Clays and Clay Minerals.
VoL 46. No. 4. 369-378. 1998.
IRON OXIDES IN A SOIL D E V E L O P E D F R O M B A S A L T
A. T. GOULART, 1'2 J. D. FABRIS, ! M . E DE JESUS FILHO,!'t J. M . D. COEY, 3 G. M . DA COSTA 4
AND E. DE GRAVE5$
Departamento de Quimica, UFMG-Pampulha, 31270-901 Belt Horizonte, MG, Brazil
2 On leave from Departamento de Qufmica, UFV, 36571-000 Viqosa, MG, Brazil
3 Department of Pure and Applied Physics, Trinity College, University of Dublin, Dublin 2, Ireland
4 Departamento de Qu/mica, UFOE 35400-000 t u r n Preto, MG, Brazil
5 Department of Subatomic and Radiation Physics, University of Gent, B-9000 Gent, Belgium
A b s t r a c t - - A dusky red Oxisol forming on a tholeiitic basalt is found to contain varying proportion of
aluminous hematite (Hm) and titanoaluminous maghemite (Mh) in the different size fractions. Maghemite
is the main iron oxide in the sand and silt fractions whereas Hm is dominant in the clay fraction, together
with gibbsite (Gb), kaolinite (Ka), rutile (Rt) (and probably anatase, An) and Mh. Maghemite is also the
major oxide mineral in the magnetic separates of soil fractions (sand, about 65% of the relative MiSssbauer
spectral area; silt, 60%). Hematite (sand, 30%; silt, 15%) and ilmenite (Im) (sand, 5%; silt, 16%) are also
significantly present in the magnetic extract. Accessory minerals are Rt and An. No magnetite (Mt) was
detected in any soil fraction. Sand- and silt-size Mh have similar nature (a0 = 0.8319 - 0.0005 nm; about
8 mol% of A1 substitution; saturation magnetization of 49 J T -* kg '), and certainly a common origin.
Lattice parameters of clay-Mh are more difficult to deduce, as magnetic separation was ineffective in
removing nonmagnetic phases. A1 content in Hm varies from 14 mol% (clay and silt) to 20 mol% (sand).
The proposed cation distribution on the spinel sites of the sand-size Mh is"
[Fe0.gzA1008]{Fe, .43Ti0.~8170.39}0
4
([7 = vacancy, [ ] = tetrahedral sites and { } = octahedral sites), with a corresponding molar mass of
208.8 g tool -1. The predicted magnetization based on this formula is tr ~ 68 J T ' kg ~, assuming collinear
spin arrangement. The large discrepancy with the experimentally determined magnetization is discussed.
Key Words--Hematite, Ilmenite, Maghemite, Magnetic Fraction, M0ssbauer, Spinel.
INTRODUCTION
B a s a l t i c b e d r o c k s tend to p r o d u c e d u s k y red m a g netic Oxisols u n d e r tropical conditions. A l t h o u g h the
m i n e r a l o g i c a l c o m p o s i t i o n o f soils d e r i v e d f r o m basalt, particularly o f the clay fraction, is k n o w n f r o m
several studies ( R e s e n d e 1976; Curi a n d F r a n z m e i e r
1987; F o n t e s a n d W e e d 1991), a c o m p l e t e d e s c r i p t i o n
o f the nature a n d origin o f their iron o x i d e s a p p e a r s
to b e n o t available. T h e p r e s e n t study is f o c u s e d o n
the iron o x i d e m i n e r a l o g y , particularly o f the c o a r s e
fractions, o f a B r a z i l i a n m a g n e t i c soil d e r i v e d f r o m
basalt, a n d a m o r e detailed d e s c r i p t i o n o f the m a g n e t i c
p h a s e s is presented.
Tropical soils c o n t a i n typically large a m o u n t s o f
iron oxides, particularly H m (ot-Fe203) a n d M t (Fe304)
or M h (~-Fe203). A l t h o u g h g o e t h i t e (o~-FeOOH, G t ) is
a w i d e s p r e a d m i n e r a l in soils, it appears in relatively
l o w e r p r o p o r t i o n s or is u n d e t e c t e d b y p o w d e r X - r a y
diffraction ( X R D ) in s o m e h i g h l y w e a t h e r e d m a g n e t i c
Oxisols f o r m i n g o n mafic r o c k s (Fontes a n d W e e d
1991). T h e c h a r a c t e r i z a t i o n o f t h e s e m i n e r a l s is imp o r t a n t for a g r i c u l t u r e land use as soil m i c r o - e l e m e n t s '
availability to plants is s o m e h o w related to their iron
o x i d e m i n e r a l o g y ( C u d a n d F r a n z m e i e r 1987; F e r r e i r a
et al. 1993). Several r e c e n t studies h a v e b e e n r e p o r t e d
o n B r a z i l i a n m a g n e t i c soils. A fully o x i d i z e d titano m a g h e m i t e f r o m a basaltic soil w i t h a s p o n t a n e o u s
m a g n e t i z a t i o n of 36 J T ' k g - ' was earlier characterized ( A l l a n et al. 1989). D i f f e r e n t M h c o m p o s i t i o n s
w e r e r e p o r t e d for o t h e r lithologies, e.g., the (A1, Ti)rich M h f r o m a d o l o m i t i c soil ( M o u k a r i k a et al. 1991)
a n d the n e w l y c h a r a c t e r i z e d M g - r i c h M h f r o m tuffite
(Fabris et al. 1995).
METHODS
T h e soil s a m p l e was c o l l e c t e d f r o m t h e B h o r i z o n
o f a d u s k y red Oxisol d e v e l o p e d f r o m a tholeiitic basalt, l o c a t e d
at g e o g r a p h i c a l
coordinates
lat
18~
l o n g 49~
in a cutting n e a r the
h i g h w a y B R 154, 2 k m s o u t h o f C a c h o e i r a D o u r a d a ,
in the r e g i o n o f Tri~ngulo M i n e i r o , M i n a s G e r a i s
State, Brazil. T h e c h e m i c a l c o m p o s i t i o n o f the p a r e n t
r o c k a n d o f the w h o l e m a s s o f the soil w e r e r e p o r t e d
b y P i n t o (1997) a n d F e r r e i r a (1995) as b e i n g (in
m a s s % ) , respectively: SIO2, 46.4 (rock), 34.2 (soil);
Fe203, 15.4, 26.8; A1203, 11.6, 15.9; T i t 2 , 3.46, 6.89;
C a t , 7.79, 0.37; M g O , 3.94, 0.91; M n O , 0.24, 0.39;
t Professor Milton Francisco de Jesus Filho died on January
2, 1996.
$ Research Director, National Fund for Scientific Research,
Belgium.
Copyright 9 1998, The Clay Minerals Society
369
370
Goulart et al.
Table 1. Chemical composition of the magnetic separates
from the sand and silt fractions.
Oxide
Sand %
Silt %
FeOt
Fe20~
AI203
Tit2
Cat
MgO
MnO
Bat
Ni203
CoO
Cut
ZnO
LOI~
1.20
77.97
5.50
8.35
0.85
0.33
0.31
0.01
0.0l
0.02
0.03
0.04
5.03
5.99
55.59
4.05
30.62
0.40
0.61
0.78
0.01
0.04
0.05
0.05
0.17
4.60
t According to Goulart et al. (1994a).
$ Loss on ignition.
P205, 0.58, Not A v a i l a b l e (NA); NazO, 2.44, N A ; K20,
1.65, N A ; loss on ignition, 6.71, 12.2; total, 100.2,
97.7. The parent rock was classified according to the
Jensen (1976) triangular plot, which correlates F e O +
TiOz, A1203 and M g O contents. Ilmenite (Im) (determined lattice parameter, a = 0.5082 n m and c =
1.4070 nm), augite and plagioclase (albite and/or anorthite) w e r e the main mineralogical phases identified
by X R D in the whole rock sample and Mt and a ferric
Im were characterized in the magnetic extract of the
tholeiitic basalt (de Jesus Filho et al. 1995). Kaolinite,
Gb, Hm, Im and M h are the main minerals in this soil.
Geologically, the sampling area is part of the Serra
Geral Formation and the basalt has its origin from a
M e s o z o i c m a g m a t i s m that occurred ca. 1 3 0 - 1 5 0 M y
ago, and which covers about 1,200,000 kin 2 in central
and southern Brazil, northwest Uruguay, northeast Argentina and southeast Paraguay (Schobbenhans et al.
1984).
The soil sample was first broken up by hand, left to
dry in air and sieved, to obtain the fine earth (mean
diameter o f particles, qb < 2 mm, saturation magnetization, cr = 3 J T -~ kg-~). The sample was dispersed
with a solution prepared by taking 50 m L NH4OH 1:1
into a final v o l u m e of 500 mL, under stirring in a
mixer. The suspension was then sieved to separate the
sand fraction (qb = 0 . 0 5 - 2 m m ; 6.7% of the soil mass).
The silt (qb = 0.002-0.05 m m ; 25.3%) and the clay (qb
< 0.002 m m ; 68.0%) fractions were separated by centrifuging the remainder (Jackson 1969). The magnetic
separates from sand and silt were obtained from a diluted slurry in water, and passed through a zigzagshaped tube placed in a magnetic field generated by a
U-shape hand magnet. Further magnetic separation
was carried out by stirring the first magnetic extract in
water in a beaker and collecting the magnetic particles
with a small hand magnet. The proportions of magnetic separate obtained were 24 mass% (sand) and 51
mass% (silt). It was o b s e r v e d that the reddish mag-
Clays and Clay Minerals
netic separates of the clay fraction still contained a
considerable amount of Hm, e v e n after repeating the
above-described procedure several times.
The chemical analysis of the magnetic separates was
carried out by fusing the samples with 1:1 Na2CO3 +
K2CO 3 mixture in a platinum crucible and dissolving
the product in diluted hydrochloric acid. The solution
was then analyzed by the standard dichromatometric
method (Jeffery and Hutchison 1981), with minor
modifications (Neves et al. 1985) to quantify Fe, and
with a Spectroflame Analytical Instruments plasma
emission spectrophotometer for the other elements.
The chemical composition is presented in Table 1.
The p o w d e r X R D patterns were obtained with a
Diffrac 500 Siemens diffractometer, using CuKc~ radiation.
The M6ssbauer spectra were collected in a constant
acceleration transmission spectrometer and a Co57/Rh
source, in the following experimental conditions to the
sample: 1) at r o o m temperature (RT) and 85 K; 2) at
RT with an externally applied field of 0.2 T and 3) at
275 K and 4.2 K with an externally applied magnetic
field of 6 T parallel to the direction of the incident ~/rays. Isomer shifts are quoted relative to metallic iron.
Magnetization measurements were made either in a
conventional vibrating sample m a g n e t o m e t e r ( V S M )
or with a portable soil magnetometer c o n c e i v e d by
C o e y et al. (1992).
RESULTS AND DISCUSSION
Petrographic Description
Optical microscopy analysis of the magnetic separate obtained from the sand fraction indicates light
rose, isotropic euhedric-subhedric particles having
white veins in 3 directions, forming triangles, which
were interpreted as being Mt under an alteration process. It will be shown later that this mineral is actually
Mh. Lamellae filled with a dark material identified as
Im in an exsolution process were also observed.
X - R a y Diffraction
X-ray diffractograms of magnetic separates from
sand and silt, as well as of the whole clay fractions,
are shown in Figure 1. Reflections due to Hm, Im, M h
or Mt (see below) and Rt were identified in the magnetic separate of the sand fraction (Figure l a). Hematite is more often an associated phase in clay (according to the definition in G u g g e n h e i m and Martin
1995), and normally it is not expected to occur as in
sand-sized grains in the B horizon of the soil studied.
A reasonable explanation for its presence in the sand
fraction could be that this H m acts as a coating or
cementing agent in smaller individual particles, forming sand-sized aggregates. The magnetic separate of
the silt fraction has additionally An and a small quantity of Ka (Figure lb). Roughly, the silt magnetic sep-
Vol. 46, No. 4, 1998
Iron oxides in a soil developed from basalt
371
Mh/Mt
81
Hi
(a)
Mh/Mt
I
Mh/Mt
sI MhlMt
,aka
(b)
5"0
20 (Degrees)
~5
eo
6'5
CuKGt
Figure 1. Powder XRD patterns (CuKc0 of (a) sand magnetic separate, (b) silt magnetic separate and (c) whole clay. An =
anatase, Gb = gibbsite, Hm = hematite, Im = ilmenite, Ka = kaolinite, Mh = maghemite, Rt = rutile, Si = silicon (internal
standard).
arate and total clay fraction were found to contain
qualitatively the same mineral composition as a whole,
but Gb, Ka and H m appear in higher proportion in the
clay fraction (Figure lc). Ruffle was detected in all
fractions.
Ditionite-citrate-bicarbonate (DCB) mixture is often
used to r e m o v e noncrystalline and s o m e crystalline
iron oxides in soil samples (Mehra and Jackson 1960).
Pedogenic M h is reported to be effectively r e m o v e d
by D C B treatment (Fine and Singer 1989; Singer et
al. 1995). In the present study, this treatment was
found to be effective in preferentially r e m o v i n g Hm,
concentrating M h and Im, in the silt and sand fractions. This enrichment is illustrated in Figure 2 for the
sand fraction.
Magnetite has been reported to be the main ferrimagnetic constituent in s o m e tropical soils (Curl and
F r a n z m e i e r 1987; Dematt~ and Marconi 1991). Furthermore, it was believed to occur m o r e often in the
coarse fractions, which are m o r e likely to contain prim a r y minerals (Moniz 1972). However, it is difficult
to identify Mr, M h and their solid solutions in natural
mixture, either by current low resolution p o w d e r X R D
(Abreu and Robert 1985), the main technique used in
those studies, or by optical m i c r o s c o p y (Anand and
Glides 1984).
In the present case, the lattice parameter a0 o f the
spinel structure in all samples (Table 2), deduced from
the Nelson-Riley extrapolation method (Klug and Alexander 1974), using the 220, 400 and 440 reflections,
are m u c h closer to that of M h (a0 -- 0.8339 nm, Joint
C o m m i t t e e on P o w d e r Diffraction Standards [JCPDS]
card #4.755) than that of pure Mt (a0 = 0.8390 nm,
J C P D S card #19.629). The lower values for the cell
parameters reported in Table 2 can be attributed to the
effect of Al-for-Fe substitution, since replacement of
Fe by titanium has no significant effect on the lattice
parameter in a fully oxidized Fe-rich spinel (Readman
and O ' R e i l l y 1972).
A n estimation of the amount o f isomorphic A1 can
be done through the linear relations given by Schwertmann and Fechter (1984), Wolska and S c h w e r t m a n n
(1989), Gillot and Rousset (1990) or B o w e n et al
(1994). The results are also shown in Table 2 for the
magnetic separate from the sand, before and after 4
successive treatments with D C B , and for the silt and
clay fractions. The X-ray pattern of M h in the w h o l e
clay fraction shows only 1 resolved 220 reflection that
372
Goulart et al.
Sl
Clays and Clay Minerals
Mh/Mt
Hrn
SI
1
Hm
i
"
Mt~~
~
......
Sl
H
(a)
Hm
I
& |Mh/Mt
Ml
~Mt
/ 1 Hm
Ka
25
3"o
"Ss . . . . .
g0 ......
20 (Degrees)
. . . . . . . 5 0 ...........
......
s'0
65
CuKa
Figure 2. Powder XRD pattern (CuK~) of the magnetic separate from the sand fraction (a) after and (b) before 4 successive
treatments with DCB. Hm = hematite, Im = ilmenite, Mh = maghemite, Si = silicon (internal standard).
does not o v e r l a p any H m peak, b u t its relatively low
intensity (Figure l c ) did not a l l o w a g o o d resolution.
H o w e v e r , a satisfactory e s t i m a t i o n c o u l d b e d o n e after
a selective attack w i t h 5 m o l L -~ N a O H , to r e m o v e
G b a n d Ka. T h e lattice p a r a m e t e r ao n o w d e d u c e d
f r o m the b e t t e r - r e s o l v e d reflections o f M h , f r o m the
X - r a y p a t t e r n (not s h o w n ) o f the c h e m i c a l l y treated
sample, is r e a s o n a b l y c o m p a r a b l e w i t h t h o s e o f the
s a n d a n d silt fractions. T h u s , the M h in all 3 fractions
h a v e a s i m i l a r a m o u n t o f i s o m o r p h i c A1 substitution.
T h e A1 substitution in the s o i l - H m (Table 3) was
d e d u c e d f r o m the lattice p a r a m e t e r a0, t h r o u g h the
equation:
A1/mol% = 100(32.4995 - 64.47a0)
[1]
t a k e n f r o m data o f S t a n j e k a n d S c h w e r t m a n n (1992),
for the s a n d b e f o r e a n d after (4 x D C B ) 4 s u c c e s s i v e
t r e a t m e n t s w i t h D C B , a n d for the silt a n d clay frac-
Table 2. A1 substitution in the Mh, estimated from the lattice
parameter (a0).
tions. T h e a0-cell p a r a m e t e r for H m w a s d e t e r m i n e d
f r o m the 300 reflection. T h e A1 substitution (14 m o l % )
o f the clay H m is in g o o d a g r e e m e n t w i t h the results
o b t a i n e d for o t h e r soil s a m p l e s f r o m the s a m e r e g i o n
(Fontes a n d W e e d 1991).
T h e m e a n crystallite d i m e n s i o n o f the H m in all
fractions is 20 --- 3 n m , as d e d u c e d f r o m the S c h e r r e r
f o r m u l a ( K l u g a n d A l e x a n d e r 1974), b y u s i n g the values o f l i n e w i d t h at h a l f h e i g h t o f the 110 reflection
peak.
M6ssbauer Spectroscopy
M/Sssbauer s p e c t r o s c o p y is, in principle, a t e c h n i q u e
c a p a b l e o f d i s t i n g u i s h i n g a m o n g different electronic
states o f Fe in the o x i d e structure, although, in certain
c i r c u m s t a n c e s , s o m e difficulties a p p e a r (da Costa, D e
Table 3. AI substitution in the Hm from the magnetic separate from the sand and silt fractions, and whole-clay fraction.
The ao-cell dimension was deduced from the d(300) reflection.
Fraction
ao/nm
Al/mol%
Fraction
d(300)/nm
a~/nm
Al/mol%
Sand
Sand (4 X DCB)
Silt
Clay
0.8318(5)
0.8319(5)
0.8319(5)
0.8326(5)
8
8
8
6
Sand
Sand (4 X DCB)
Silt
Clay
0.1447
0.1446
0.1449
0.1449
0.5013(4)
0.5010(4)
0.5019(2)
0.5019(1)
18(3)
20(3)
14(1)
14(1)
Vol. 46, No. 4, 1998
I
|
Iron oxides in a soil developed from basalt
I
I
I
I
|
|
I
I
I
373
Table 4. M 6 s s b a u e r parameters at RT of the magnetic separates of the sand and silt fractions and of the whole clay.
Fraction
Subspectrum
~ / m m s -1
2~Q, AE~/
mm s J
BJT
A/%
A2/%
Sand
Mh
Hm
Im
0.31
0.38
1.03
0.37
0.00
-0.18
0.68
0.67
48.3
50.4
---
65
29
2.8
3.5
65
30
1.4
3.6
Silt
Mh
Hm
Im
Doublet
0.33
0.37
1.03
0.28
0.38
0.00
-0.18
0.69
0.35
0.68
48.0
50.4
----
60
16
8.6
2.9
12
57(3)
15
13
3.0
11
Mh
Hm
Doublet
0.32
0.38
0.38
0.00
-0.18
0.60
45.7
49.3
--
19
51
29
25
56(3)
19
Clay
Key: g = isomer shift; AEQ = quadrnpole splitting; 2 % =
quadrupole shift; Bhf = hyperfine field, A~ = relative area of
the subspectrnm, without externally applied magnetic field,
A 2 = relative area under an external field of 0.2 T, M h =
maghemite, H m = hematite, Im = ilmenite.
-B
-4
0
+4
VELOCITY (mm/s)
§
Figure 3. M 6 s s b a u e r spectra of magnetic separates from the
sand at (a) RT and (b) 85 K; from the silt at (c) RT and (d)
85 K; and of the whole clay fractions at (e) RT and (f) 85 K.
G r a v e et al. 1 9 9 5 ) . F i g u r e 3 s h o w s t h e M 6 s s b a u e r
s p e c t r a at R T a n d at 85 K o f t h e m a g n e t i c s e p a r a t e s
o b t a i n e d f r o m s a n d a n d silt f r a c t i o n s a n d o f t h e w h o l e
c l a y f r a c t i o n . T h e s e s p e c t r a h a v e b e e n fitted as a s u p e r p o s i t i o n o f 2 s e x t e t s , r e l a t i v e to M h a n d H m , tog e t h e r w i t h 2 q u a d r u p o l e d o u b l e t s to a c c o u n t f o r ilm e n i t e . T h e R T s p e c t r a o f b o t h silt a n d c l a y f r a c t i o n s
r e q u i r e d a t h i r d d o u b l e t t h a t c o u l d b e r e l a t e d to a f r a c tion of Mh and/or Hm under superparamagnetic relaxation. T h e fitted s p e c t r a a r e n o t p e r f e c t , b u t t h i s w a s
expected considering the complex nature of these systems.
As mentioned above, only one sextet was included
to a c c o u n t f o r M h , b u t in r e a l i t y t h e r e a r e 2 c o n t r i b u t i o n s , o n e d u e to F e a t o m s o n t h e t e t r a h e d r a l s i t e s
a n d a n o t h e r o n e d u e to F e a t o m s o n t h e o c t a h e d r a l
o n e s . A n o t h e r s h o r t c o m i n g o f t h i s f i t t i n g p r o c e d u r e is
t h a t m o s t p r o b a b l y all c o m p o n e n t s s h o w a d i s t r i b u t i o n
o f h y p e r f i n e f i e l d s a n d q u a d r u p o l e s p l i t t i n g s d u e to t h e
i s o m o r p h i c s u b s t i t u t i o n b y A I a n d Ti. I n s p i t e o f t h a t ,
some semiquantitative conclusions can be drawn from
t h e r e s u l t s l i s t e d in T a b l e s 4 a n d 5. S o m e r o o m - t e m perature M6ssbauer parameters for bulk Mt, Hm, Mh
and Im were compiled by Murad and Johnston (1987),
B o w e n et al. ( 1 9 9 3 ) a n d M u r a d ( 1 9 9 6 ) , a n d a r e p r e s e n t e d in T a b l e 6 f o r c o m p a r a t i v e p u r p o s e .
Based on the values of the relative subspectral areas,
it is s e e n t h a t M h is i n d e e d t h e m a j o r c o m p o n e n t in
t h e s a n d a n d silt f r a c t i o n s , w h e r e a s H m d o m i n a t e s in
the clay fraction. The quadrupole shift of the latter
r e m a i n s u n c h a n g e d o n g o i n g f r o m R T to 85 K , i n d i c a t i n g t h a t t h i s H m is in t h e w e a k l y f e r r o m a g n e t i c
state, i.e., t h e r e w a s n o M o r i n t r a n s i t i o n . T h e m o r e
i n t e n s e a b s o r p t i o n in t h e c e n t r a l p a r t o f t h e s p e c t r u m
Table 5. M 6 s s b a u e r parameters at 85 K of the magnetic separates of the sand and silt fractions and of the whole clay.
Fraction
Subspectrum
?~/mm s 1
2%,AE~/
m m s -:
BJT
A/%
Sand
Mh
Hm
Im
0.43
0.50
1.14
0.41
0.00
-0.18
1.16
0.68
51.4
53.2
---
60
34
3
3
Silt
Mh
Hm
Im
0.42
0.51
1.18
0.45
0.00
-0.18
1.16
0.68
51.0
52.8
---
57
26
11
6
Clay
Mh
Hm
0.43
0.48
0.48
0.00
-0.18
0.68
49.0
52.7
--
18
77
5
Key: ~ = isomer shift, AEQ = quadrupole splitting; 2 % =
quadrupole shift, Bhf = hyperfine field, A = relative area.
374
Goulart et al.
Table 6. Room temperature MOssbauer parameters for bulk
Mt, Hm, Mh and Im (adapted Murad and Johnston (1987),
Bowen et al. (1993) and Murad (1996)).
Clays and Clay Minerals
l
I
I
I
I
I
I
I
I
I
I
2~QaEd
Mineral
Formula
Mt
Fe303
Hm
Mh
ctFe203
Im
FeTiO3
~/FezO3
8/mrn
s J
0.26
0.67
0.37
0.23
0.35
1.07
mm
s i
-----10.021
-<10.021
-0.20
--<10.021
-<10.021
0.68
B.dT
49.2
46.1
51.8
50.0
50.0
--
at 85 K o f the silt is attributed to the ferric I m
Fe~.~sTi0.8503, d e s c r i b e d b y G o u l a r t et al. (1994a).
T h e A1 substitution in the clay H m , e s t i m a t e d u s i n g
the equation:
Beff (RT) = 51.7 - 7.6 A1 - 3 2 / M C D
[2]
( M u r a d a n d S c h w e r t m a n n 1986) a n d c o n s i d e r i n g
M C D = 2 0 n m , is 11 m o l % . T h e c o r r e c t i o n s for smallparticle effects c a n b e n e g l e c t e d at 85 K, a n d the equation:
Berf = 532 - 0.39x
[3]
(De G r a v e et al. 1982) g i v e s x = 13 m o l % . B o t h resuits are in r e a s o n a b l e a g r e e m e n t w i t h that e s t i m a t e d
f r o m X R D , n a m e l y , x = 14 m o l % . S i m i l a r c a l c u l a t i o n
c o u l d n o t b e d o n e for the silt a n d s a n d H m d u e to its
s m a l l c o n t r i b u f i o n to the overall spectral area.
A s for the M h c o m p o n e n t , n o c o n c l u s i o n c a n b e
d r a w n w i t h r e s p e c t to the i s o m o r p h i c substitution. T h e
h y p e r f i n e fields listed in Tables 4 a n d 5 m u s t b e considered as a r o u g h e s t i m a t i o n , since they only represent the a v e r a g e b e t w e e n the A - a n d B-site c o n t r i b u tions. Also, the p r e s e n c e o f H m in large quantifies seriously influences the fitted p a r a m e t e r s . In all likelih o o d , t h e s e m e a s u r e m e n t s clearly s h o w that any Mt,
if p r e s e n t in t h e s e samples, r e p r e s e n t s only a m i n o r
fraction a n d that M h is b y far the m a i n f e r r i m a g n e t i c
component.
It h a s b e e n s h o w n that an e x t e r n a l m a g n e t i c field o f
0.2 T at R T w o u l d b e sufficient to i m p r o v e the s e m i q u a n t i t a t i v e e s t i m a t i o n o f the relative M 6 s s b a u e r abs o r p t i o n areas o f the 2 s u b s p e c t r a in synthetic m i x t u r e s
o f H m - M h ( G o u l a r t et al. 1994b). H o w e v e r , the results
p r e s e n t e d in Table 4 i n d i c a t e that the M 6 s s b a u e r analysis o f these natural iron o x i d e p h a s e s is only m o d erately i m p r o v e d u n d e r such an externally m o d e r a t e
applied m a g n e t i c field (Figure 4).
T h e influence o f the 4 x D C B t r e a t m e n t o n the
M 6 s s b a u e r spectra in an e x t e r n a l field o f 0.2 T o f the
s a n d a n d silt fractions are s h o w n in F i g u r e 4. T h e residual intensities o f lines 2 (rightside line relative to
the l e f t m o s t line 1) a n d 5 (leftside line relative to the
r i g h t m o s t line 6) are m a i n l y due to H m , as these absorptions w o u l d t e n d to a l m o s t v a n i s h in the case o f
pure M h with particle sizes o f the order o f t h o s e in
Figure 4. M6ssbauer spectra of the sand fraction magnetic
separate (a) before and (b) after 4 successive treatments with
DCB (4 • DCB), of the silt fraction (c) before and (d) after
4 x DCB and (e) of the whole clay fraction, all under an
externally applied magnetic field of 0.2 T.
the p r e s e n t study. T h e D C B - t r e a t e d s a m p l e s s h o w
m u c h l o w e r intensities o f lines 2 a n d 5 as c o m p a r e d
to the u n t r e a t e d samples, i n d i c a t i n g a d e c r e a s e o f the
H m content, w h i c h is c o n s i s t e n t w i t h the X R D results.
A s a general trend, an i n c r e a s e o f the M h / H m ratio in
b o t h s a n d a n d silt fractions is o b s e r v e d after D C B
treatment, i n f e r r i n g that this c h e m i c a l t r e a t m e n t is
preferentially r e m o v i n g H m . A similar result was earlier r e p o r t e d b y Taylor a n d S c h w e r t m a n n (1974). T h i s
preferential r e m o v a l o f H m c o u l d suggest that treating
the s a m p l e s a few m o r e t i m e s w i t h D C B w o u l d lead
to a h i g h e r e n r i c h m e n t in the ferrimagnefic c o m p o nent, b u t it was o b s e r v e d that s o m e M h is actually
attacked b y the D C B . T h i s is clearly seen f r o m the
saturation m a g n e t i z a t i o n values for the s a n d - m a g n e t i c
Vol. 46, No. 4, 1998
i
I
I
|
Iron oxides in a soil developed from basalt
w
i
I
v
w
i
i
J
I
100
|
i
i
IHm
I Mh A
IMhB
Table 7. M6ssbauer parameters of the magnetic separate of
the sand fraction under an external field of 6.0 T.
Hm
Mh
T(K)
273
t
(a)
BJT
49.2
50.2
51.2
53.3
AI%
24
43
26
45
s -1
BherF
AI%
0.33
-0.18
51.2
23
0.43
-0.20
53.7
26
~/mm
s -t
mm
98
!
100
o
I--
I
s -~
2,4
Key: ~ = isomer shift; 2~o = quadrupole shift; Bar = hyperfine field, A = relative area.
t<
r.
~/mm
0.21
0.33
4.2 0.31
0.44
9g
.o
375
9g
§
(b)
g8
g7
'-~0'-a -'e'-'4'-'2'6 '~ ' i '~ '8 ' ~ '
m y (ram/9)
Figure 5. M6ssbauer spectra of the sand fraction magnetic
separate at (a) 273 K and (b) 4.2 K under an externally applied magnetic field of 6 T.
separate before (35 J T -~ kg -t) and after D C B treatment (33 J T -~ kg-~), and for the corresponding siltmagnetic separate values (16 J T ~ kg -~ and 12 J T -1
kg -j, respectively).
In a stronger external field o f 6 T, the M h spectrum
splits into 2 subspectra relative to the tetrahedral and
octahedral sites, because the A-site hyperfine field
adds to and the B-site subtracts from the applied external field. At 4.2 K, no important superparamagnetic
relaxation effect is observed, and hence the assignment o f the hyperfine parameters for M h and H m can
be substantially improved. Two example spectra at 273
K and 4.2 K are displayed in Figure 5 for the sandfraction's magnetic separate. The presence o f 3 magnetic components is evident, plus a central absorption,
which is m o r e pronounced in the 273-K spectrum. N o
attempts were m a d e to fit the central absorption in the
273-K spectrum because it lacks any fine structure.
Probably this c o m p o n e n t is in fact a triplet since Im
is paramagnetic at that temperature. In the 4.2 K, this
central contribution almost disappeared, but it was not
found possible to include another sextet to account for
the magnetic splitting of the Im.
As for the 3 main sextets, the following restrictions
were used in the fitting procedure: pure Lorentzian-
shaped sextets with the isomer shift for H m coupled
to that of the B-site of Mh; line-area ratios as 3:x: 1
for M h and 3:3.6:1 for H m (De G r a v e et al. 1992);
quadrupole shifts ~Q = 0 for both sites of Mh. S o m e
numerical parameters derived from this fitting routine
are given in Table 7. Taking an average value of the
relative spectral area for Im from all spectra measured
without the strong field, the ratio o f the relative areas
M h : H m : I m is found to be 71:26:3. This ratio is in
disaccord with those derived from the zero-field measurements (RT and 85 K), showing that the latter measurements are not adequate to fully characterize this
kind of material. Furthermore, e v e n the hyperfine
fields are affected by a large uncertainty due to the
strong overlap of all 3 subspectra. Thus, any attempt
to correlate the low values of these hyperfine fields
with isomorphic substitution would only have a qualitative meaning. A difference of about 0.12 m m s -~
was found both at 4.2 K and 275 K, in agreement with
the values reported by da Costa, De Grave, Vandenberghe et al. (1994) and da Costa, De Grave, B r y a n
and B o w e n (1994).
Another important observation is that the high intensities of the middle lines of the hematite c o m p o n e n t
(3:3:4:1), together with a negative value for the quadrupole shift, indicate that this oxide is in the weakly
ferromagnetic state, e v e n at 4.2 K (De G r a v e et al.
1992; B o w e n and De G r a v e 1995). The suppression
o f the Morin transition is presumably due to the presence of A1 and/or Ti in the H m structure, to which this
transition is extremely sensitive. A 1% Ti-substitution
is sufficient to eliminate the antiferromagnetic state
(Ericsson et al. 1986), while approximately 9% o f A1
is required to create the same effect (Coey 1988).
Based on the A1 content in H m and M h as deduced
from X R D , and on the proportions o f these 2 phases
derived from the in-field M6ssbauer spectra, the overall allocation gives about 5.3 mass% A1203 in the sand
fraction. This result is in good agreement with the total
5.50 mass% A1203 found in the c h e m i c a l analysis (Table 1). Diagenetic H m is likely to retain only trivalent
structural cations in its structure. Di and tetravalent
cations are expelled from the lattice on passing f r o m
M h to H m (Sidhu et al. 1980), an expected process in
376
Goulart et al.
s o i l - m i n e r a l genesis. Thus, d i s c a r d i n g a n y e v e n t u a l Ti
in the H m , the M h w o u l d c o n t a i n 7 m a s s % TiO2, taking into a c c o u n t that 1.3 m a s s % TiO2 m u s t b e alloc a t e d to the ferric Im. T h e c o m b i n e d effect o f b o t h A1
a n d Ti c a n e x p l a i n the a b s e n c e o f the M o r i n transition
d o w n to 4.2 K.
T h e q u e s t i o n n o w is h o w to distribute Fe, A1, Ti
a n d v a c a n c i e s b e t w e e n the 2 sublattices o f M h . T h e
A - to B-site area ratio as d e r i v e d f r o m the 6 T M 6 s s b a u e r spectra is 37:63, but this ratio m a y b e u n d e r e s t i m a t e d as H m lines o v e r l a p w i t h t h o s e o f A-site. Several r e c e n t studies h a v e s h o w n that AI h a s n o site preference, its l o c a t i o n b e i n g d e p e n d e n t o n the synthesis
p r o c e d u r e a n d o n its c o n c e n t r a t i o n . Actually, there are
reports in w h i c h all AI are located e x c l u s i v e l y o n the
A sites (da Costa, D e G r a v e , B r y a n a n d B o w e n 1994),
o n the B sites (de Jesus Filho et al. 1992; A l l a n et al.
1989; B o w e n et al. 1994) or on b o t h sites (Wolska a n d
S c h w e r t m a n n 1989; d a Costa, L a u r e n t et al. 1995). In
the p r e s e n t case, f r o m F e distribution, it m a y b e occ u p y i n g only A-sites. O n the o t h e r h a n d , it is k n o w n
that Ti h a s a strong B-site p r e f e r e n c e ( W a y c h u n a s
1991). A s for the vacancies, it is usually a s s u m e d that
in relatively w e l l - c r y s t a l l i z e d M h , w h i c h is the case
for the p r e s e n t s a m p l e s , they are located o n B-sites
( L i n d s l e y 1976; B o w e n et al. 1994). T h e s u g g e s t e d
s t o i c h i o m e t r i c spinel f o r m u l a , b a s e d u p o n the d e d u c e d
e l e m e n t a l c o m p o s i t i o n o f A1203 (8%) a n d TiO2 ( 7 % )
a n d FezO 3 ( 8 5 % , the c o m p l e m e n t to 100%) for the
s a n d - M h , a n d a s s i g n i n g AI o n A a n d Ti a n d v a c a n c i e s
o n the B site is:
[Fe092A1008] {Fel.43Ti0.t 8[~0.39}04
([--] = v a c a n c y , [ ] = tetrahedral sites a n d { } = oct a h e d r a l sites), w i t h a c o r r e s p o n d i n g m o l a r m a s s of
208.8 g mo1-1.
T h e saturation m a g n e t i z a t i o n o f the s a n d m a g n e t i c
separate is 35 J T 1 k g - i , w h i c h gives a r o u n d 49 J T -~
k g -] for the soil M h . T h e a b o v e f o r m u l a c o r r e s p o n d s
to a m a g n e t i z a t i o n o f 68 J T -~ kg -1, a s s u m i n g a m o m e n t o f 5 IxB o n the Fe ions a n d a c o l l i n e a r ferrimagnetic spin structure. Several effects c a n c o n t r i b u t e to
the d i s c r e p a n c y b e t w e e n the d e t e r m i n e d m a g n e t i z a t i o n
(49 J T ] kg -1) relative to the predicted v a l u e f r o m the
allocated f o r m u l a , as for e x a m p l e , small-particle sizes
(MCrup a n d Tops~e 1976), inter- a n d intra-particle int e r a c t i o n s ( M ~ r u p and Tronc 1994), surface effects
( B o u d e u l l e et al. 1983) a n d spin r e d u c t i o n due to cov a l e n c e ( G r e a v e s 1983). Spin c a n t i n g h a s an i m p o r t a n t
influence o n the m a g n e t i z a t i o n o f r a n d o m l y substituted
spinel ferrites ( C o e y 1987). In addition, the site occ u p a n c y a n d the a m o u n t o f substitution are not k n o w n
w i t h great precision, m a k i n g it e x t r e m e l y difficult to
d e r i v e the true c o m p o s i t i o n . M u c h s t r o n g e r e x t e r n a l
fields, o f the o r d e r o f 15 T, w o u l d b e v e r y useful in
o r d e r to c o m p l e t e l y separate the A - a n d B-site contrib u t i o n s o f M h f r o m that o f H m .
Clays and Clay Minerals
CONCLUSIONS
T h e p r e s e n t study o n the m i n e r a l o g y o f a B r a z i l i a n
d u s k y r e d m a g n e t i c O x i s o l w a s f o c u s e d o n the characterization o f the F e - b e a r i n g m i n e r a l s o f the m a g n e t i c
separate f r o m the sand a n d silt fractions. H e m a t i t e is
m a i n l y p r e s e n t in the w h o l e clay fraction, t o g e t h e r
w i t h G b , Ka, Rt (and p r o b a b l y A n ) a n d Mh. M a g h e mite, w h i c h c o n t r i b u t e s b y a b o u t 6 % to the w h o l e soil
mass, is the d o m i n a n t m i n e r a l in the m a g n e t i c separate
(in sand, a b o u t 6 5 % o f the relative M ~ s s b a u e r spectral
area; silt, 60%). H e m a t i t e (sand, 30%; silt, 15%) a n d
I m (sand, 5%; silt, 16%) are also significantly p r e s e n t
in the silt fraction. A c c e s s o r y m i n e r a l s are R t and An.
N o M t was detected in the m a g n e t i c extract o f a n y soil
fraction. Sand- and s i l t - M h h a v e similar cell p a r a m e ters (a0 = 0.8319 -+ 0.0005 n m ) a n d saturation m a g n e t i z a t i o n s o f 49 J T 1 kg ~. H e m a t i t e , an associated
p h a s e in clay, was also d e t e c t e d in the s a n d - m a g n e t i c
separate. T h e A1 c o n t e n t in H m varies f r o m 14 m o l %
(clay- a n d silt-Hm) to 20 m o l % (sand). T h e p r o p o s e d
cation distribution a m o n g the spinel sites o f the soilM h leads to a n e x p e c t e d m a g n e t i z a t i o n ~ ~- 82 J T ]
k g 1, a s s u m i n g c o l l i n e a r spin a r r a n g e m e n t .
ACKNOWLEDGMENTS
This paper is dedicated to the memory of Professor M. E
de Jesus Filho. The authors are indebted to D. Prudente Santana (CNPMS/EMBRAPA) and N. Curi (ESAL) for their help
in selecting and collecting samples and to G. Pereira Santana
(postgraduate student at Department of Chemistry, UFMG,
Brazil) for his technical support on the M~ssbauer measurements. We are also indebted to M. B. Harmendani Vieira,
Technology Centre, Vale do Rio Doce Co., Santa Luzia, MG,
Brazil, for the optical microscope analysis work financially
supported by CNPq, FINEP and Fapemig (Brazil), ISC-European Commission (CT90-0856) and the Fund for Joint Basic Research, Belgium (G 2014.93).
REFERENCES
Abreu MM, Robert MN. 1985. Characterization of maghemite in B horizons of three soils from Southern Portugal.
Geoderma 56:97-108.
Allan JEM, Coey JMD, Sanders IS, Schwertmann U, Friedrich G, Wichowski A. 1989. An occurrence of a fully oxidized natural titanomaghemite in basalt. Mineral Mag 53"
299-304.
Anand RR, Glides RJ. 1984. Mineralogical and chemical
properties of weathered magnetite grains from lateritic saprolite. J Soil Sci 35:559-567.
Boudeulle M, Batis-Landoulsi H, Leclerq CH, Vergnon P.
1983. Structure of 3,Fe203 microcrystals: Vacancy distribution and structure. J Solid State Chem 48:21-32.
Bowen LH, De Grave E. 1995. M~Sssbauer spectra in external
field of highly substituted aluminous hematites. J Magn
Magn Mater 139:6-10.
Bowen LH, De Grave E, Bryan AM. 1994. Mt~ssbauer studies
in external field of well crystallized Al-maghemites made
from hematite. Hyperfine Interact 94:1977-1982.
Bowen LH, De Grave E, Vandenberghe RE. 1993. M~Sssbauer
effect studies of magnetic soils and sediments. In: Long
GJ, Grandjean E editors. M6ssbauer spectroscopy applied
to magnetism and material science. New York: Plenum Pr.
p 115-159.
Vol. 46, No. 4, 1998
Iron oxides in a soil developed from basalt
Coey JMD. 1987. Noncollinear spin structures. Can J Phys
65:1210-1232.
Coey JMD. 1988. Magnetic properties of iron in soil iron
oxides and clay minerals. In: Stucki JW, Goodman BA,
Schwertmann U, editors. Iron in soil and clay minerals.
Dordrecht: Reidel. p 397-465.
Coey JMD, Cugat O, McCauley J, Fabris JD. 1992. A portable soil magnetometer. Revista de Ffsica Aplicada e Instrumenta~o 7:25-30.
da Costa GM, De Grave E, de Bakker PMA, Vandenberghe
RE. 1995. Influence of nonstoichiometry and the presence
of maghemite on the M6ssbauer spectrum of magnetite.
Clays Clay Miner 43:656-668.
da Costa GM, De Grave E, Bryan AM, Bowen LH. 1994.
M6ssbauer studies of nano-sized aluminium-substituted
maghemites. Hyperfine Interact 94:1983-1987.
da Costa GM, De Grave E, Vandenberghe RE, Bowen LH,
de Bakker PMA. 1994. The center shift in the M6ssbauer
spectra of maghemite and aluminium maghemites. Clays
Clay Miner 42:628-633.
da Costa GM, Laurent CH, De Grave E, Vandenberghe RE.
1995. A comprehensive M6ssbauer study of highly-substituted aluminum maghemites. Mineral spectroscopy: A tribute to R. G. Burns. Dyar MD, McCammon C, Schaefer
MW, editors. Geochem Soc Spec Publ 5:93-104.
Curi N, Franzmeier DE 1987. Effect of parent rocks on chemical and mineralogical properties of some oxisols in Brazil.
Soil Sci Soc A m J 51:153-158.
De Grave E, de Bakker PMA, Bowen LH, Vandenberghe RE.
1992. Effect of crystallinity and A1 substitution on the applied-field M6ssbauer spectra of iron oxide and oxyhydroxides. Z Pflanzenern~ihr Bodenk 155:467-472.
De Grave E, Bowen LH, Weed SB. 1982. M6ssbauer study
of aluminum-substituted hematites. J Magn Magn Mater
27:98-108.
Dematt6 JLI, Marconi A. 1991. A drenagem na mineralogia
de solos desenvolvidos de diab~isio em Piracicaba. R Bras
Ci Solo 15:1-8.
Ericsson T, Krisnhamarthy A, Srivastava, BK. 1986. Morin
transition in Ti-substituted hematite: A M6ssbauer study.
Phys Scripta 33:88-90.
Fabris JD, Coey JMD, Qinian Qi, Mussel W da N. 1995.
Characterization of Mg-rich maghemite. A m Mineral 80:
664-669.
Ferreira BA. 1995. Caracteriza~o fisico-qufmica de minerals
ferruginosos de um pedossistema desenvolvido de basalto
[MSc thesis]. Belo Horizonte, Brazil: Federal Univ of Minas Gerals. 99 p.
Ferreira SAD, Santana DE Fabris JD, Curi N, Nunes Filho
E, Coey JMD. 1993. Rela~6es entre magnetizaqao, elementos tra9os e litoiogia de duas sequ~ncias de solos do Estado
de Minas Gerais. Rev Bras Ci Solo 18:167-174.
Fine P, Singer MJ. 1989. Contribution of ferrimagnetic minerals to oxalate- and dithionite-extractable iron. Soil Sci
Soc A m J 53:191-196.
Fontes MPF, Weed SB. 1991. Iron oxides in selected Brazilian
Oxisols. I: Mineralogy. Soil Sci Soc A m J 55:1143-1149.
Gillot B, Rousset A. 1990. On the limit of aluminum substitution in Fe304 and "yFe20~. Phys Status Solidi (a) 118:K5K8.
Goulart AT, de Jesus Filho ME Fabris JD, Coey JMD. 1994a.
Characterization of a soil ilmenite developed from basalt.
Hyperfine Interact 91:771-775.
Goulart AT, de Jesus Filho ME Fabris JD, Coey JMD. 1994b.
Quantitative M6ssbauer analysis of maghemite-hematite
mixtures in external applied field. Hyperfine Interact 83:
451-455.
377
Greaves C. 1983. A powder neutron diffraction investigation
of vacancy ordering and covalence in ~/Fe203. J Solid State
Chem 49:325-333.
Guggenheim S, Martin RT. 1995. Definition of clay and clay
mineral: Joint report of the AIPEA nomenclature and CMS
nomenclature committees. Clays Clay Miner 43:255-256.
Jackson ML. 1969. Soil chemical a n a l y s i s - - A d v a n c e d
course. Madison, WI: ML Jackson. 894 p.
Jeffery PG, Hutchison D. 1981. Chemical methods of rock
analysis. Oxford: Pergamon Pr. 379 p.
Jensen LS. 1976. A new cation plot for classifying subalkalic
volcanic rocks. Ontario: Department of Mines, Misc Paper
66.
de Jesus Filho ME Fabris JD, Goulart AT, Coey JMD, Ferreira BA, Pinto MCE 1995. Ilmenite and magnetite of a
tholeiitic basalt. Clays Clay Miner 43:641-642.
de Jesus Filho ME Mussel W da N, Qinian Qi, Coey JMD.
1992. Magnetic properties of aluminium-doped ~Fe203. In:
Yamaguchi T, Abe M, editors. Proc 6th Int Conf on Ferrites
(ICF 6); The Japan Society of Powder and Powder Metallurgy; Tokyo, Japan. p 126-128.
Klug HE Alexander LE. 1974. X-ray diffraction procedures
for polycrystalline and amorphous materials. New York: J.
Wiley. 966 p.
Lindsley DH. 1976. The crystal chemistry and structure of
oxide minerals as exemplified by the Fe-Ti oxides. Rev
Mineral 3:L1-L60.
Mehra OP, Jackson ML. 1960. Iron oxide removal from soils
and clay a dithionite-citrate system buffered with sodium
bicarbonate. Clays Clay Miner 7:317-327.
Moniz AA. 1972. Elementos de pedologia. Sao Paulo: Editora
Poligono. 459 p.
M~rup S, Tops~e H. 1976. M6ssbauer studies of thermal excitations in magnetically ordered microcrystals. Appl Phys
11:63-66.
MOrup S, Tronc E. 1994. Superparamagnefic relaxation of
weakly interacting particles. Phys Rev Lett 72:3278-3281.
Moukarika A, O'Brien E Coey JMD, Resende M. 1991. Development of magnetic soil from ferroan dolomite. Geophys Res Lett 18:2043-2045.
Murad E. 1996. Magnetic properties of microcrystalline
iron(III) oxides and related materials as reflected in their
M6ssbauer spectra. Phys Chem Minerals 23:248-262.
Murad E, Johnston JH. 1987. Iron oxides and oxyhydroxides.
In: Long GJ, editor. M6ssbauer spectroscopy applied to inorganic chemistry, vol. 2. New York: Plenum Pr. p 5 0 7 582.
Murad E, Schwertmann U. 1986. Influence of A1 substitution
and crystal size on the room temperature M6ssbauer spectrum of hematite. Clays Clay Miner 34:1-6.
Neves AA, Goulart AT, Garotti FV. 1985. Caracterizaqfio da
interfer~ncia da platina, na amilise de ferro em amostras
naturals. Qufmica Nova 8:152-154.
Pinto MCE 1997. Caracterizaqfio e establidade ligogen6tica
da magnetica de rochas m~ificas [MSc thesis]. Belo Horizonte, Brazil: Federal Univ of Minas Gerais. 75 p (in Portuguese).
Readman PW, O'Reilly W. 1972. Magnetic properties of oxidized (cation-deficient) titanomagnetites (FeTi_)304. J
Geomagn Geoelect 24:69-90.
Resende M. 1976. Mineralogy, chemistry, morphology and
geomorphology of some soils of the central plateau of Brazil [Ph.D. thesis]. Purdue Univ. Diss Abstracts 37:4803.
Schobbenhaus C, Campos D de A, Derze GR, Asmus HE.
1984. Formaq~o Mata da Corda (kmc). In: Minist6rio das
Minas e Energia/DNPM. Geologia do Brasil: Texto explicativo do mapa geol6gico do Brasil e da ~irea oceanica
adjacente incluindo dep6sitos minerals. Brasflia. p 348.
378
Goulart et al.
S c h w e r t m a n n U, Fechter H. 1984. The influence of a l u m i n u m
on iron oxides: XI. Aluminum-substituted in soils and its
formation. Soil Sci Soc A m J 48:1462-1463.
Sidhu PS, Gilkes RJ, Posner A M . 1980. The behavior of Co,
Ni, Zn, Cu, Mn, and Cr in magnetite during alteration to
m a g h e m i t e and hematite. Soil Sci Soc A m J 44:135-138.
Singer MJ, B o w e n LH, Verosub KL, Fine P, TenPas J. 1995.
M o s s b a u e r spectroscopic evidence for citrate-bicarbonatedithionite extraction of m a g h e m i t e from soils. Clays Clay
Miner 43:1-7.
Stanjek, H, S c h w e r t m a n n U. 1992. The influence of alumin u m on iron oxides. Part XVI: Hydroxyl and a l u m i n u m
substitution in synthetic hematites. Clays Clay Miner 40:
347-354.
Clays and Clay Minerals
Taylor RM, Schwertmann U. 1974. M a g h e m i t e in soils and
its origin. 1. Properties and observation on soil m a g h e mites. Clays Clay Miner 10:289-298.
W a y c h u n a s GA. 1991. Cystal chemistry of oxides and oxhydroxides. In: Lindslay GH, editor. Rev Mineral 25, Oxide
minerals: Petrologic and magnetic significance. Mineral
Soc A m . p 12-68.
Wolska E, Schwertmann U. 1989. The vacancy ordering and
distribution of a l u m i n u m ions in (Fe,A1)203. Solid State
Ionics 32/33:214-218.
(Received 28 June 1996; accepted 13 August 1997; Ms.
2786)