Clay Minerals (1987) 22, 83 92
THE INFLUENCE
OF ALUMINIUM
ON IRON
OXIDES: XIII. PROPERTIES
OF GOETHITES
SYNTHESISED
I N 0-3 M K O H A T 2 5 ~
D. G . S C H U L Z E
AND U. S C H W E R T M A N N *
Agronomy Department, Purdue University, West LaJayette, Indiana 47907, USA, and *Institut J~r Bodenkunde,
Technische Universiti~t Miinchen, 8050 Freising-Weihenstephan, FRG
(Received 20 May 1986; revised 16 September 1986)
A B S T R A C T : Aluminium-substituted
goethites were synthesised in 0.3 MKOH at either 25~or
70~ The 25~ goethites had unit-cell a and c dimensions about 0-1-0.2~ larger than the 70~
goethites of comparable A1 substitution. Unit-cell b dimensions were similar regardless of
synthesis temperature. The 25~ goethites contained larger amounts of A1 in their structures
than the 70~ goethites synthesised at the same initial A1/(A1 + Fe) mole ratio in solution. The
25~ goethites had lower dehydroxylation temperatures and smaller differences between the two
OH-bending bands than comparable 70~ goethites. These differences between the two sets of
goethites are believed to be caused by the presence of more structural defects in the 25~
goethites than in the 70~ goethites. Goethites with the lowest AI substitutions consisted of
crystals with several coherently diffracting domains, while at high A1 substitutions the crystals
were mainly monodomainic. Surface areas of the 25~ goethites varied in a complex manner as a
function of thickness along [100], character of the domains, and physical size.
Al-substituted goethites can be synthesised in a few days by using high O H concentrations
and temperatures of 70~ (Lewis & Schwertmann, 1979a,b). The properties of these goethites
have been studied in considerable detail (Schulze & Schwertmann, 1984). Goethite forms at
much lower temperatures during weathering and soil formation. Not much is known about
synthetic Al-substituted goethites formed at lower temperatures. Goethites have previously
been synthesised at 25~ only by oxidation of mixed Fe(II)-A1 solutions. These goethites
usually have very small crystals and more structural defects than goethites formed from
Fe(III) at higher temperatures and OH concentrations (Fey & Dixon, 1981; G o o d m a n &
Lewis, 1981; Schulze & Schwertmann, 1984).
The purpose of this study was to produce and characterize somewhat better crystalline AIgoethites synthesised by transforming freshly precipitated ferrihydrite in an alkaline
aluminate solution (0.3 M K O H ) at 25~ The goethites are compared to Al-goethites made by
the same procedure but at 70~
MATERIALS
AND
METHODS
A n aluminate solution was prepared by slowly pouring 1.51 of 0.5 N AI(NO3) 3 into 900 ml of 5
M K O H while constantly stirring. The required amounts of this aluminate solution plus
additional 5 M K O H (Table 1) were poured into 5 1 polypropylene bottles. Then, for each
bottle, 225 ml of 1 M Fe(NO3) 3 was quickly added, the bottle was swirled to thoroughly mix
9 1987 The Mineralogical Society
84
D. G. Schulze and U. Schwertmann
TABLE1. Volumesof aluminate and 5 MKOH used
for each sample (ml).
Sample
Aluminate
5 MKOH
35/o
35/1
35/2
35/3
35/4
35/5
35/6
35/7
35/8
35/9
0
22.5
45
67-5
112-5
180
270
337.5
450
675
405
402
399
397
391
383
371
363
349
321
the solutions, then quickly filled to 4-5 1 with deionized H20 from another large bottle, and
then stirred with a large plastic spoon. The elapsed time from the beginning of the Fe(NO3)3
addition to the achievement of the final 4.5 1 volume was < 30 s. The solutions were then
placed in an oven at 70~ for 14 days (Series 34) or in a constant-temperature room at 25~ for
1310 days (Series 35). The solutions at 70~ were stirred once each day and those at 25~ were
stirred at irregular intervals every few weeks. At the end of the synthesis period the samples
were washed with deionized H20 and then dried at 50~
Total Fe (Fet) was determined after dissolving the sample in concentrated HCI. Oxalateextractable Fe (Feo) was determined after treatment with NH4-oxalate at pH 3 in the dark for
2 h (Schwertmann, 1964). Iron was determined by the e,e'-sulfosalicylic acid procedure
(Koutler-Anderson, 1953).
Samples for X-ray diffraction were mixed with either 25~o (Series 35) or 50~o (w/w)
c~-A1203 (Series 34) as an internal standard. Instrument settings and preparation of the
powder mounts were as described by Schulze (1984). The Series 35 samples were scanned at a
rate of 1/2~ 20/min and the patterns recorded on a strip chart. The Series 34 samples were
step-scanned and a curve-fitting procedure used to determine line positions (Schulze, 1984)
which, for both series were measured relative to the positions of nearby c~-A1203 lines. A
check using three samples showed that both methods gave essentially the same results.
Widths at half-height (WHH) were corrected for instrumental broadening by a folding
procedure (H. Stanjek, Institut ftir Bodenkunde, Technische Universit~it Mfinchen,
unpublished). Calcined hematite showing no particle-size broadening was used to obtain the
instrument broadening profile. Mean crystallite dimensions (MCD) of both series were
calculated from the 110 and l 1l lines using the Scherrer formula (Klug & Alexander, 1974).
Goethite unit-cell dimensions were calculated from the 110, 130 and 111 line positions
(Schulze, 1984). Hematite a dimensions were calculated from the 110 and 300 lines.
The procedures used for infrared absorption spectroscopy (IR), differential thermal
analysis (DTA), transmission electron microscopy (TEM), and specific surface area
determinations have been described by Schulze & Schwertmann (1984).
Properties of synthetic goethites
85
RESULTS AND DISCUSSION
Samples 35/0 through 35/5 of the 25~ series had an Feo/Fet ratio of < 0.005 at the end of the
1310 day period (Table 2), indicating essentially complete conversion to well-crystallized
phases. Samples 35/6 through 35/9, however, still contained appreciable quantities of
oxalate-soluble material. The greater the A1 addition, the greater was the Feo/Fet ratio,
reflecting the increasing inhibition by A1 of the transformation of ferrihydrite to goethite or
hematite. The same trend held for Feo/Fet of the 70~ series but the values at higher A1
substitution were much lower, indicating more complete conversion.
More AI was incorporated into the goethite structure at 25~ than at 70~ at the same
AI/(A1 + Fe) mole ratio in the initial solution. This indicates that the goethite crystals can
accommodate more AI if they grow slowly than if they grow quickly. Substitutions of > 30
mole~ A1 are common under natural conditions, perhaps in part due to the slow growth of the
crystals.
Schulze (1984) showed that the unit-cell c dimension of 81 synthetic goethites decreased
linearly with increasing AI substitution. The relationship Al(mole~) = 1730 - 572.0c
(dashed line in Fig. 1) was found to predict A1 substitution to within + 2.6 mole~ at the 95~o
confidence level. This equation predicts A1 substitution of the goethites in this study within
the stated accuracy. A closer examination of the cell dimensions, however, shows that the c
dimensions are slightly different depending on synthesis temperature (Fig. 1). The c
dimensions of the goethites synthesised at 25~ are slightly above the line given by Schulze
(1984), whereas the goethites synthesised at 70~ fall slightly below the line. Linear
3. 025 !
"~',,~ -- ""',~
3.015
"~~'~7,
25' goethites
C=3.0265-0.00t8t5 A1
r=-O.998
70' goethites
- ~-,,~ ~"-~
c=_3.o2 -o.oo 2 AZ
n=7, r=-O .993
"~.-.
3.005
Schu lze" --
2.9950
"
A1
Ib
.
.
.
.
(mole %)
FIG. 1. Unit-cell c dimensions vs. AI substitution.
.
{5
"
"
D. G. Schu~e and U. Schwertmann
86
I
~
~
~
~
~
'
~
!=
o
~.~.
~o.
I,..~(~.~l~.r~.~l
o. o. ~ m .
r~
~.
~..
~2
.o
0
e~
',.7
~
~
. . . . .
,-._,
~
~
" ~
2.-,
~
~
9
9
~
.o
e-i
,.o
~1
~'~Z
~,~
"~
~
.o
~.
.
Properties of synthetic goethites
87
0.04'
0.03'
25'
goethi
W
w
"-" 0 . 0 2 '
r
~a
0.0t
oo . . . . .
1'o
ab . . . . .
A1
(mole
3'o
%)
FIG. 2. Aa vs. AI substitution.
regression analysis shows that the intercepts of the two lines differ by 0.0037 A. This
difference is significant at the 95% level. The slopes, however, are not significantly different.
Thus, some of the uncertainty in estimating AI substitution from the c dimension using the
relationship given by Schulze (1984) is due to small variations in the c dimension itself and
not due to measurement error, If Schulze's (1984) regression line is used, the c dimension
over-estimates AI substitution by about 1 mole% for the 25~ goethites, while it underestimates it by about the same amount for the 70~ goethites within the range of 0 to ~ 10
mole% A1.
Chemically determined A1 is used in subsequent discussions for the samples which
contained only goethite. Samples 34/7, 34/8, 34/9, 35/7, 35/8, and 35/9 contained hematite.
The A1 substitution of these samples was determined by extrapolation of the two linear
relationships given in Fig. 1.
The unit-cell a dimensions (Table 2) were 0.006-0.014 A larger for the samples synthesised
at 25~ than for those synthesised at 70~ The larger a dimensions may indicate that the
samples synthesised at 25~ contain more structural defects than those synthesised at 70~
(Schulze, 1984). Schulze & Schwertmann (1984) proposed Aa, the deviation of the observed a
dimension from the a dimension predicted by the Vegard Rule, as a measure of structural
defects. Aa is defined by the relationship Aa = aob~ -- (4.608 -- 0.00212A1), where ao~ is the
observed a dimension, and AI is AI substitution in mole%. The value of 4-608 A is for a wellcrystallized goethite without AI substitution (ASTM card 29-713(starred)). This equation
differs slightly from the equation given by Schulze & Schwertmann (1984) because the
previous ASTM card for goethite, 17-536(starred), has been replaced by card 29-713(starred).
88
D. G. Schulze and U. Schwertmann
-200
I
50.
150
,~
I
ID1
E
"E
40"
r-
I1D
lO0
0
0
=K
~
L
ID
U
'4--
I
20
f...
50
Surface
Area /
Goethite
0
l'0
Goethite
+
Hematite
2'0
AI
"
O3
3100
(mole %)
FIG. 3. MCD110, MCD111 and surface area vs. AI substitution for the 25~ goethites (Series 35).
Fig. 2 shows that the greater the AI substitution, the greater the value of Aa, suggesting that
there are more structural defects at higher A1 substitutions than at lower ones. Aa is
significantly larger for the 25~ goethites than for the 70~ goethites at the same level of AI
substitution. This is in agreement with the results of Schwertmann et al. (1985) for a synthesis
series in which only temperature was varied. They found that the a dimension was about 0.02
A larger for goethites synthesised between 4 and 30~ than for goethites synthesised between
50 and 80~ The higher energy input during the synthesis procedure apparently reduces
structural defects, resulting in a approaching the ideal value of 4.608 A.
The b dimensions were similar for both synthesis series, with the exception of sample 34/9
(Table 2). Schwertman et al. (1985) also found the b dimensions of goethites to be similar
regardless of synthesis temperature.
The EGME surface area of the 25~ series first decreased from 52 to 26 m 2 g-~ as A1
increased from 0 to 11-6 mole~o, then increased at higher AI substitutions (Fig. 3). The
decrease between 0 and 11-6 mole~ A1 can be explained by (i) an increase in domain size (and
crystal thickness) along [100] and (ii) a decrease in number of domains per crystal. Electron
microscopy shows lath-shaped crystals that have about the same width but become shorter as
AI substitution increases from 0 to 11.6 mole~o (Fig. 4, samples 35/0 to 35/5). Electron
diffraction has shown that lath-shaped goethite crystals tend to orientate with the (100) face
perpendicular to the electron beam (Cornell et al., 1983). Thus, only changes along [010] and
[001] are normally observed by EM; changes in thickness along [100] are not observed. Both
MCD, 10 and MCD~ ~ are, however, strongly dependent on the thickness of the coherently
diffracting domains along [100] and are an indicator of the thickness along [100] not
Properties of synthetic goethites
89
FIG.4. Selected transmission electron micrographs for the 25~ goethites (Series 35).
observable by EM. Schulze & Schwertmann (1984) have shown that for similar acicular
goethites, surface area is inversely related to MCD~, the thickness along [100]. The increase in
MCD~0o and M C D I ~ between 0 and 20 mole~o A1 (Fig. 3, Table 2) is accompanied by a
decrease in surface area, at least up to 11.6 mole~ A1 substitution. Additional surface area
decrease is due to the character of the domains in the crystals. At low A1 substitution each
physical crystal consists of several domains (Fig. 4, sample 35/0) as observed by
Schwertmann (1984), Schulze & Schwertmann (1985) and Schwertmann et al. (1985). These
domains often grow to different lengths along the c axis to give crystals with jagged ends.
Both the jagged ends and the gaps which often occur between adjacent domains (arrows in
Fig. 4) can increase the surface area. The loss of the gaps and jagged ends as A1 substitution
increases (Fig. 4, compare samples 35/0, 35/3 and 35/5) contributes to the decrease in surface
area. Above 11.6 mole~ A1, each crystal consists of one domain, but the crystals become
physically smaller (Fig. 4, sample 35/9) and surface area increases.
The 70~ goethites also show a general increase in MCD with increasing A1 substitution,
except for sample 34/9 (Table 2). The surface area is lower than for the 25~ goethites,
90
D. G. Schulze and U. Schwertmann
120
T
E
(J
110
70*goethite
"1cz)
goethites
I
"r-
100
901
0
I
I'0
2'0
3O
AI (mole %)
FIG. 5. Difference between the two OH-bending bands (~OH and ~,OH) vs. A1 substitution.
particularly at low substitution. There is a weak minimum at about 8 mole~ AI substitution
and higher values for the last two samples.
The OH-stretching band positions (vOH) were approximately 20 wavenumbers higher for
the 25~ goethites than for the 70~ series for samples with < 12 mole~ A1 substitution. This
increase in vOH is consistent with the larger a dimensions. The separation of the two OHbending bands increased with increasing A1 substitution and was consistently lower for the
25~ goethites than for the 70~ goethites, except for sample 34/9 (Fig. 5). Band separation is
determined mainly by the amount of A1 substitution, but it is also influenced to some extent
by structural defects. More structural defects in the 25~ goethites leads to a slight decrease in
the band separation as compared to the 70~ goethites. Pure goethites synthesised between 4
and 40~ were found to have a band separation of about 93 cm-i while goethites synthesised
between 50 and 80~ had a separation of about 97 cm -1 (Schwertmann et al., 1985).
The average dehydroxylation temperature was similar for both series and increased
linearly with A1 substitution for samples with < 12 mole~ A1 substitution (Fig. 6).
The hematite which was associated with goethite in samples 35/8 and 35/9 was also A1substituted. A1 substitution in hematite was estimated from the a dimension using the
relationship Al(mole~) = 676(5.0417 - a) (n = 7; r = 0.997; a in A) found for a series of
hematites produced at 25~ and pH 7.0. Sample 35/8 had 2.8 and sample 35/9 had 4.4 mole~
A1 in the hematite structure. This is much less than the goethites in the same samples (22-9
and 27.3 mole~o AI respectively). The (130) line of goethite and the (104) line of hematite are
therefore clearly separated, although they coincide at 2.67 A when the two phases are
Properties of,"synthetic goethites
91
360"
0
c~ 340
I:D
"0
70* goethites
t3..
E 320
g,J
4-J
0
x
o
25* goethites
300'
0
0
280:
>
I
2600
2'0
l'O
AI
I
30
(m01e %)
FIG. 6. Average dehydroxylation temperature vs. A1 substitution.
unsubstituted. The lower A1 substitution of hematite than that of the associated goethite and,
therefore, the line separation, has also been observed in soils (Schulze, 1981 ; Schwertmann &
K~impf, 1985; Schwertmann & Taylor, 1984).
CONCLUSIONS
Goethites synthesised from ferrihydrite in 0.3 M K O H at 25~ contain more structural defects
than goethites synthesised at 70~ The structural defects give rise to larger unit-cell a and c
dimensions, larger OH-stretching band positions, and smaller separations between the OHbending band positions. The goethites synthesised at low A1 additions consist of crystals
which contain several coherently diffracting domains, while the goethites synthesised at
higher A1 additions consist mainly of crystals which are single domains. The surface areas of
the 25 ~ goethites varied as a function of both domain size and physical size.
ACKNOWLEDGMENTS
The technical assistance of Mrs B. Gallitscher and Mrs U. Maul from the Institut fiir Bodenkunde, Technische
Universit~it Mfinchen, in carrying out the experimental work, is gratefully acknowledged. This is journal
article no 10712 of the Agricultural Experiment Station, Purdue University.
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92
D. G. Schulze and U. Schwertmann
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