydroxide sheet with a quadrupole splitting of c. 2.3 rams -1 p r o p o s e d by M u r a d & Taylor
(1984). The line-widths for the ferrous iron magnetic field components and the distributions
for the ferric iron magnetic hyperfine fields p r o b a b l y reflect local variations in the
distributions of the two iron species.
COMMENTS
AND CONCLUSIONS
The sulphate green rusts form well-ordered crystalline strcutures with characteristic
M/)ssbauer spectra at both 77 K and 4.2 K. The spectra at r o o m t e m p e r a t u r e are similar to
those at 77 K but usually show absorption by oxidised c o m p o n e n t s as well as those of the
green rusts. The narrow line-widths observed at 77 K (0-32 mms -1) imply either a single
ferrous iron site, or a distribution of sites with different numbers of F e 3+ neighbours which
are not very sensitive to the nature of the ferrous ion/ferric ion distributions. The larger linewidths observed at 4.2 K for the magnetic hyperfine components of both the ferric and the
ferrous ions are attributed to a combination of relaxation and differences in the local
magnetic fields resulting from variations in the ferrous/ferric ion distributions. The
ferrous : ferric ion ratio for the green rusts varies around a m e a n of 2.25. This figure does not
include any ferric iron magnetic hyperfine c o m p o n e n t observable in the 77 K M6ssbauer
spectra as this c o m p o n e n t is variable or even absent from some samples, particularly those
p r e p a r e d at lower concentrations of ferrous and ferric iron.
MOssbauer study of green rust
299
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-6
I
I
I
-2
I
I
2
VELOCITY
I
6
MMS-I
FIG. 4. MOssbauerspectrum at 4.2 K of a "ferric gel".
The nature of the interaction between the ferrous and ferric species is not known except
that the mechanism appears to be one of adsorption followed by charge exchange. This
would explain the approximately constant ratio of ferrous to ferric ions in the paramagnetic
phase observed at 77 K. The occasional appearance of a ferric iron magnetic hyperfine field
of variable intensity implies that it is not part of the green rust structure and indicates that
complex mechanisms of formation occur. These may be influenced by differences in
technique e.g., the effect of high alkali concentrations would have been to precipitate some
ferrous hydroxide which would have redissolved as the alkali mixed into the bulk solution.
Where the solutions were magnetically stirred, and for low concentrations, the ferric iron
magnetic hyperfine c o m p o n e n t appeared to be lower, due possibly to faster mixing with a
consequent reduction in precipitation of ferrous hydroxide. Similarly, the rate at which the
TABLE3. Line positionsand intensitiesof the ferrous iron magnetichyperfinecomponent at 4.2 K.
SamplelLinenumber
1
Freshly prepared sample
Position rams-1
-1-090
Intensity %
78
Stored frozen sample
Position mms-1
-1.122
Intensity %
78
2
3
4
5
6
7
8
-0.865
61
0-358
30
0-583
48
1.527
70
2.975
39
3-092
9
4.540
100
-1.062
61
0.371
30
0-432
48
1-313
70
2.807
39
3.084
9
4.577
100
300
A.H.
Cuttler e t al.
f e r r o u s i r o n s o l u t i o n w a s t a k e n t o p H 7 w o u l d h a v e a f f e c t e d t h e q u a n t i t y o f b a s i c salts
f o r m e d . T h e s e s u b t l e t i e s m a k e it i m p o s s i b l e t o d e f i n e t h e p r e c i s e m e c h a n i s m o r
mechanisms of formation.
T h e M 6 s s b a u e r q u a d r u p o l e s p l i t t i n g f o r t h e s u l p h a t e g r e e n r u s t s a p p e a r s to b e slightly
larger than those for similar green rusts prepared from chloride and carbonate solutions.
W h i l s t t h e p a r a m e t e r s a t 77 K m i g h t b e u s e d as a g u i d e to t h e p r e s e n c e o f g r e e n r u s t s , t h e
s i m i l a r i t y o f t h e s e to o t h e r m i n e r a l s c o n t a i n i n g i r o n m i g h t limit t h e i r a p p l i c a b i l i t y . T h e u s e
o f d i f f e r e n c e t e c h n i q u e s for s p e c t r a o f r e d u c e d a n d o x i d i s e d s a m p l e s m i g h t p r o v i d e t h e
necessary information.
ACKNOWLEDGMENTS
The authors wish to thank Mr L. Becker of AEA Technology (formerly AERE), Harwell for his assistance in
running the M6ssbauer equipment and recording the spectra at 4.2 K. Dr Cuttler wishes also to acknowledge the
receipt of a Nuffield Foundation Small Science Grant to cover travel and liquid helium costs.
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