Clay Minerals (1980) 15, 369-382.
F O R M A T I O N A N D P R O P E R T I E S OF FE(II)FE(III)
H Y D R O X Y - C A R B O N A T E A N D ITS POSSIBLE
S I G N I F I C A N C E IN SOIL F O R M A T I O N
R. M. TAYLOR
CSIRO Division of Soils, Private Bag 2, Glen Osmond, South Australia 5064
(Received 19 February 1980)
ABSTRACT: Small amounts of Fe(IlI) induce precipitation of Fe(II) from carbonate solutions
around neutral pH values. The reaction involves the formation of Fe(II)Fe(IlI) hydroxycarbonate, a green, layer-structured compound of the pyroaurite group of minerals. Variations in
the concentration of Fe(II) in the solution phase, and in the form and amounts of the total Fe(III)
in the system, may cause other phases such as magnetite and siderite to form also. Freeze-dried
Fe(II)Fe(III) hydroxy-carbonate undergoes topotactic alteration to a magnetic phase, !~resumably maghemite, when stored under vacuum. The compound is readily oxidized in air in either the
moist or dry state, rapidly becoming yellow or yeUow-brown in colour. Oxidation leads to the
formation of either goethite, lepidocrocite, ferrihydrite or mixtures of these phases, depending on
the mode of oxidation and the impurities present. The similarity between the rapid colour changes
and the oxidation products of the blue-green precipitates in gleyed soil horizons and this
Fe(II)Fe(III)hydroxy-carbonate leads to the speculation that this and related compounds may be
the meta-stable precipitates in such soils, rather than a previously postulated hydro-magnetite
phase. Eh and pH conditions for the formation of Fe(II)Fe(III)hydroxy-carbonate are given, and
are in the range encountered in the soil environment.
It is necessary to utilize systems t h a t a p p r o x i m a t e to those f o u n d in p e d o g e n i c environm e n t s i f an u n d e r s t a n d i n g o f the f o r m a t i o n a n d m u t u a l a s s o c i a t i o n o f soil iron oxides is to
be g a i n e d f r o m synthesis experiments. A d d i t i o n a l l y , it is i m p o r t a n t to be a w a r e o f possible
i n t e r m e d i a t e , m e t a - s t a b l e phases t h a t m a y f o r m in soils, since the t r a n s f o r m a t i o n p r o ducts o f these phases m a y also d e p e n d o n e n v i r o n m e n t a l c o n d i t i o n s . These i n t e r m e d i a t e
phases w o u l d n o t be o b s e r v e d d u r i n g the n o r m a l l y m o r e direct syntheses involving high
o r low p H values a n d elevated t e m p e r a t u r e s .
T a y l o r & S c h w e r t m a n n (1974) s h o w e d t h a t the c o m m o n soil i r o n oxides c o u l d all f o r m
f r o m an F e ( I I ) c h l o r i d e s o l u t i o n at a m b i e n t t e m p e r a t u r e s a n d n e a r neutral p H values, the
p r o d u c t s d e p e n d i n g on the rate o f o x i d a t i o n a n d the F e ( I I ) c o n c e n t r a t i o n in solution. In a
further s i m u l a t i o n o f soil c o n d i t i o n s these two a u t h o r s ( T a y l o r & S c h w e r t m a n n , 1978)
used F e ( I I ) c a r b o n a t e dissolved in CO2-charged water. T h e y were able to ~ynthesize
lepidocrocite, goethite o r ferrihydrite (a p r e c u r s o r o f h e m a t i t e a n d goethite) d e p e n d i n g o n
c o n c e n t r a t i o n a n d rate o f o x i d a t i o n o f the F e ( I I ) in solution, a n d d e m o n s t r a t e d the effects
o f soluble AI h y d r o x y species on the f o r m a t i o n o f these phases. H o w e v e r , m a g h e m i t e , a
further c o m m o n p e d o g e n i c i r o n oxide, was n o t f o r m e d f r o m the c a r b o n a t e system.
A s a n extension to this w o r k , T a y l o r & M c K e n z i e (1980) p r o d u c e d an F e ( I I ) A I ( I I I )
h y d r o x y - c a r b o n a t e u n d e r c o n d i t i o n s t h a t c o u l d be e n c o u n t e r e d in certain soils. This
c o m p o u n d was i s o - s t r u c t u r a l with h y d r o t a l c i t e , a m i n e r a l o f the p y r o a u r i t e g r o u p , b u t
was very u n s t a b l e a n d t r a n s f o r m e d , u n d e r a q u e o u s o x i d a t i o n , to A l - s u b s t i t u t e d goethite.
0009-8558/80/1200-0369502.00
9 1980 The Mineralogical Society
370
R. M. Taylor
This transformation could explain the observed upper limit of 33 m o l ~ A1 substitution
found in natural and synthetic samples of this mineral.
Other members of the pyroaurite group, viz. the Fe(II)Fe(III) hydroxy-chlorides and
-sulphates, commonly referred to as 'green rusts' (Bernal et al., 1959), also transform to
iron oxides. The Fe(II)Fe(III) hydroxy-carbonate is more likely to form in most pedogenic environments than the basic chloride or sulphate. This carbonate is also isostructural with hydrotalcite and was found by Stampfl (1969) as a corrosion product
associated with siderite.
This research not only describes the synthesis of basic Fe(II)Fe(III) carbonate under Eh
and p H regimes encountered in the soil environment, but also demonstrates the unstable
character of this compound and shows that it converts rapidly to other c o m m o n soil iron
oxides. This work is also directed towards the elucidation of the formation conditions of
maghemite from an essentially carbonate system.
EXPERIMENTAL
TECHNIQUES
A similar technique to that described by Taylor & McKenzie (1980) for the formation of
Fe(II)AI(III) hydroxy-carbonate was used. CO2-saturated solutions of Fe(II) carbonate
were made by dissolving either freshly precipitated and washed Fe(OH)2 or FeCO3 in
aqueous suspensions by the addition of solid CO2. Whilst maintaining solid CO2 in the
filter funnel, these suspensions were then filtered, and the clear filtrates collected in
volumetric flasks containing further CO2. The filtrates were then stoppered and stored at
about 2~ until required. On standing, these solutions generally formed a fine yellow
precipitate, but directly before use they were again filtered as before. At this stage a
portion of the filtrate was analysed for Fe(II) by atomic absorption techniques. The
filtered solution was made up to 200 ml with CO2-saturated water, and Fe(III) added
either directly as weighed amounts of soluble ferric nitrate, or as suspensions of washed
ferrihydrite, precipitated from ferric nitrate solutions with either Na2CO3 or NH4OH.
The resultant suspension, initially with an excess of COz present, was then stirred* while
150 ml/min of high purity N2 were passed through until the p H had risen from its initial
value of 5.5-6.5 (depending on the manner in which the Fe(III) was added) to 7.3. In other
preparations, definite amounts of Fe(III) were not added. Instead, the carbonated FeCO3
solutions were stirred, and air was passed above the solutions at a fixed flow rate ( ~ 100
ml/min) until some oxidation, indicated by a yellow coloration or precipitation, had
occurred. The air flow was then stopped, and N2 introduced until the p H rose to 7.3 as
before.
The N2 was then passed above the solution in the closed titration vessel and the stirring
rate reduced.* The pH, and in most cases, Eh variations of the suspension over the next
16-20 h were recorded. The Eh was measured using a calomel-Pt electrode system, and
0.244 V was added to all measured values.
The precipitates formed after 16-20 h were quickly centrifuged in stoppered tubes,
washed once in acetone and then freeze-dried from di-ethyl ether. Part of the dried
product was sealed into evacuated glass ampoules, while the remainder was examined by
X-ray diffraction (as soon as possible after freeze-drying) either by powder photography
* While the pH was being raised to 7.3 in the initial stage, the fastest speed (~ 1000 rpm) of a Radiometer
TTA3 titration unit stirrer (smallblade) was used. The slowestspeed (~ 500 rpm) was used for the next 16-20 h.
The speed and efficiencyof the stirring are important because of their influence on the rate of removal of CO2
from the system.
Formation of Fe ( II) Fe (III) hydro x y-carbonate in soils
371
(5.73 cm diameter camera) or diffraction recording techniques using Co K~ radiation.
The products of aerial or aqueous oxidation were also identified by either of these
techniques. In a few cases, a portion of the green precipitate was oxidized in the solution in
which it had formed without centrifugation or washing. IR spectra were obtained on a
Perkins Elmer 521 spectrophotometer using KBr disks (1.5 mg sample to 200 mg KBr).
RESULTS
The products formed by the interaction of the Fe(III) hydroxy species and the Fe(II)
carbonate solution under N2 varied from a black magnetic compound in the magnetitemaghemite range, to a dark green Fe(II)Fe(III) hydroxy-carbonate. The latter was
identified by XRD which gave spacings analogous to those of a basic Fe(II)Fe(III)
carbonate (Stampfl, 1969) and a green compound reported by McGill et al. (1976a) (see
Table 1), both compounds being derived from the corrosion of cast iron. These compounds are iso-structural with hydrotalcite, the Mg-A1 hydroxy-carbonate member of the
pyroaurite group, and also with the similarly prepared Fe(II)AI(III) hydroxy-carbonate
produced by Taylor & McKenzie (1980). Stampfl (1969) found the average composition
of the basic carbonate to be Fe(II)4Fe(III)2(OH)12CO3, but noted that the Fe(II)/Fe(III)
ratio was variable. Further confirmation that the green compound formed in the present
experiments is a member of the M(II)M(III) hydroxy-carbonate group is given by the
similarity between its IR spectra and that of hydrotalcite (Fig. 1 and Table 2).
TABLE 1. X-ray diffraction spacings o f
Fe(II)Fe(IIl) hydroxy-carbonate
Sample 79-34
d/n*
,~
Intensity
7.554
3.736
VSS
S
2.647
2.536
2"441
2.332
2.203
2.085
1.953
VS
VVW
VW
S
VW
VW
S
VW
W
W
W-M
W-M
W broad
1-739
1.631
1.575
1.542
1-453
Corrosion product of
McGill et al. (1976a)
d/n
A
(hkl)
7.504
3.755
2.718
2-666
003
006
101
012
2'462
2.343
104
015
2.086
1.964
107
018
1.740
1.641
1"584
1"549
1"462
10,10
01,11
110
113
116
V = Very strong; S = strong; W = weak;
M = moderately weak.
* Measured on a 5.73 cm diameter X-ray
powder photograph.
372
R. M. Taylor
100
"•P
yd rotalcite
8O
z~
60
o
~..-
-
~ 40
z
~
20
I
I
I
I
4000
I
3500
r~ J
I
I
I
3000
i
I
I
I
I
i
I
I
2500
FREQUENCY
CM
I
2000
I
i
I
i
I
1500
1~3S~
I
I
I
1000
I
I
I
I
600
1
FIG. 1. Infrared spectra of hydrotalcite and Fe(II)Fe(III) hydroxy-carbonate.
TABLE 2.
Infrared adsorption bands for hydrotalcite and
hydroxy-carbonate
Fe(II)Fe(III) hydroxy-carbonate
Hydrotalcite
Wavenumber
cm- 1
Wavenumber
cm- 1
Assignments
3380
3300
3500
H20 stretch
3000
1625
StrongH-bonded OH
H20 deformation
1380
1370 }
1110 /
1080
1000
Symmetrical C032-
Fe(II)Fe(III)
stretch
/
/
Assignments
H20 stretch
3000
1590
1525
H-bonded OH
OH deformation
1350
SymmetricalCO32- stretch
CO3 stretch
Lattice Mg and A1-OH
Mg,AI-O stretch or
Mg,A1-OH bend
83z~
775 J
Fe-O-H deformation
When the Fe(III) was added as ferric nitrate, the products were dependent on the
amount of Fe(II) initially present as well as on the ratio of Fe(II)/Fe(III) added (Table 3).
For similar Fe(II) contents of about 1.4 mmol, hydroxy-carbonate was the only phase
detected at higher Fe(II)/Fe(III) ratios, while increasing amounts of magnetite were
produced as the ratio decreased. Magnetite was not observed when the initial Fe(II)
content was increased to 2.3 mmol, but a trace of siderite appeared at the higher
Fe(II)/Fe(III) ratio. The higher initial Fe(II) content also resulted in higher final p H
values.
This tendency to form siderite at higher initial values of the Fe(II)/Fe(III) ratio is also
suggested from the results of those experiments (79-51, 79-52 and 79-55, Table 3) where
Fe(III) was not initially added, but was formed through partial oxidation of the Fe(II) in
solution. In these cases, the suspensions at p H 7.3 were not as dense as in those
experiments where Fe(III) had been added, so that much higher values of the Fe(II)/
Fe(III) ratio would be expected. In these experiments siderite was the dominant phase.
When the Fe(III) was added as a suspension of freshly precipitated ferrihydrite,
hydroxy-carbonate was produced in much lower amounts and in a poorly crystalline
373
Formation of Fe ( II) Fe (III) hydroxy-carbonate in soils
9.0
pH
7.0
6.O I
I
I
I
I
Eh
(mY)
200
I
SAMPLE No.
79-48
~
0
-100
I
~"
~
79-47
79-51
79 38
"'....~'-..
.... . , ~ _
I
[
I
I
I
PRODUCT FORMED
Fe(ll) Fe(lll) hydroxy-carbonate
....
.......
----
Predominantly magnetite and hydroxy carbonate
Predominantly siderite and hydroxy-carbonate
Hydroxy carbonate and magnetite
...............
..............................
-3o0
....
--
..........
22 ..........
...........
-400
I
I
I
I
I
I
I
I
I
I
I
2
4
6
8
10
12
14
16
18
20
22
TIME (hours)
FIG. 2. V a r i a t i o n in E h a n d p H w i t h time d u r i n g the f o r m a t i o n o f F e ( l l ) F e ( I I I ) h y d r o x y carbonate.
form. Generally it changed rapidly to a dark brown colour, even during the acetone wash.
Magnetite, siderite and goethite were also formed in various preparations (Table 3).
The changes in Eh and pH which occur during the progress of some of these reactions
are shown in Fig. 2. Where the Fe(III) was added as a soluble nitrate, the variations in
these parameters are similar (e.g. samples 79-47 and 79-48), regardless of the difference in
final composition--in these cases predominantly magnetite with some hydroxycarbonate. Where the Fe(III) was obtained from oxidation on the Fe(II) solution (79-51),
there was an initial increase in pH, but subsequent changes were very similar to those
observed in the previous two samples. In sample 79-51 the Eh increased between 4 and
10 h, whereas it decreased continuously in samples 79-47 and 79-48. Where the Fe(III) was
added as ferrihydrite (sample 79-38) the pH went to higher, and the Eh to lower, final
values than with sample 79-47 although the products, a mixture of hydroxy-carbonate
and magnetite, were similar. These variations between the different reactions may be due
to precipitation of components at different stages in the various systems, but no definite
explanation can be given.
In the formation of Fe(II)AI(III) hydroxy-carbonate (Taylor & McKenzie, 1980) the
pH remained almost constant; the tendency towards lower pH values arising from
hydrolysis of Fe(II) being balanced by the increase due to the loss of CO2 from the system.
The increase in pH in the present formation of the Fe(II)Fe(III) hydroxy-carbonate
suggests that the lower solubility of the Fe(III) hydroxy species, compared with that of the
analogous A1 species, results in less Fe(II) hydrolysis.
The colour of freshly prepared hydroxy-carbonate varied from a bright to a dark
green, possibly influenced by the Fe(II)/Fe(III) ratio in the compound. The stability of
colour and structure also varied between the different preparations, again suggesting
some dependence on the actual initial composition. Even samples sealed in evacuated
glass ampoules sometimes showed colour transitions after a few days to yellow-brown.
R. M. Taylor
374
"o
"0
~
9o
~
.~
~a " o
t.,
~9 e.,
0
?
~
o
?
"0
,
+
"=~
=
0
k~
0
tt~
"0
I
I
0
Z
Z
I
o,0
t.z-
o,,
t":-
"7.
~
t~
6.-
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6,,
,.~
6~
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,~
~.
,-y.
eq
o'~
,.-y
o'~
]
-~
9
9
9
,-q
eq
rq
[....,
"o
a)
375
Formation of Fe(II)Fe(III) hydroxy-carbonate in soils
0
c~
ee~
c~
0
?
a
=',"
~
. o .o ~
~
,-~
~
-~
r--- N
o~
o
I
!
oo
o
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.,c:
~'~.~ ~
?
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.-'~~
-o~
,o
","
0
8
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N~o
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g
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R. M. Taylor
376
(003)
SAMPLE 79 50
A
4I~e counts
r second
_••h
z
J
after preparation
~..
l
11
l
12
l
13
I
14
I
15
24 hours
after preparation-stored in air
i
16
I
9 28
I
24
I
25
I
26
I
27
I
28
I
29
I
30
l
31
I
Co Ko Radiation
FIG. 3. Variation in basal spacings and intensities with time during the exposure to air of an
Fe(II)Fe(III) hydroxy-carbonate.
This colour change was accelerated by exposure to air. In many cases the colour change
was superficial, as the X R D pattern showed that the hydroxy-carbonate was still the only
detectable phase present, although the diagnostic (003) basal reflection was reduced in
intensity and shifted to lower values, consistent with the oxidation of Fe(II) to the smaller
Fe(III) cation. The (006) reflection, however, appeared to lose its intensity much more
rapidly (see Fig. 3), and in some cases was not visible, although the basal spacing was still
obvious.
In many cases, especially where the hydroxy-carbonate was present in higher concentrations, the material stored under vacuum developed a degree of ferro-magnetism due to
a solid state transformation to maghemite or magnetite, whereas, if left exposed to air,
this did not occur9 In some samples the transformation occurred within a few days, while
in others the green material remained non-magnetic for about two months and then
rapidly darkened and became magnetic. This comparison between the alteration under
vacuum and in air is shown in Tables 3 and 4. Although the Fe(II)/Fe(III) ratio in the
magnetic material was not determined, the term magnetite was used in these tables to
denote a phase within the magnetite-maghemite range9 Samples 79-53 and 79-54, containing siderite, magnetite and hydroxy-carbonate, were heated at 105~ for 1 and 24 h
respectively9 In both cases the colour changed to red-brown and the X R D spacings of the
magnetic component shifted to lower values9 Both these factors indicate that oxidation
towards the composition of the maghemite end member of the series has occurred. During
heating, the diffraction lines of the associated hydroxy-carbonate and siderite disappeared (with the possible formation of hematite although this was not positively
identified).
The products formed by oxidation of these preparations were influenced by the nature
377
Formation of Fe ( II) Fe (III) hydro xy-carbonate in soils
o
~
0
0b
~
o
0
._
9~ o
~
~
-
~
. . ~
o
~
o~
~
"0
~_=~ ~ o . ~
o
~
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~=
~:~ "~ "~
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-o ~ ~ ' ~~' - - ~
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.,,~
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e~
o
o
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x
~.~
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'
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R
0
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"0 0
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"0
e~
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=.~ ~.~
R
o ~
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r~
9
<
o
~
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X
R
aZ+
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o
o
a=-I-
~'0
"EL
o
&
r~
1......
&
,It-
378
R. M. Taylor
of the impurities present and the method of oxidation (Table 3). In air the green
freeze-dried precipitate rapidly became brown or yellow-brown, but the structures of the
original compounds present were still detectable by XRD. The siderite impurity remained
unaltered during aerial oxidation whereas the magnetic phase generally showed a shift to
lower diffraction spacings, indicative of an approach to the fully oxidized maghemite,
7-Fe203. With increased time of exposure to air the Fe(II)Fe(III) hydroxy-carbonate was
observed to alter to ferrihydrite. The (012) hydroxy-carbonate spacing at 2.64 A (see
Table 1) contracts with the oxidation of Fe(II) to Fe(III) and becomes the (110) ferrihydrite spacing, as indexed by Chukhrov et al. (1973), with a value within the range
2.50-2.56 A. This contraction is linearly related to the simultaneous contraction in the
basal (003) spacing (see Fig. 4) suggesting that the same oxidation rate of Fe(II) to Fe(III)
is involved. Whereas ferrihydrite produced by hydrolysis from solution generally gives a
broad assymetric (110) diffraction peak, structural breakdown following aerial oxidation
of the hydroxy-carbonate gave rise to much sharper line profiles for this peak, suggesting
that the product was more likely to be feroxyhite rather than ferrihydrite (Carlson &
Schwertmann, 1980).
Alteration products formed during oxidation in water depended on whether the
precipitate had been dried and, to some extent, whether an undried fresh precipitate had
been washed free of the original solution in which it had formed (Table 4). Because the
magnetic phase would be essentially unaltered during the aerial or aqueous oxidation
techniques used, samples that were initially predominantly magnetite are not included in
Table 4. Siderite impurities generally disappeared during aqueous oxidation, and
appeared to aid the transformation of the hydroxy-carbonate to goethite. Samples 79-52
and 79-55 (Table 4), which contained dominant amounts of siderite, also completely
transformed to goethite, whereas 79-50, containing no siderite, still had residual hydroxycarbonate even after a longer period of agitation in water. This effect is possibly due to the
faster dissolution of the hydroxy-carbonate and ferrihydrite (formed from the oxidation
of the hydroxy-carbonate) in the presence of Fe(II) ions provided by the relatively soluble
7.5
SAMPLE79 50 (SeeTable2)
within 1 hour of freeze-drying
7.4
doo3
~
hoursin air
7.3
~
7.2
2.52
i
I
2.54
hours in air
i
I
2.56
i
I
2.58
i
I
2.60
d012
FIG. 4. Variation in the X-ray diffractionspacings of the (003) and (012) reflectionsduring
exposureto air of an Fe(II)Fe(III)hydroxy-carbonate.
Formation of Fe ( II) Fe (III) hydroxy-carbonate in soils
379
siderite. This influence of Fe(II) ions on the dissolution rate of Fe(III) solid phases has
been previously reported by Lieser & Schroeder (1959) who found that these ions
accelerated the solution of anhydrous Fe(III) sulphate, and by Schwertmann & Taylor
(1973) who demonstrated that Fe(II) ions accelerated the transformation via solution of
lepidocrocite to goethite. Additional experiments were carried out to determine whether a
siderite impurity, acting as a source of Fe(II) ions, would accelerate the dissolution of the
hydroxy-carbonate.
The freeze-dried precipitate 79-39, consisting of hydroxy-carbonate with a small
amount of siderite impurity, transformed to goethite with a trace of residual hydroxycarbonate after shaking in water for 26 days. However, the same precipitate in water
containing an added 0.13 mmol of FeCO3 in solution was completely converted to
well-crystalline goethite after the same period of oxidation. Added FeCO3 solution also
increased the relative amounts of goethite to lepidocrocite in some oxidation products.
This supports the observations of Fischer (1972) that Fe(II) ions can overcome the kinetic
hindrance to goethite formation in systems that would otherwise result in lepidocrocite
formation. This effect is shown in sample 79-40. After 20 days shaking in water, the
washed precipitate oxidized to goethite with a trace of lepidocrocite. However, the
addition of 0.13 mmol of FeCO3 solution caused only goethite to form after the same
period of oxidation.
The well-crystalline, freeze-dried hydroxy-carbonate did not appear to decompose
more rapidly at higher pH values. Sample 79-31 gave the same products, ferrihydrite (or
feroxyhite) and residual hydroxy-carbonate, after 15 days in either H20 or 1 : 1 NH4OH
solution.
In all cases where the precipitate was not freeze-dried, the hydroxy-carbonate did not
persist during the aqueous oxidation, but transformed to either goethite or lepidocrocite.
Lepidocrocite formation appeared to be more prevalent if the precipitate was not washed
free of the solution in which it had formed (compare samples 79-34 to 79-40, Table 4).
The results demonstrate that this mixed Fe(II)-Fe(III) basic carbonate can alter under
different environmental conditions to form goethite, lepidocrocite, maghemite, and
ferrihydrite, a possible precursor of hematite. The Fe(II)AI(III) hydroxy-carbonate
described by Taylor & McKenzie (1980), however, transformed during aqueous oxidation to Al-substituted goethite, agreeing with the observations of Taylor & Schwertmann (1978) that A1 can inhibit ~-phase iron oxide formation.
DISCUSSION AND CONCLUSIONS
At least two different processes appear to be involved in these syntheses, depending on the
composition of the added Fe(III) solid phase. In two syntheses (79-53 and 79-54) where
the Fe(III) was added as a suspension of rapidly formed ferrihydrite, the suspensions
became brown-black and magnetic within two hours. Moreover, the pH dropped to lower
values than in the reactions where Fe(III) was added as a soluble nitrate. One possibility
to account for this pH variation is that Fe(II) hydroxy species are rapidly adsorbed from
solution by the solid phase ferrihydrite, thus causing further hydrolysis of Fe(II) and a
consequent drop in pH. This adsorption of Fe(II) hydroxy species by ferrihydrite possibly
leads to a different precursor phase than in the soluble ferric nitrate systems. In these latter
systems, the brownish-black magnetic phase was not encountered within two hours,
although compounds in the magnetite-maghemite series were sometimes produced later.
380
R. M . Taylor
Instead, the suspensions of hydrolysed Fe(III) in the Fe(II) carbonate solution became
dark green, and, at this stage, the dominant reaction between the Fe(II) and Fe(III)
species may be in the solution phase. According to Taylor (1973), the formation of such
mixed M(II)M(III) hydroxy salts proceeds quite readily at near neutral pH values.
The observations in the present experiments provide an alternative explanation for the
results of Ponnamperuma et al. (1967), who proposed the existence of a green hydromagnetite or ferrous-ferric hydroxide phase in soils. The presence of this compound was
postulated on the basis ofEh, pH and Fe(II) concentrations in flooded soils, but it has not
been isolated or synthesized. The presently formed green Fe(II)Fe(III) hydroxycarbonate or the green Fe(II)AI(III) compound described by Taylor & McKenzie (1980),
while not yet isolated from soils, has been formed under natural environments (Stampfl,
1969), and synthesized under conditions approaching those expected in soils. The Eh and
pH fields encountered during the formation of the Fe(II)Fe(III) hydroxy-carbonate,
siderite and maghemite in these present experiments fall within the range of natural
aqueous environments and geologically important organisms reported by Baas Becking
et al. (1960). The formation of Fe(III) by aerial oxidation at the carbonate solution-air
interface approximates to the conditions at a watertable surface in soils. Moreover, these
compounds lead to the formation of all common soil iron oxides or their precursors,
whereas no information is available as to the alteration products of the postulated
meta-stable hydro-magnetite.
McGill et al. (1976a) studied the corrosion of cast iron in carbonate solutions and
observed epitaxial growths of magnetite on the basal faces of a 'green rust', a compound
shown to be isostructural with pyroaurite and analogous to the basic Fe(II)Fe(III)
carbonate of Stampfl (1969). Mackay (1976) suggested that the magnetite obtained by
McGill and co-workers might have formed from a topotactic transformation of the green
rust, as was previously reported to occur with the chloride 'green rust I' (Bernal et al.,
1969). McGill et al. (1976b), however, refute this and state that the magnetite and green
rust phases precipitate independently from solution during the corrosion process. The
observations made during the present experiments complement both these ideas. For
example, the solid state transformation of the Fe(II)Fe(III) hydroxy-carbonate in vacuo
to magnetite would tend to support Mackay's argument, whereas the co-formation of
magnetite and hydroxy-carbonate in syntheses in which the Fe(III) was added as a nitrate
in higher concentrations relative to the Fe(II) might explain the epitaxial formation of the
two phases in McGill et al. (1976a) corrosion studies.
The Fe(II)Fe(III) hydroxy-carbonate is considered to form a solid series with the
similarly produced iso-structural Fe(II)AI(III) hydroxy-carbonate described by Taylor &
McKenzie (1980). The blue-green colour of the series, the conditions under which it is
formed, the rapidity of its colour change on exposure to air and the nature of the products
of its oxidation suggest very strongly that these compounds might be responsible for the
blue-green precipitates in gley soils which are formed under similar conditions, oxidize
rapidly in air to a yellow-brown colour and give rise to the soil iron oxides described.
Additionally at lower concentrations, or in soils which possess localized environments of
temporary oxygen depletion, these compounds may also exist as meta-stable precursors
of the common soil iron oxides without imparting their distinctive green coloration. The
ability to form maghemite from the carbonate system around neutral pH values (Table 3)
is a further approach to pedogenic conditions, and advances the initial work of Taylor &
Schwertmann (1974).
Formation o f Fe ( I I ) Fe ( I I I ) hydro xy-carbonate in soils
381
ACKNOWLEDGMENTS
The IR spectroscopy was carried out by Mr L. J. Janik of the CSIRO Division of Soils, Adelaide, South
Australia. The author also acknowledges the helpful discussions with Dr M. Raupach and Mr R. M. McKenzie,
also of this Division.
REFERENCES
BAASBECKINGL.G.M., KAPLANI.R. & MOORED. (1960) Limits of the natural environment in terms ofpH and
oxidation-reduction potential. J. Geol. 68, 243-284.
BERNALJ.D., DASGUPTAD.R. & MACKAYA.L. (1959) The oxides and hydroxides of iron and their structural
interrelationships. Clay Miner. Bull. 4, 15-30.
CARLSONL. & SCnW~TMANN U. (1980) Natural occurrence of feroxyhite. Clays Clay Miner. 28, 272-280.
CHUKI-mOV F.V., ZVYAGIN B.B., ERMILOVAL.P. & GORSnKOV A.1. (1973). New data on iron oxides in 'the
weathering zone. Proc. Int. Clay Conf. Madrid, 1,397-404.
FISCHER W.R. (1972) Die Wirkung yon Zweivertigen Eisen auf Aufl6sung und Umwandlung yon Eisen(III)hydroxiden. Pp. 37-44 in: Pseudogleyand Gley. Trans. Int. Soil Sci. Soc. Comm. V and VL Stuttgart, Verlag
Chemie, Weinheim.
LIESERK.H. & SCHROEDERH. (1959) Kinetics of solution of anhydrous Fe(III) sulphate in solutions containing
Fe(II) ions. Z. Elektrochem. 64, 252-257.
MACKAY A.L. (1976) Green Rust: A pyroaurite type structure. Comments. Nature 263, 353.
MCGILL I.R., MCENANEY B. & SMITH D.C. (1976a) Green Rust: A pyroaurite type structure. Nature 259,
200-20 I.
MCGILL I.R., MCENAN~Y B. &SMITH D.C. (1976b) Green Rust: A pyroaurite type structure. Reply to
comments. Nature 263, 353-354.
PONNAMPERUMAF.N., TIANCOESTRELLAM. & TERESITAL. (1967) Redox equilibria in flooded soils: 1. The iron
hydroxide systems. Soil Sci. 103, 374-382.
STAMPFL P.P. (1969) Ein basisches Eisen-II-III-Karbonat irn Rost. Corros. Sci. 9, 185-187.
SCHWERTMANN U. & TAYLOR R.M. (1973) The transformation of lepidocrocite to goethite. Proc. Int. Clay.
Conf. Madrid, 1,343-350.
TAYLOR H.F.W. (1973) Crystal structures of some double hydroxide minerals. Miner. Mug. 39, 377-389.
TAYLOR R.M. & McKErqZIE R.M. (1980) The influence of Al on iron oxides. VI. The formation of Fe(II)-Al(III)
hydroxy-chlorides, -sulphates and -carbonates as new members of the pyroaurite group and their possible
significance in soils. Clays Clay Miner. 28, 179-187.
TAYLORR.M. & SCHWERTMANNU. (1974) Maghemite in soils and its origin. II. Maghemite syntheses at ambient
temperatures and pH 7. Clay Miner. 10, 299-310.
TAYLOR R.M. & SCHWERTMANNU. (1978) The influence of A1 on iron oxides. 1. The influence of A1 on Fe oxide
formation from the Fe(II) system. Clays Clay Miner. 26, 373-383.
RI~SUMI~: Des petites quantit6s de Fe (III) provoquent la pr6cipitation de Fe (II) ~i partir de
solution de carbonate pour des valeurs de pH proches de la neutralit& La r6action provoque la
formation de Fe (II) Fe (III) hydroxy-carbonate, un min6ral vert fi structure feuillet6e du groupe
de la pyroaurite. Des variations de la concentration en Fe (II) duns la phase solution et duns la
forme et les quantit6s du Fe (III) total du syst6me, peuvent provoquer la formation d'autres
phases telles que la magn6tite et la sid6rite.
KURZREFERAT: Geringe Mengen von Fe(III) bewirken die Ausf'~illung yon Fe(lI) aus Karbonatl6sungen im neutralen pH-Bereich. Diese Reaktion bringt ebenso die Bildung yon Fe(II)Fe(III) Hydroxy-Karbonat mit sich, einer grfinen Verbindung mit Schichtgitterstruktur aus der
Pyroaurit-Gruppe. Ver~inderungen der Fe(II)-Konzentration in der L6sung, ebenso wie die Form
und die Menge des gesamten Fe(III) im System, k6nnen zur Bildung anderer Phasen, wie
Magnetit und Siderit fiihren.
Unter Vakuum wird gefriergetrocknetes Fe(II) Fe(III) Hydroxy-Karbonat topotaktisch zu
einer magnetischen Phase, vermutlich Meghemit, umgewandelt. Die feuchte oder auch trockene
Verbindung oxidiert sehr leicht an Luft und wird schnell gelb oder gelbbraun. Durch Oxidation
kommt es zur Bildung yon entweder Goethit, Lepidokrokit und Ferrihydrit oder zu einem
382
R. M. Taylor
Gemisch dieser Phasen. Das h/ingt von dem Grad der Oxidation und von vorhandenen Verunreinigungen ab. Der schnelle Farbumschlag und die Oxidationsprodukte von blaugr/inen Ausf'~illungen in Bodenhorizonten sind/ihnlich den Reaktionen, die oben f/Jr das Fe(II)Fe(III)-HydroxyK a r b o n a t beschrieben wurden. Das fiihrt zu der Ansicht, da6 diese und verwandte Verbindungen
eher metastabile Ausf'~illungen in solchen B6den sind als, wie vorl/iufig postuliert, Hydromagnetit
Phasen.
Es werden die EH und pH-Bedingungen f/ir die Bildung von Fe(II)Fe(III) HydroxidKarbonaten angegeben. Sie liegen im gleichen Bereich, wie sie fiir B6den a n g e n o m m e n werden.
R E S U M E N : Cantidades pequefias de Fe(III) inducen la precipitaci6n de Fe(II) de las soluciones
de carbonatos con valores pH de alrededor del punto neutro. La reacci6n provoca la formaci6n
de hidroxicarbonato Fe(II)Fe(III), un compuesto verde con estructura de capas del grupo de
minerales designado piroauritas. Las variaciones en la concentraci6n de Fe(II) en la fase de
soluci6n, y e n la forma y las cantidades del total de Fe(III) en el sistema pueden dar lugar a que
se formen tambi6n otras fases como la magnetita y la siderita.
E1 hidroxicarbonato Fe(II)Fe(III) liofilizado sufre alteraci6n topot/tctica a u n a fase magn&ica,
que es de suponer que sea maghemita, cuando se almacena en u n vacio. El compuesto se oxida
f~cilmente al aire en el estado hflmedo o seco, y se vuelve r~ipidamente amarillo o marr6n
amarillento. La oxidaci6n conduce a la formaci6n de geotita, lepidocrocita, ferrihidrita o mezclas
de estas fases, dependiendo del m o d o de oxidaci6n y de las impurezas presentes. La similaridad
entre los r/tpidos cambios de color y los productos de la oxidaci6n de los precipitados verdeazulados en horizontes de suelos arcillosos y este hidroxicarbonato Fe(II)Fe(III) conduce a la
especulaci6n de que 6ste y los compuestos con 61 relacionados pueden ser precipitados metaestables en esos suelos, mils bien que u n a fase de hidromagnetita como se habia postulado
anteriormente.
Se indican las condiciones del Eh y p H para la formaci6n de hidroxicarbonato Fe(II)Fe(III) y
que estfin comprendidas en la g a m a que se encuentra en el ambiente de los suelos.