Clays and Clay Minerals, Vol.44, No. 5, 658-664, 1996.
T H E N A T U R A L O C C U R R E N C E OF E T A - A L U M I N A ('q-AlzO 3) IN B A U X I T E
DAVID B. TILLEY AND RICHARD A. EGGLETON
Centre for Australian Regolith Studies, Australian National University, Canberra, ACT, 0200, Australia
Abstract--Approximately 20 wt.% of the bauxite from Andoom in northern Queensland, Australia is
composed of material that cannot be accounted for by identifiable well-crystallized phases. This poorlydiffracting material (PDM), found within the core of bauxitic pisoliths, has similar characteristics to that
of eta-alumina (-q-A1203); a cubic form of alumina. A differential XRD pattern of the PDM displayed a
series of broad diffraction maxima attributed to eta-alumina with a mean crystal size of 9 nm. Unit cell
refinement, on the basis of a cubic cell, gave a lattice parameter of a = 7.98 A for Andoom eta-alumina.
TEM and selected-area electron diffraction revealed the PDM to be composed of minute (10 nm wide),
randomly oriented crystals of eta-alumina in close association with Al-hematite. Chemical analysis using
a nanoprobe showed Andoom eta-alumina to be almost pure alumina with < 2 M% Fe, < 1 M% Si and
< 1 M% Ti. The closely associated Al-hematite may contain as much as 22 M% A1, however a value
closer to the theoretical limit of 17 M% is more likely. A broad absorption band at 3450 cm -1 and 1630
cm 1 in the infra-red spectrum of the PDM indicates the presence of a substantial quantity of I-I/O, strongly
adsorbed onto the surface of the crystals. This is presumably due to "q-AlzO3's large surface area of
approximately 2200 m2/g. The natural occurrence of "q-A1203 in bauxite may be the result of low H20
activities within the micro-environment of pores at the time of crystallization. The epigenetic replacement
of kaolinite with "q-A1203 and Al-hematite is put forward as an explanation for the formation of bauxitic
pisoliths at Andoom.
Key Words----Akdalaite, Bauxite, Chi-alumina, Eta-alumina, Gamma-alumina, Pisoliths, Poorly-diffracting material, Tohdite.
INTRODUCTION
Two c r y s t a l l i n e a l u m i n a p h a s e s h a v e r e c e n t l y b e e n
r e p o r t e d in A u s t r a l i a n bauxites. Tilley a n d E g g l e t o n
(1994) r e p o r t e d that b a u x i t i c pisoliths f r o m Weipa,
N o r t h Q u e e n s l a n d c o n t a i n e d material w h i c h yielded
v e r y b r o a d diffraction m a x i m a that c o u l d n o t b e acc o u n t e d for b y identifiable w e l l - c r y s t a l l i z e d minerals.
Such material, termed poorly-diffracting material
( P D M ) , was s h o w n to b e c o m p o s e d p r e d o m i n a n t l y o f
tohdite (5A1203.H20). A r e c e n t letter f r o m the International Mineralogical Association Commission on
N e w M i n e r a l s a n d M i n e r a l N a m e s ( C N M M N ) disc o u r a g e d the use o f the n a m e " t o h d i t e " in any prop o s e d p u b l i c a t i o n s due to it b e i n g an i n v a l i d m i n e r a l
n a m e . T h e n a m e akdalaite (4AI2Oa.H20) was a p p r o v e d
b y the C N M M N for the m i n e r a l c o r r e s p o n d i n g to tohdite, due to their similarity, a n d as a c o n s e q u e n c e ,
W e i p a tohdite will n o w b e r e f e r r e d to as akdalaite.
S i n g h a n d Gilkes (1995) r e p o r t e d the o c c u r r e n c e o f
p o o r l y c r y s t a l l i n e chi a l u m i n a (•
in the b a u x ites o f D a r l i n g R a n g e , W e s t e r n Australia. T h e authors
d e m o n s t r a t e d that x-A1203 f o r m e d b y the d e h y d r a t i o n
o f a n a l u m i n o u s gel-like material. E x a m i n a t i o n o f
b a u x i t i c pisoliths f r o m A n d o o m , w h i c h is 15 k m n o r t h
o f Weipa, h a s r e v e a l e d the p r e s e n c e o f a n o t h e r poorlydiffracting phase, distinct f r o m akdalaite a n d x-A1203.
Differential X R D a n d t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y ( T E M ) o f the P D M s h o w e d that the p h a s e has
similar characteristics to that o f "q-Al203. S t u m p f et al.
(1950) d e s c r i b e d e t a - a l u m i n a as h a v i n g a cubic, spin e l - t y p e structure w i t h a lattice p a r a m e t e r o f 7.94 A.
Copyright 9 1996, The Clay Minerals Society
658
Selected area electron diffraction o f synthetic "q-A1203
b y L i p p e n s a n d de B o e r (1964) r e p o r t e d it to b e tetragonally d e f o r m e d w i t h c/a r a n g i n g b e t w e e n 0.985
a n d 0.993. Z h o u a n d S n y d e r (1991) d e m o n s t r a t e d that
the crystal structure o f ~q-AlzO3 is essentially cubic,
h o w e v e r s o m e w h a t spinel d e f o r m e d w i t h a = 7.914
A. B r o w n et al. (1953) s h o w e d that "q-A1203 is a transition p h a s e p r o d u c e d b y the t h e r m a l d e c o m p o s i t i o n
o f poorly c r y s t a l l i n e b o e h m i t e ( A 1 0 ( O H ) ) or wellcrystallized b a y e r i t e ([3-Al(OH)3). F u r t h e r h e a t i n g o f
"q-A1203 leads to the f o r m a t i o n o f t h e t a - a l u m i n a (0A1203) f o l l o w e d b y c o r u n d u m (ct-A1203).
EXPERIMENTAL METHODS
T h e core o f a b a u x i t i c pisolith f r o m A n d o o m (1 to
1.5 m depth) w a s s a m p l e d u s i n g a dental drill. T h e
extracted material w a s finely g r o u n d u s i n g a m o r t a r
and pestle a n d t h e n applied to a l o w - b a c k g r o u n d holder. A n X R D p a t t e r n o f the s a m p l e was o b t a i n e d b y
s c a n n i n g o v e r a p e r i o d o f 12 h. A differential X R D
pattern was p r o d u c e d b y s u b t r a c t i n g the calculated
pattern for e a c h o f the w e l l - c r y s t a l l i z e d c o m p o n e n t s ,
in p r o p o r t i o n to t h e i r w e i g h t fraction. T h e resulting
differential X R D p a t t e r n was c o m p a r e d w i t h m i n e r a l s
a n d i n o r g a n i c c o m p o u n d s in the P o w d e r D i f f r a c t i o n
File p u b l i s h e d b y the J o i n t C o m m i t t e e o n P o w d e r Diffraction Standards. W h e n it was f o u n d that the d-spacing values o f the b r o a d e s t p e a k s in the X R D pattern
m a t c h e d well w i t h t h o s e o f eta-alumina, q u a n t i t a t i v e
analysis o f the X R D pattern was u n d e r t a k e n u s i n g
S I R O Q U A N T , w h i c h is a R i e t v e l d - t y p e quantitative-
Vol. 44, No. 5, 1996
2000
Eta-alumina in bauxite
659
,,,,,,,,,i,,,,,,,,,i,,,,,,,,,i,,,,,,,,,i,,,,,,,,,I,,,,,,,,,I,,,,,,,,,
h
h
1500-
t'-
= 1000-
g
o
0
h
k
g
h
a r
h
h
h
500 -
0
h
lilll 'lllrLllJi]llll
10
20
h
J'llllllllilllliLIlillllfil'~llliltlJlLil'iJIIJll
30
40
~
50
CuKo
60
70
80
Figure 1. An XRD of PDM-rich bauxite extracted from the core of a pisolith from Andoom in northern Queensland. Key:
g = gibbsite, h = hematite, k = kaolinite, a = anatase and r = rntile.
X R D phase analysis program (Taylor and Clapp 1992).
S I R O Q U A N T uses the cubic (Fd3m) structure determined by Shirasuka et al. (1976) when calculating an
X R D pattern for eta-alumina. Refinement o f the unit
cell parameter from a starting value of a = 7.906
was undertaken during the analysis.
The same p o w d e r that was used for X R D , was ultrasonically dispersed in ethanol and then pipetted onto
a holey carbon-coated grid. T E M was p e r f o r m e d on
the sample using a J E O L 2 0 0 C X transmission electron
microscope, operating at 200 kV. Selected-area electron diffraction ( S A E D ) patterns of bauxite particles
were obtained at a camera length of 74 cm. An S A E D
pattern displaying a series of diffraction rings was indexed and c o m p a r e d to X R D and calculated d-spacing
values.
Analytical T E M ( A E M ) was undertaken using a
nano-probe with a b e a m diameter o f 10 n m in conj u n c t i o n with e n e r g y d i s p e r s i v e X - r a y analysis
( E D X A ) on a Philips 4 3 0 E M transmission electronmicroscope, operating at 300kV.
An infra-red (IR) spectrum of the P D M was obtained using a Perkin E l m e r 1800 F T I R spectrophotometer.
RESULTS
The X R D pattern (Figure 1) displays relatively
sharp diffraction m a x i m a associated with well-crystallized hematite, gibbsite, anatase, rutile, kaolinite and
quartz while broad diffraction m a x i m a are attributed
to PDM.
The procedure for obtaining a differential X R D pattern was reasonably effective in r e m o v i n g the well defined peaks associated with gibbsite, hematite and other well-crystallized phases, e v e n though s o m e residual
peaks are still evident. The differential X R D pattern
displays a series of broad diffraction maxima, similar
in intensity and breadth to Weipa akdalaite, but the
diffraction lines do not coincide with those of Weipa
akdalaite (Figure 2). The broad peaks have m a x i m a
that match well with those of -q-A1203 (Stumpf et al.
1950) (Table 1). The 2.0 .~ peak has a width (at half
m a x i m u m ) o f 1.31 ~
which gives a m e a n crystallite
660
Clays and Clay Minerals
Tilley and Eggleton
1400t''''
I''''
I'''' < I''''
1200
rl -alumina
,~
<
-
o,i
1000
,<
r
t-"
0
tO
800
o<
-
~c.o
600 ~
'~
400-1
I''''
I''''
~
o
o<
oo
i''''
E
o~
h ~
cJ
c,i
~
h
~
~_
"..i.
-
ill
~,
200
0
akdalaite
I
,
,
,,
10
I
,
I
20
,,
I,
,,
30
,
1,
,,
,
I
,
40
50
~
CuKo~
,,
,
I'
,
,
60
J
I
70
'
~
''
I
80
Figure 2. A differential XRD pattern of Andoom -q-A1203 compared with Weipa akdalaite. Under- and over-calculated
hematite peaks (h) are labeled.
size o f 9 n m w h e n u s i n g the S c h e m e r equation. Q u a n titative X R D u s i n g S 1 R O Q U A N T indicates that the
core o f the a n a l y z e d pisolith c o n t a i n s greater t h a n 70
wt.% xi-A1203 (Table 2 a n d F i g u r e 3). U n i t cell refine-
Table 1. Comparison between the d-spacing values of the
PDM determined by electron diffraction, differential XRD
and those calculated by Stumpf et al. (1950).
PDM
electron diffraction
I/I~t
20
80
20
90
20
100
10
20
10
10
10
dA
2.79
2.40
2.28
1.96
1.52
1.39
1.19
1.13
0.98
0.88
0.80
PDM
differential XRD
.q-alumina
(Stumpf et al. 1950)
UII~
d A
Ijl I
d/~
hkl
10
30
80
20
90
20
100
4.62
2.82
2.45
2.29
2.01
1.55
1.41
40
20
60
30
80
20
100
10
20
10
4.6
2.8
2.40
2.27
1.97
1.52
1.40
1.21
1.14
1.03
111
220
311
222
400
333
440
533
444
553
t Intensities by visual estimation.
merit o f A n d o o m -q-A1203, o n the basis o f a c u b i c cell,
gives a lattice p a r a m e t e r o f a = 7.98 ,~.
E x t r e m e l y fine crystals o f a p p r o x i m a t e l y 10 n m in
w i d t h are e v i d e n t in the T E M i m a g e (Figure 4). T h o s e
crystals that h a v e lattice fringes o f 2.3 a n d 2.4 ,~ are
associated with "q-A1203. T h e crystallite size o f - q A1203 d e t e r m i n e d f r o m the T E M i m a g e is close to that
calculated u s i n g the S c h e r r e r e q u a t i o n ( ~ 9 n m ) a n d is
slightly larger t h a n that o f W e i p a akdalaite ( ~ 6 nm).
T h e d - s p a c i n g values o b t a i n e d f r o m the S A E D pattern
(Figure 5) c o i n c i d e well w i t h the X R D pattern o f the
p o o r l y - d i f f r a c t i n g material, i n d i c a t i n g that the finely
Table 2. quantitative XRD analysis of the PDM-rich material
using SIROQUANT.
Phase
Wt%
Error %
eta-alumina
Al-hematite
gibbsite
kaolinite
rutile
quartz
anatase
71.6
16.7
8.6
0.7
1.6
0.3
0.5
0.26
0.18
0.18
0.11
0.07
0.04
0.05
Vol. 44, No. 5, 1996
Eta-alumina in bauxite
I
4000
I
I
661
I
[
I
30002000cO
0
1000-
_
calculated
_
difference
10
Figure 3.
I
I
20
30
I
I
40
50
o20 CuKo
I
I
60
70
80
Rietveld-type quantitative-XRD analysis of the bauxite sample containing approximately 70 wt.% -q-A1203.
crystalline material observed using TEM is the same
material identified in the X R D pattern.
A E M of randomly selected crystals in the PDM-rich
sample revealed the presence of 2 mineral phases: 1)
an aluminium-bearing ferruginous phase; and 2) an almost pure aluminous phase (Table 3). The most ferruginous particle analyzed contained 22 M% AI while
the most aluminous one had < 2 M% Fe.
A transmittance infra-red spectrum of the material
containing approximately 70 wt.% PDM, shows a
broad absorption band at 3450 cm -~ due to O-H
stretching and at 1630 cm -I associated with O-H bending vibrations (Figure 6).
DISCUSSION
The XR D patterns of synthetic eta- and gamma-alumina are known to very similar (Spider and Pollack
1981; Zhou and Snyder 1991). However, there are
small but measurable differences that can be used to
distinguish between the 2 phases. The most useful discriminator is at approximately 1.98 _~, where -q-AlzO3
has 1 strong line while ~-A120 ~ has a doublet consisting of a strong line at 1.98 .~ and a strong shoulder at
1.95 A. We conclude that PDM from Andoom is com-
posed of -q-A1203 rather than ~/-A1203 because although
slightly asymmetric, the 1.98 .~ peak appears single.
Though as yet undiscovered, the existence of ~/-A1203
in bauxites is highly probable.
The aluminium-bearing ferruginous phase analyzed
by A E M is ascribed to Al-hematite while the almost
pure aluminous phase is attributed to "q-A1203. A EM
indicates the Al-hematite may contain up to 22 M%
A1. Unit cell refinement of the hematite gave cell parameters of a, b = 5.0118 A and c = 13.7120 .~,
indicating an A1 substitution of between 20 and 23
M% (Schwertmann 1988a; Stanjek 1991), assuming a
formation temperature of between 25 and 40 ~ However, Schwertmann (1988b) reported that no soil hematites have been found that contain greater than 18
M% AI. In fact, Al-substitution in hematite may be
limited to 76 of the possible octahedral positions, that
is, 17 M% A1 (Schwertmann 1988a). Although every
effort was made to ensure that the analyses were obtained from individual crystals, it is possible that characteristic AI X-rays from an adjacent alumina grain
may have led to an apparently high A1 content for the
hematite. Due to this inconclusiveness, a value of 18
M% A1 was chosen for hematite's site occupancy. A
662
Tilley and Eggleton
Clays and Clay Minerals
Figure 4. A TEM of one particular area in the poorly diffracting material. The crystal size of the PDM is approximately 10
nm. Lattice fringes of 2.3 and 2.4 ~ indicate -q-AI203.
close match between the observed and calculated X R D
patterns was obtained using this value. Quantification
of the bauxite's mineralogy was also performed using
the same site occupancy.
Figure 5.
An SAED pattern of Andoom "q-A1203.
The H20 contained within the crystal lattices of
gibbsite is 34.6 wt.%, boehmite is 15.0 wt.%, and akdalaite is 4.4 wt.%. The theoretical crystalline structure of ~-A1203 contains no chemically-bonded water.
B o t h akdalaite and a3-A1203 contain a p p r e c i a b l e
amounts of strongly adsorbed H20 as evidenced by
their transmittance IR spectra, but the actual amount
is not determinable from the spectra alone. The reason
for the large amount o f adsorbed water is presumed to
be mainly to their large surface areas. For example,
the surface area o f 1 g of l]-A1203 (p --" 3.63 g / c m 3)
assuming it consists of an aggregate of cubic crystals,
is approximately 2200 m2/g.
The d i s c o v e r y o f akdalaite and -q-A1203 in the
bauxites of Weipa and A n d o o m has greatly e x p a n d e d
our k n o w l e d g e and understanding o f bauxite mineralogy and the role n 2 0 activity m a y play in the e v o lution of bauxitic pisoliths. The majority of concentrically-banded bauxitic pisoliths and those with
P D M - r i c h cores e v o l v e in a similar m a n n e r to that
p r o p o s e d by Tardy and N a h o n (1985), except that ak-
Vol. 44, No. 5, 1996
Eta-alumina in bauxite
663
Table 3. Nano-probe analyses of individual crystals in the PDM-rich bauxite sample. The analytical total has been recalculated
to 100%.
eta-alumina
eta-alumina + Al-hematite
Al-hematite
1
2
3
4
5
6
7
8
9
10
11
wt%
A1203
SiO2
TiO 2
Fe203
96.4
0.6
0.7
2.3
95.7
0.8
1.1
2.4
95.5
1.4
0.9
2.2
95.1
0.8
1.0
3.1
91.1
0.0
0.1
8.8
88.5
0.9
0.0
10.6
36.3
0.0
1.3
62.5
29.8
0.6
2.6
67.0
23.0
0.3
2.2
74.4
18.6
0.0
2.4
78.9
15.4
0.0
2.4
82.3
cation %
A1
Si
Ti
Fe
97.6
0.5
0.4
1.5
97.0
0.7
0.7
1.5
96.8
1.2
0.6
1.4
96.6
0.7
0.6
2.0
94.1
0.0
0.1
5.8
92.1
0.8
0.0
7.1
47.2
0.0
1.0
51.8
39.9
0.7
2.2
57.3
31.9
0.4
2.0
65.8
26.4
0.0
2.2
71.4
22.2
0.0
2.2
75.7
d a l a i t e or "q-A1203 m a y e p i g e n e t i c a l l y r e p l a c e k a o l i n ite d u r i n g n o d u l e d e v e l o p m e n t as w e l l as A l - h e m a tite. T r o l a n d a n d Tardy ( 1 9 8 7 ) s h o w e d t h a t at 25 ~
the c h e m i c a l a c t i v i t y o f w a t e r w a s a m a j o r f a c t o r in
d e t e r m i n i n g t h e d e g r e e o f h y d r a t i o n in b a u x i t e m i n erals. I f the k a o l i n i t e g r a i n s w i t h i n t h e d e v e l o p i n g
n o d u l e are v e r y small, t h e n the i n t e r c r y s t a l l i n e v o i d s
are c o r r e s p o n d i n g l y so. A s the w e a t h e r i n g profile
dries out, w a t e r is r e t a i n e d in p r o g r e s s i v e l y s m a l l e r
p o r e s w i t h c o r r e s p o n d i n g l y l o w e r H 2 0 activities (Tardy a n d N a h o n 1985). T h e a c t i v i t y o f H20 w i t h i n
p o r e s m a y b e so l o w t h a t a n h y d r o u s p h a s e s s u c h as
A l - h e m a t i t e a n d -q-Al203 m a y f o r m . W h e r e p o r e sizes
are a little l a r g e r a n d a slightly h i g h e r H 2 0 activity
prevails, b o e h m i t e b e c o m e s the d o m i n a n t A i - r i c h
m i n e r a l . In the l a r g e s t v o i d s g i b b s i t e n o r m a l l y c r y s tallizes, as the H20 a c t i v i t y is u s u a l l y t o o h i g h for
t h e less h y d r a t e d p h a s e s to f o r m .
100
80--1
tD
tO
t.-
60-
E
t./}
t-
40-
I.-
I--
203 4 5 0 c m -1
0
4000
i
3500
~
3000
i
2500
I
2000
I
1750
I
1500
I
1250
I
1000
I
750
Wavenumber (cm-1)
Figure 6.
A transmittance infra-red spectrum of bauxitic material containing approximately 70 wt.%
xI-A1203.
I
400
664
Tilley and Eggleton
ACKNOWLEDGMENTS
This work was supported by a research grant from COMALCO Aluminium Limited, for which we are most appreciative. SEM was performed using the facilities of the EM unit
at the Research School of Biological Sciences, ANU. D. Bogshnyi of the Research School of Chemistry, ANU acquired
the infrared spectrum. Analytical TEM using the nano-probe
was undertaken at the Research School of Earth Sciences,
ANU.
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(Received 29 June 1995; accepted 31 December 1995; Ms.
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