Clay Minerals (1983) 18, 301-312
T H E I N F L U E N C E OF A L U M I N I U M
S U B S T I T U T I O N A N D C R Y S T A L L I N I T Y ON THE
M ( ) S S B A U E R S P E C T R A OF G O E T H I T E
E. M U R A D
AND U. S C H W E R T M A N N
Lehrstuhl fiir Bodenkunde, TU Miinchen, D-8050 Freising- Weihenstephan, FR G
(Received 11 January 1983; revised22 March 1983)
ABSTRACT: Both aluminium substitution and poor crystallinity reduce the magnetic
hyperfine field of goethite. M6ssbauer spectra taken at 4.2 K show that the effect of poor
crystallinity is similar to that of AI substitution, i.e. it reduces the saturation hyperfinefield. A
multiple correlation was found to exist between the magnetic hyperfine field at 4.2 K as a
dependent variable vs AI substitution and crystallinity as independent variables. If a hyperfine
field is to be interpreted with respect to either A1 substitution or crystallinity, it is therefore
necessary to have knowledgeof the other variable.
Natural iron oxides often exhibit isomorphous substitutions of other cations for Fe 3+.
These substitutions may occasionally be quite high, resulting in significant deviations from
the theoretical composition. Substitutions can also be produced during the synthesis of
iron oxides in the laboratory, enabling their effects on physical properties to be studied.
The substitution of Fe a+ by AP +, which is well documented for natural goethites
(Correns & Engelhardt, 1941; Norrish & Taylor, 1961; Janot et al., 1968; Davey et al.,
1975; Nahon et al., 1977; Bigham et al., 1978; Golden et al., 1979; Torrent et al., 1980;
Fitzpatrick & Schwertmann, 1982), can reach about 33 mole%. In monomineralic samples
the identity and concentrations of elements substituting for Fe can be readily determined
by chemical analysis. In mineralogically complex samples, specific chemical procedures-e.g. treatment with sodium dithionite-citrate-bicarbonate (DCB) (Mehra & Jackson,
1960)-- are necessary to extract the iron oxides and other elements contained in them
prior to chemical analysis. The reagents used in such procedures may, however, also
release Al from minerals other than iron oxides and the iron oxides may appear to be more
highly Al-substituted than they really are. Enrichment procedures which concentrate iron
oxides by removing other minerals (e.g. boiling with 5 y N a O H (Norrish & Taylor, 1961))
can, under adverse conditions (Kfimpf & Schwertmann, 1982), also affect the iron oxides.
Physical techniques sensitive to A1 substitution, which give information not on the bulk
properties of a sample but on properties of the individual phases, are therefore preferable
for their analysis.
The technique most frequently employed for this purpose is X-ray diffraction (XRD).
Because AI 3+ is smaller than Fe 3+ (ionic radii 0.51 A and 0.64 A respectively (Ahrens,
1952)), Al-substituted goethites have a smaller unit cell and lower d-spacings than pure
goethite (Thiel, 1963; J6nfis & Solymar, 1970; Schulze, 1982). In samples containing
sufficient amounts of goethite (_> 10%), X R D can therefore give a reliable indication of the
extent of A1 substitution.
9 1983 The Mineralogical Society
302
E. M u r a d and U. Sehwertmann
If samples contain only minor amounts of iron oxides (< 10%), the XRD lines of these
may be masked by lines from other minerals. This can render accurate measurement of the
X-ray parameters difficult if not impossible. The sensitivity of XRD can be considerably
improved by using a differential technique (Schulze, 1981). In this, the XRD information
obtained from a sample from which iron oxides have been removed is subtracted from the
data obtained from an untreated sample using a computer procedure. At even lower iron
oxide concentrations, however, especially if the iron oxides are poorly ordered, this method
may also fail to give the required information.
An alternative physical method, which is sensitive only to iron-containing components,
is 57Fe M6ssbauer spectroscopy. Fe 3+ is a paramagnetic ion with an electronic structure
corresponding to that of an Ar core plus five 3d-electrons. In the high-spin state this gives
a (maximum) magnetic moment of 5 Bohr magnetons. The electronic structure of A13+, in
contrast, corresponds to that of Ne. This is therefore a diamagnetic ion with no net
magnetic moment. In the magnetically ordered state, iron oxides can be identified by
parameters (magnetic hyperfine field H~ and quadrupole splitting AEQ) which are
characteristic of each species.
Pure, well-crystallized goethite orders magnetically below its N6el-temperature (TN) of
~400 K (Woude & Dekker, 1966; Forsyth et al., 1968). Below Tu the variation of the
magnetic hyperfine field with temperature is reasonably well approximated by a Brillouin
function (Woude & Dekker, 1966), with a saturation field of about 506 kOe at 0 K (see
later). Substitution of Fe a+ by A13+ (magnetic dilution) will lower TN. The reduced
temperature, T / T N, is therefore higher for substituted than for pure goethites. This causes
Al-substituted goethites to have significantly lower hyperfine fields at temperatures
corresponding to a T / T N ratio >0.3. An additional reduction of the hyperfine field at all
temperature s follows from the occasional replacement of Fe 3+ by A13+ ions in the crystal
lattice. This will result in a reduction of the contribution of neighbouring Fe 3+ ions, i.e. that
field which is 'supertransferred' via intermediate O 2- ions from one Fe 3+ ion to another.
The hyperfine field is therefore not only suited to identify goethites, but also to characterize
them with respect to A1 substitution (Janot et al., 1968; Hogg et al., 1975; Golden et al.,
1979; Fleish et al., 1980; Goodman & Lewis, 1981; Fysh & Clark, 1982).
On the other hand, magnetic hyperfine field reductions have also been observed for
unsubstituted goethites of poor crystallinity due to their very small particle size (Kraan &
Loef, 1966; Yamamoto, 1968; Golden et al., 1979; Murad, 1982a). This observation has
been questioned by Fysh & Clark (1982) who considered the effects of particle size on the
magnetic hyperfine field of goethites to be solely a relaxation phenomenon, which should
be negligible at 4.2 K.
The purpose of the present investigation was to study the dependence of the magnetic
hyperfine field on AI substitution and on crystallinity, and, if possible, to establish a
quantitative relationship between these factors.
EXPERIMENTAL
PROCEDURE
For the M6ssbauer spectra a 57Co/Rh source of ~25 mCi was mounted on a
loudspeaker-type drive system providing a sinusoidal motion. Sample quantities with an
average Fe density of about 6 mg/cm 2, packed into a Plexiglas holder, served as absorbers.
Spectra were taken at room temperature and with both source and absorber cooled to 125
and 4.2 K in a bath cryostat. The transmitted radiation was recorded with a Kr
Mfssbauer spectra of goethite
303
proportional counter and stored in a 1024-channel analyser. The recorded spectra were
monitored with an oscilloscope and, when sufficiently good statistics had been attained,
punched out on tape and processed with a CDC Cyber 175 computer.
In order to improve the accuracy of the 4.2 K spectra, which showed only relatively
minor variation of the hyperfine field with A1 substitution, these were taken with a 6/zm
iron foil attached to the sample holder. The iron spectrum, which was recorded together
with that of the goethite contained within the holder, served as an internal standard for
velocity calibration and as an isomer shift reference; the hyperfine field of iron was taken
as 339.0 kOe (Violet & Pipkorn, 1971). As this technique virtually eliminates systematic
errors, only statistical errors averaging less than _+ 0.2 kOe remained. All 4.2 K spectra
were therefore fitted with two sextets; three very Al-rich samples containing some hematite
were fitted with three sextets.
Some Fe contamination in the M6ssbauer spectrometer (probably a minor constituent
of aluminium foils used for thermal insulation and/or beryllium windows of the counter)
resulted in a broad, weak peak near zero velocity, visible in the 4.2 K spectra and some of
the 125 K spectra (Figs 4 and 3, respectively). These peaks are of no relevance for this
study, and have not been fitted separately.
MATERIALS
STUDIED
Several batches of freshly prepared 1 M Fe(NO3) 3 were mixed with AI(NO3) 3 solutions of
concentrations up to 0.24 M (series 12), or with different amounts of a 0.5 ~ AI(NO3) 3
solution (series 31). The mixtures were precipitated at pH 7.5 using N H 3 (series 12) and
5 M KOH (series 31). The precipitates ('proto'-ferrihydrites) were stored in 2 M KOH
(series 12) and 0.3 M K O H (series 31) at 70~ for 27 and 14 days, respectively. After this
period the precipitates were centrifuged, washed, and dried at 60~
The third set of
goethites (series 3 + 4) used in this study was prepared via an intermediate 'green rust'
phase (Goodman & Lewis, 1981). Starting materials for this procedure were mixed
solutions of FeC12 + A1C13 containing 0-02 M (Fe + A1). NaHCO 3 solutions were added to
these initial mixtures to an excess of 5 and 100 mmole NaHCO 3 (series 3 and 4,
respectively). Air was bubbled through these mixtures at room temperature and pH 7-9.
Following oxidation (after 48 h) the precipitates were washed free of salts and dried.
The A1 contents of the samples were determined by chemical analysis. The samples were
further characterized by X-ray diffraction (using C o - K a radiation, Philips PW 1130
diffractometer, and a graphite diffracted-beam monochromator), surface area (EGME)
measurements, and transmission electron microscopy (Zeiss EM 10; 80 kV).
Electron micrographs showed that the goethites had a wide range of particle sizes and
morphologies. Even the unsubstituted end-members varied considerably in their crystal
dimensions, the crystals becoming successively smaller in the order 31/0 - 12/0 - 3/0
(Fig. 1).
Quantitative parameters suitable for the characterization of the crystallinity of these
samples are the mean crystal diameters (MCDs) in specific crystallographic directions, e.g.
[ll0] or [ l l l ] , and the surface area. These parameters also showed considerable
variations between the different series, and a smaller variation as a function of A1
substitution within each series. The most obvious trend was that of decreasing crystallinity
of the series in the order 31 - 12 - (3 + 4); average MCDs 3_ [ 111] determined by X-ray
diffraction were 70, 30, and 10 nm, respectively. This is in accordance with particle-size
304
E. Murad and U. Schwertmann
06
ea
O
e~
d~
e4
ee~
e~
0
E
e.
o
Q
,.d
t~
M6ssbauer spectra of goethite
305
observations by TEM (Schulze & Schwertmann, in preparation). Within the individual
series, crystallinity was found to be reasonably constant for series 12 and 31, but
decreased with increasing A1 substitution for the series 3 + 4.
RESULTS
Effects of A 1 substitution
Room-temperature M6ssbauer spectra even of pure goethites usually show broadened
resonant lines, which can be interpreted as the outcome of magnetic hyperfine field
distributions, with maximum absorption at about 380 kOe (Murad, 1982a). Fig. 2 shows
room-temperature M6ssbauer spectra of samples with different A1 contents from series 12.
Even minor incorporation of AI into the goethite lattice results in superparamagnetic
relaxation of the M6ssbauer spectra and/or pronounced effects of hyperfine field
distribution (12/5 and 12/10 in Fig. 2). Attempts to quantify the hyperfine field reduction
at room temperature as a function of A1 substitution must therefore be considered
somewhat tentative. From A1 substitutions (AI, = A1/(Fe + A1)) of about 0.12 upwards,
goethite is paramagnetic at room temperature, i.e. TN < T. The M6ssbauer spectra then
consist of just one doublet with non-specific parameters characteristic of Fe 3+ in
octahedral coordination (6(Fe) = 0.36; AE o = 0.52 mm s-l; Fig. 2d).
::
9
-.i.,
.:
9
,
9.A ..
'
,5,
9
9
%
"
..,~
z
9
C
c
.
.9
:"Z' ' : .e. . .
~"
.?
.~
:.
;
9
.o
9
":
1210
" " ? A~!
..
":
FeOOH
12/5
Fe~gsAto~sOOP
~ x
p"
q
n
a
p ~
%
t
a
/
\
J
.."
9
-
.:,
....;...q
."
12/10
-I0'.00
..
.
'
'
0'.00
h
-5'.O0
VELOCITY (MM/SEC)
Feo.91ALo.o900H
,
5.00
,
12/15
,
10 ~ -10.
:
_SJ
i
Feoes Ato lz OOH
n
.00
0'.00
VELOCITY (MM/SEC)
5'.00
i
295K
FIG. 2. Room-temperature M6ssbauer spectra of samples of series 12 with A1 substitutions of
0-00, 0-05, 0.09, and 0.12.
I
10.00
306
E. M u r a d and U. Schwertmann
q
12/0
i
12110
-I0100
-s.oo
i
FeOOH
i
i
i
i
Feo91 A[o09 OOH
0'.00
VELOCITY ~MM/SEC)
5,00
12/5
i
Feo 9sALo,os OOH
12115
10
10.
-5.00
r
Feo 88 ALo lz 0014
oJ. oo
VELOCITY (MM/SEC)
5'.00
10.00
125K
FIG. 3. 125 K M6ssbauer spectra of samples of series 12 with AI substitutions of 0.00, 0.05,
0.09, and 0.12.
The effects of relaxation and/or hyperfine field distribution can be reduced by cooling
the absorber~ usually using either liquid nitrogen or helium. Cooling the absorber to 125 K
produces magnetically ordered spectra to A1s values of about 0.3. A linear correlation
exists between the magnetic hyperfine fields measured at 125 K for series 12 (Fig. 3) and
A1, in the range from 0-00 to 0.16. The equation derived from a regression analysis of this
data is:
H i (series 12/125 K) = 485.3 -- 140Al~
(n = 5, r 2 = 0-996)
(1)
Lorentzian curves fitted to the 125 K M6ssbauer spectra, however, show deviations
from the data that become more pronounced as the extent of A1 substitution increases
(Fig. 3). These discrepancies may result either from distributions of magnetic hyperfine
fields (that are still apparent at 125 K), possibly incipient superparamagnetic relaxation, or
a combination of these two effects. They are best overcome by cooling the absorber to
even lower temperatures, e.g. to 4.2 K using liquid helium. Because the M6ssbauer spectra
of many natural goethites are paramagnetic or only partly magnetically ordered when
cooled by liquid nitrogen (Simopoulos et al., 1975; Schwertmann et al., 1982; Murad,
1982b), magnetic ordering of these requires lower temperatures anyway.
At 4.2 K all samples of the series 31, 12 and (3 + 4) have magnetically ordered
M6ssbauer spectra. The spectra of series 31 and 12 (Fig. 4) have lines that do not deviate
noticeably from Lorentzian shape. The spectra of series (3 + 4), however, still show such
deviations, which become more pronounced as AI~ increases. For each of these series a
Mfssbauer spectra of goethite
307
r
12/0
FeOOH
12/5
Feo 9sA[oos OOH
i
12/10
-1 o~.oo
-5~. 00
~
VELOCITY
OJ.O0
Feo91A[oo900H
5~.0-0
10'.
10.
[MM/SEE]
12115
FeossA[o1200H
-5.00
0.00
5.00
VELOCITY (MM/SEC)
10.00
4.2K
FiG. 4. 4.2 K M6ssbauer spectra of samples of series 12 with AI substitutions of 0.00, 0.05,
0.09, and 0.12.
separate linear regression can be calculated which relates H i to A1s over the whole A1
substitution range. The regressions for series 31, 12, and (3 + 4), respectively, are given
by:
H i (series 3 I/4 K) = 505.1 - 42A1 s
(n
0"996)
(2)
H i (series 12/4 K) = 504.1 -- 44A1 s
(n = 7, r 2 = 0.994)
(3)
H t (series (3 + 4)/4 K) = 500.0 -- 52Als
=
7, r 2 =
(n = 6, r 2 = 0.996)
(4)
Effects of crystallinity
Room-temperature M6ssbauer spectra of unsubstituted goethites of different crystallinities range from magnetically-ordered sextets with lines of almost Lorentzian shape, over
sextets with broad asymmetric lines indicative of hyperfine field distributions (Murad,
1982a), to spectra that are predominantly to totally paramagnetic (Kraan & Loef, 1966;
Shinjo, 1966).
At 4.2 K all unsubstituted goethites were magnetically ordered; the hyperfine fields and
line widths, however, varied as a function of crystallinity. Line widths increased as
crystallinity decreased; the unsubstituted end-members of the goethite series 31/0, 12/0
and 3/0 had line widths of 0.32, 0.35, and 0.53 m m s -1, respectively. The spread of
magnetic hyperfine fields observed for these goethites ranged from 505.2 kOe (31/0) to
499.6 (3/0). On the basis of the regression equations given above, this difference of 5.6
E. Murad and U. Schwertmann
308
kOe would correspond to an A1s difference of about 0.13. It is thus obvious that reductions
of the magnetic hyperfine field, even at 4.2 K, cannot be unequivocally related to A1
substitution, because the same effect can be brought about by poor crystallinity.
Combined effects of A 1 substitution and crystallinity
As mentioned above, at 4.2 K the samples gave magnetically-ordered Mrssbauer
spectra over the whole A1 substitution range covered (AI s = 0.00-0.31). Each series
showed a linear dependence of the hyperfine field on AI substitution. Because the hyperfine
field depends not only on A1 substitution but also on crystallinity, the regression lines of
the individual series do not coincide, but are displaced approximately parallel to one
another, such that at any A1 substitution the series with the best crystallinity (series 31) has
a higher hyperfine field than series 12, and this in turn has a higher field than series (3 + 4).
Poor crystallinity is probably responsible for the deviations between the 4.2 K
Mfssbauer spectra of series (3 + 4) and Lorentzians fitted to these. The deviations,
which give lower hyperfine fields for the Al-substituted samples than the hyperfine fields of
maximum absorption, probably cause the steeper slope (0.52 kOe/% Als) of the regression
calculated for series (3 + 4) than for the other two series.
Confidence intervals of 95% show that the differences between the regression lines
calculated for the three series (equations 2 to 4) are statistically significant (Fig. 5).
505
498
,$
0
31
"1"
12
491
~
4840
'
0.~06
'
0'.12
'
0'.18
0.24
AI/(AI + Fe)
FIG. 5. Magnetic hyperfine fields of series 31, 12, and (3 + 4) at 4-2 K in dependence of the AI
substitution. Data points of each series are joined by linear regression lines (equations 2 to 4 in
the text), which are bordered by 95% confidence intervals.
3,4
M6ssbauer spectra o f goethite
309
Consequently, the validity of an equation relating the magnetic hyperfine fields of goethites
of different crystallinities to A1 substitution only (Fysh & Clark, 1982), must be doubted.
For all the samples examined here (EGO, this equation, given by
H i (EGt/4 K) =- 504.1 - 55A1 s
(n = 20, r 2 = 0.702)
(5)
is characterized by a much poorer correlation than equations 2-4.
Quantitative evaluation of the information given above obviously requires analysis by
multiple regression, relating the magnetic hyperfine field to A1 substitution and
crystallinity. On the basis of a study of an Al-substituted goethite series and unsubstituted
goethites of different crystallinities, Golden et al. (1979) presented an equation relating the
hyperfine fields of goethites at 77 K to AI substitution and surface area:
H,. = 498 -- 136A1 s -- 0. l l S
(6)
In the present study, multiple regressions were calculated with H t at 4.2 K as the
dependent variable, and A1s and a measure of the crystallinity, e.g. surface areas (S) or
MCDs obtained from a suitable X R D line, as independent variables for the series 12, 31,
and (3 + 4). The MCDs calculated from the half-widths of the (111) X R D line were
considered suitable for this purpose because this plane has components in all axial
directions. The regressions gave the following equations:
H i = 505.4 - 33A1 s - 0.036S
H i = 506.5 - 42A1 s - 87/MCDtH~j
(n = 19, r 2 = 0.967)
(n = 20, r 2 = 0.969)
(7)
(8)
Entering the second variable (surface area or MCD) into the regressions is justified by
improvements of the multiple coefficients of determination, r 2, which, compared to
equation (5), are significant at the 99.9% level. The reason for using the reciprocal mean
crystal diameter in equation (8) is that this will give a rational hyperfine field value for
infinitely big ('bulk') crystals, whereas the opposite possibility (MCD = 0) is not realistic.
A correlation matrix shows that AI s and the reciprocal MCDtH n are not (r = 0.36)
correlated, whereas A1s and surface area are only weakly correlated (r = 0.52). The
multiple coefficient of determination when relating H i to A1s and 1/MCDoH ~ (r 2 = 0.969)
is higher than when AI s and 1/MCDtH0) are used (r 2 = 0-937), justifying the preference of
MCD~H~) over MCD~I~0 ~.
DISCUSSION
The slopes of the regression lines given by equations (2) to (4), and the AI coefficients of
equations (7) and (8), correspond to the hyperfine field reduction if there is no
supertransfer from any Fe 3+ ion to another, i.e. at a hypothetical AI substitution of 100%.
Using equations (2), (3), and (8), which, for reasons given above, are considered the most
dependable, this supertransferred hyperfine field is calculated as 42 + 3 kOe. This value is
similar to the supertransferred hyperfine field in hematite at 77 K, given by DeGrave et al.,
(1982) as 39 + 4 kOe.
The significant contributions of surface area and mean crystal dimensions in equations
(7) and (8), respectively, indicate poor crystallinity to play a role similar to that of AI a§
ions. A particle-size reduction from bulk to a MCDtIH) of 21 nm, for example, results in
the same hyperfine field reduction (4.2 kOe at 4.2 K) as an AI substitution of 10~ This
310
E. M u r a d a n d U. S c h w e r t m a n n
can also be interpreted as the outcome of hyperfine field supertransfer reduction due to
missing neighbours, because increasingly m a n y Fe 3+ ions come to lie near the surface as
the particle size decreases, and, in addition, the concentration of defects within the whole
crystal may increase as the particle size decreases (Schroeer, 1970).
Since two variables cannot be determined from only one equation, it is obviously
impossible to determine the A1 substitution and crystallinity from one M6ssbauer
spectrum, even if this is taken at a temperature at which relaxation effects are supposedly
absent (e.g. at 4.2 K). Additional information (from X R D , chemical, or other methods) is
therefore necessary to characterize a goethite with respect to either A1 substitution or
crystallinity.
The situation may be further complicated by the fact that numerous other elements have
also been detected in natural goethites (Johnston & Norrish, 1981). It is not altogether
clear whether these elements are incorporated in the goethite lattice, isomorphously
substituting for Fe 3+ (as A13+), or form submicroscopic foreign inclusions. Si, for example,
has been detected in natural goethites to the extent of 5.4 mol% (Johnston & Norrish,
1981). On the other hand, Schwertmann & Taylor (1972) were able to incorporate only
0.3 to 0.1 atom% Si in structural sites in synthetic goethites. Nevertheless, hyperfine field
reductions in natural goethites of good crystallinity that contain foreign elements other
than A1 (Murad, 1979; Johnston & Norrish, 1981) indicate that the influence of such
elements cannot be neglected a priori.
The applicability of equations (7) and (8) to natural systems is therefore based on the
premise that foreign ions other than A13+ are not incorporated in the goethite structure in
significant amounts. Equations (7) and (8) show that, if the effects of crystallinity and
possible substitutions of elements other than A1 are ignored, a too high value for the A1
substitution may be obtained. This (hypothetical) A1 substitution would cover the sum of
all deviations from ideal crystallinity and chemical composition.
Conversely, the observation of well-developed M6ssbauer spectra of high hyperfine
fields (~380 kOe at room temperature, 498 kOe at 77 K, and 506 kOe at 4.2 K,
respectively) will always indicate a goethite to be of good crystallinity and free from
foreign element substitutions.
ACKNOWLEDGMENTS
The authors are indebted to Prof. F. E. Wagner (Physik Department, T.U. Mfinchen in Garching) for making
the M6ssbauer measurements at 125 and 4.2 K possible. Further thanks are due to Dr D. G. Lewis for
preparation of the goethite series (3 + 4), to Dr H. Ch. Bartscherer for taking the electron micrographs, to Dr
A. S. Campbell and Prof. F. E. Wagner for critical reviews of the manuscript, and to the Deutsche
Forschungsgemeinschaftfor financial support.
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312
M~ssbauer spectra of goethite
R E S U M E : Des substitutions par l'aluminium et une mauvaise cristallinit+ r+duisent toutes les
deux le champ magn~tique hyper-fin de la goethite. Des spectres M6ssbauer enregistr6s/l 4.2 K
montrent que l'effet d'une mauvaise cristallinit6 est semblable/t celui de la substitution par AI,
c'est-/~-dire qu'elle r6duit la saturation du champ hyper-fin. On a montr+ qu'il existe une
corr61ation multiple entre le champ magn&ique hyper-fin /t 4.2 K qui admet comme variable
d6pendante la substitution par A1 et le degr6 de cristallinit+ en tant que variable ind+pendante. Si
on doit interpr6ter le champ hyperfin par rapport ~ la substitution A1 ou la cristallinit6, il est donc
n6cessaire de conna~tre l'une ou l'autre des deux variables.
K U R Z R E F E R A T : Sowohl AI Substitution als auch geringe Kristallqualit~it verkleinern das
rnagnetische Hyperfeinfeld von Goethit. M6fSbauerspektren bei 4.2 K zeigen, dal3 der Effekt
geringer Kristallqualit~it ~ihnlich dem von Al-Substitution ist, also das S~.ttigungshyperfeinfeld
verringert. Zwischen dem magnetischen Hyperfeinfeld bei 4.2 K (als Zielgr613e) und der AlSubstitution und der Kristallinit~it (als unabhS.ngige Variablen) wurde eine multiple Korrelation
gefunden. Bevor ein Hyperfeinfeld im Hinblick auf Al-Substitution oder Kristallqualit~it interpretiert wird, ist es daher notwendig, auch die Gr6f3e der jeweils anderen Variablen zu kennen.
R E S U M E N : Tanto la sustituci6n de hierro por aluminio como la baja cristalinidad redueen
el campo magn6tico hiperfino de la goethita. Los espectros M6ssbauer obtenidos a 4.2 K
muestran que el efecto de la baja cristalinidad es similar al de la sustituci6n de aluminio, es decir
reduce el campo hiperfino de saturaci6n. Se encontr6 que existe una correlacibn mflltiple entre el
campo magn+tico hiperfino a 4.2 K como variable dependiente y la sustituci6n de aluminio y la
cristalinidad como variables independientes. Por 1o tanto, para interpretar los valores del campo
hiperfino es necesario tener en cuenta las dos variables citadas.