Journal of Asian Earth Sciences 52 (2012) 53–62
Contents lists available at SciVerse ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes
Mineralogical characteristics of the superlarge Quaternary bauxite deposits
in Jingxi and Debao counties, western Guangxi, China
Xuefei Liu a, Qingfei Wang a,⇑, Qizuan Zhang b, Yuewen Feng a, Shuhui Cai a
a
b
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, PR China
Guangxi Bureau of Geo-Exploration and Mineral Development, Nanning 530023, PR China
a r t i c l e
i n f o
Article history:
Received 12 June 2011
Received in revised form 23 February 2012
Accepted 28 February 2012
Available online 24 March 2012
Keywords:
Karstic bauxite
XRD
SEM/EDS
DTA
EPMA
Ore-forming environment
a b s t r a c t
In recent decades, more than 0.5 billion tons of ores scattered in the Quaternary laterite in western Guangxi, China have been explored. The ores were derived from a bauxite horizon in Permian via physical
break-up and re-sediment process. Utilizing various test methods, i.e., XRD, DTA, TG/DTG, SEM/EDS
and EPMA, the mineralogical characteristics of the Quaternary bauxite ores in Jingxi and Debao counties
were investigated. XRD was used together with TG/DTG to obtain relatively accurate ore mineral abundance. Diaspore is the major phase, whereas hematite, kaolinite, anatase, chamosite, gibbsite, goethite,
illite and rutile are minor. Diaspore is characterized by a small particle size, low degrees of crystallinity
and complex chemical composition. Both gibbsite and goethite have a varied particle size, and goethite
crystals contain high Al substitution and Si. It is clarified that diaspore, chamosite and anatase
were formed in a mildly reduced and alkaline depositional environment in Permian, while gibbsite,
hematite, goethite and part kaolinite were precipitated from Al3+-, Si4+- and Fe3+-enriched solutions
within an Quaternary oxidized environment. The ions Al3+, Si4+ and Fe3+ are mostly released from chamosite in its dissolution process. The different physicochemical conditions between the Permian depositional and the Quaternary weathering periods resulted in a complex mineral assemblage in the
Quaternary bauxite.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Based on bedrock lithology, bauxite is divided into karstic and
lateritic types (Bárdossy, 1982; MacLean et al., 1997; Mameli
et al., 2007; Calagari and Abedini, 2007; Deng et al., 2010). Gibbsite
is a dominant hydrated alumina oxide in lateritic bauxite, whereas
boehmite and diaspore principally occur in karstic bauxite
(Bárdossy and Aleva, 1990; Mongelli and Acquafredda, 1999;
Bogatyrev and Zhukov, 2009). The bauxites in western Guangxi,
China occur in Permian Heshan Formation and Quaternary laterite,
and both belong to karstic diaspore-type (Liu et al., 2008, 2010).
The Quaternary bauxite, transformed from Permian bauxite via a
break-up and re-sediment process in Quaternary weathering
(Wang et al., 2004; Zhang, 2011), has a great deal of economic
value, and more than 0.5 billion tons of ore of this type have been
explored in the last 20 years (Wang et al., 2011; Zhang, 2011).
Ore compositions and ore-forming process of the bauxite in
western Guangxi have been studied by several researchers. It has
been proposed that the magmatic rocks related to the Emeishan
⇑ Corresponding author. Address: State Key Laboratory of Geological Processes
and Mineral Resources, China University of Geosciences, No. 29, Xueyuan Road,
Beijing 100083, PR China.
E-mail address: wqf@cugb.edu.cn (Q. Wang).
1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jseaes.2012.02.011
plume in western Guangxi and the underlying carbonates in the
Maokou Formation are considered to provide the material for
Permian bauxite (Deng et al., 2010; Liu et al., 2010; Wang et al.,
2010). Liu et al. (2010) investigated the mineralogical and geochemical features of the Dajia bauxite deposit in the westernmost
part of Guangxi province. Mineral assemblage and ore textures of
the Dajia bauxite suggest that the Permian ore-forming environment is close to a phreatic situation. And Wang et al. (2010) suggested that the condition of Permian bauxite formation varies
from acid to alkaline due to the occurrence of churchite and parisite. Wang et al. (2011) further demonstrated that the Quaternary
ores was reworked in the Quaternary oxidized conditions via a
geo-mathematical approach. Despite these studies, the abundance,
occurrence, chemical properties and genesis of the major minerals
in the Quaternary ores remain obscure, and the mineralogical
change during the transformation from Permian bauxite to Quaternary bauxite is still ambiguous.
In this paper, we systematically utilized X-ray diffraction (XRD),
differential thermal analysis (DTA), thermogravimetry and derivative thermogravimetry (TG/DTG), scanning electron microscope
and energy dispersive spectrometer (SEM–EDS), and electron
microprobe analyzer (EPMA), and carried out a detailed mineralogical study on the Quaternary bauxite in Jingxi and Debao counties
to further reveal ore-forming process.
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X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
2. Geological setting
The western Guangxi in Southwestern China is geologically a
part of the South China plate (Deng et al., 2011) (Fig. 1a). Bauxite
in western Guangxi was widely distributed in Napo (Dajia),
Pingguo, Jingxi, Debao and Tianyang counties. The bauxite in Jingxi
and Debao counties was in the middle part of western Guangxi and
contains more than 350 million tons of ore.
The stratigraphic succession of the Jingxi and Debao counties,
from the oldest to the youngest, includes Early Devonian sandstone, Middle to Late Devonian carbonates and shale, Carboniferous carbonates, Early to Middle Permian limestone, Late Permian
siliceous limestone and bauxite, Early Triassic sandstone and carbonate, as well as Quaternary laterite containing bauxite ore blocks
(Fig. 1b).
At the end of Middle Permian, the Emeishan LIP-induced uplift
led to the exposure of the magmatic rocks related to the Emeishan
plume and the Permian Maokou Formation carbonates. These rocks
suffered strong weathering forming abundant bauxitic materials.
In a following transgression, the bauxitic materials were transported and deposited in karst depressions forming Permian bauxite, which is mainly comprised of diaspore, chamosite, anatase
and pyrite (Wang et al., 2011; Zhang, 2011). And then, a few diaspores experienced dissolution–recrystallization and some kaolinites
formed via silica replacement of the alumina in diaspore in epigenetic process (Liu et al., 2010). At the end of the Middle Triassic, the
Permian bauxite was exposed and subject to weathering again; it
was partly broken up and accumulated in the karstic depressions
randomly, transforming into the Quaternary bauxite (Zhang,
2011). In summary, the Quaternary bauxite was primarily formed
in Permian and then reworked in Quaternary.
The Quaternary orebodies are with varied thickness (0–31 m)
and situated in the middle part in the Quaternary lateritic profile
(Fig. 2); the ores, with various surface colors, including red, brown-
ish red and light gray, are mostly characterized by clastic, nodular,
pisolitic and ooidic textures. The studied area is divided into the
Batou, Mayi, Xinxu and Lvtong ore sections from north to south.
3. Sampling and analytical methods
According to the ore surface color, texture and structure, 14 typical Quaternary bauxite samples were collected from 11 orebodies.
The samples are widely distributed in the study area (Fig. 1b).
For whole-rock geochemical analyses, the samples were
crushed into a 200-mesh powder using an agate mill. The major
elements were determined at the Geological Survey and Laboratory Center of Langfang, China. The whole-rock abundances of
the major constituents (except FeO, H2O+ and CO2) were determined by X-ray fluorescence (XRF) using a Philips Model 1480
spectrometer. The FeO content was analyzed through a volumetric
method; H2O+ was determined by a gravimetric method, and CO2
was determined using potentiometry (Deng et al., 2010). The
detection limit was 60.1 wt.% for these major components.
XRD analyses were carried out at the Petroleum Geology Research and Laboratory Center of Beijing using a Rigaku D/Mac-RC
and Cu Ka1 radiation with the following operating conditions:
voltage, 40 kV; beam current, 80 mA; graphite monochromator;
continuous scanning; scanning speed, 8°/min; slit, DS = SS = 1°;
ambient temperature, 18 °C and 30% humidity. The mass percentage contents (mass%) of the main mineral phases identified were
semi-quantified (Snyder and Bish, 1989).
Simultaneous TG/DTG and DTA measurements were carried out
using Derivatograph D-1500 equipment from Hungarian Optical
Works in the Laboratory of Orogenic Belt and Crustal Evolution
at Peking University. Pulverized samples were heated over the
range of 20–1200 °C at a rate of 10 °C/min under static atmospheric conditions. Alumina was used as a reference. Semi-quanti-
Fig. 1. (a) An index map of South China showing the location of the bauxite deposits in Jingxi and Debao counties. (b) A geologic map illustrating the geological features of the
bauxite deposits in Jingxi and Debao counties.
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X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
Fig. 2. Two cross-sections of the Jingxi and Debao bauxite deposit (for position see Fig. 1 ‘‘A–A0 and B–B0 ’’). BT-31, BT-18, XX-16 and XX-03 are the orebody numbers in the
diagram.
Table 1
Major element compositions and CIA of the bauxite samples from the Jingxi and Debao counties.
Sample
M-01
M-02
SiO2 (%)
Al2O3
Fe2O3
FeO
MgO
CaO
Na2O
K2O
MnO
P2O5
TiO2
H2O+
CO2
Total
CIA
1.68
67.44
7.19
0.19
0.256
0.015
4.746
0.080
0.005
0.210
3.69
13.55
1.13
100.17
93.3
1.56
57.73
20.85
0.30
0.086
0.000
0.517
0.003
0.022
0.204
2.60
15.37
1.01
100.26
99.11
L-04
X-01
X-03
4.28
74.23
2.08
0.35
0.055
0.001
0.516
0.002
0.016
0.038
3.83
14.34
0.42
100.15
99.31
0.81
70.19
10.87
0.35
0.069
0.000
0.401
0.001
0.005
0.036
3.04
13.34
0.93
100.04
99.43
0.95
67.04
15.01
0.40
0.042
0.018
0.184
0.032
0.037
0.057
2.83
12.59
0.82
100.01
99.65
SiO2 (%)
Al2O3
Fe2O3
FeO
MgO
CaO
Na2O
K2O
MnO
P2O5
TiO2
H2O+
CO2
Total
CIA
M-05
7.66
49.85
17.96
4.70
0.349
0.008
0.328
0.000
0.006
0.075
4.11
13.73
0.65
99.42
99.33
B-01
B-03
L-01
L-03
3.90
75.09
2.22
0.20
0.063
0.006
0.623
0.209
0.005
0.061
3.93
13.89
0.42
100.62
98.9
2.13
52.92
27.29
0.35
0.096
0.003
0.472
0.001
0.009
0.043
3.26
12.43
0.83
99.83
99.11
5.51
62.70
15.08
0.25
0.073
0.000
0.454
0.007
0.006
0.071
2.92
12.81
0.24
100.12
99.27
X-04
X-08
X-14
gxB31
2.05
67.33
12.20
0.55
0.049
0.000
0.341
0.000
0.004
0.046
3.68
13.37
0.84
100.46
99.5
2.54
61.63
14.84
0.40
0.073
0.000
0.118
0.000
0.005
0.103
4.82
14.44
1.09
100.06
99.81
8.11
55.84
10.60
7.00
0.100
0.000
0.245
0.000
0.007
0.025
3.37
13.81
0.62
99.71
99.56
4.58
66.35
10.23
0.55
0.081
0.051
0.678
0.128
0.016
0.118
3.56
12.25
0.85
99.44
98.72
5.56
55.63
13.56
6.00
0.281
0.007
0.407
0.004
0.020
0.041
3.44
13.88
0.75
99.58
99.25
CIA = Al2O3/(Al2O3 + CaO + Na2O + K2O) 100.
tative estimates of aluminum and iron hydroxide were made based
on TG/DTG results. The detailed calculation procedure is described
in Liu et al. (2008).
SEM–EDS and EPMA analyses were carried out at the China University of Geosciences (Beijing) using a Hitachi S-3400N SEM
equipped with a Link Analytical Oxford IE 350 ED X-ray spectrometer and a Shimadzu EPMA-1600 with the following operating conditions: accelerating voltage, 15 kV; beam current, 1 108 A;
lifetime, 50 s and a beam diameter of 1 lm.
4. Results
4.1. Ore geochemistry
The major elements of the bauxite samples are listed in Table 1.
The ores contains Al2O3 (49.85–75.09%), Fe2O3 (2.08–27.29%), SiO2
(0.81–8.11%) and FeO (0.19–7.00%), and TiO2 (2.60–4.82%). Fe2O3
and SiO2 show the largest variations. Al2O3 vs. Fe2O3 present negative correlations with a correlation coefficient of 0.78. The
chemical index of alteration (CIA) (Nesbitt and Young, 1982) of
all samples (except M-01, which is 93.3) is higher than 98, suggesting a complete weathering (Table 1).
4.2. Mineral compositions
Mineralogical investigations revealed that diaspore is the main
mineral; hematite, kaolinite, anatase, goethite, gibbsite, chamosite
and illite are minor accessories, and rutile is a relict mineral
(Table 2 and Figs. 3 and 4).
Semi-quantitative XRD results revealed that diaspore (52–83%),
hematite (0–18%), kaolinite (0–21%), goethite (0–14%) and gibbsite
(0–14%) vary widely in their abundance (Table 2). Anatase
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X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
Table 2
Semi-quantitative mineral abundances (mass%) determined by XRD in the bauxite samples.
Sample
Diaspore
Anatase
Chamosite
Kaolinite
Hematite
Goethite
Gibbsite
Illite
Rutile
M-01
M-02
M-05
B-01
B-03
L-01
L-03
L-04
X-01
X-03
X-04
X-08
X-14
gxB31
82
55
52
73
72
67
69
77
83
75
76
68
60
57
6
4
5
6
5
6
6
6
7
6
7
10
5
10
–
–
22
–
–
–
–
–
–
–
–
–
–
22
3
2
3
3
8
1
18
14
2
3
7
4
21
–
4
10
–
8
2
18
7
2
5
12
6
5
5
–
2
13
14
–
2
2
–
–
1
2
–
7
3
11
2
14
2
2
3
5
–
1
1
–
4
6
4
–
–
1
–
8
7
1
–
–
–
2
–
–
–
–
1
1
2
–
1
–
–
–
1
–
–
–
2
–
–: Not determined;
Fig. 3. XRD patterns of six typical bauxite ore samples. (D – diaspore, H – hematite, K – kaolinite, A – anatase, G – gibbsite, Go – goethite, I – illite, R – rutile).
abundance (4–10%) is relatively consistent. The illite abundance in
some samples ranges from 1% to 8%, and chamosite accounts for
22% and 22% in samples M-05 and gxB31, respectively.
Mass losses of diaspore, gibbsite and goethite were calculated
based on TG/DTG curves (Table 3). The diaspore and gibbsite
contents are 57.3–88.2% and 0–11.6%, respectively. Six samples
contain 3.3–19.6% goethite. The contents of diaspore, goethite
and gibbsite estimated by XRD roughly equals to those derived
from TG/DTG.
4.3. Occurrence of major minerals
Diaspore is widely distributed in ooids, pisolites and matrix as
cryptocrystalline particles (Fig. 4). Chamosite mainly coexists with
diaspore (Fig. 4a and b); in BSE images, it is observed that most
chamosite is dissolving, forming abundant voids with various sizes
(Fig. 4c and d). Part kaolinites fill in the void spaces within diaspore
aggregates as flakes or aggregates, indicating an epigenetic origin
(Fig. 4f), and the rest coexist with hematite as tabular or flake
X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
57
Fig. 4. Backscattered SEM micrographs of bauxite ores. (a) An ooid formed by aggregates of diaspore and chamosite core and a banded cortex of diaspore; (b) an ooid formed
by aggregates of diaspore and chamosite core and a banded cortex of diaspore and chamosite; (c) a goethite vein filling the matrix and abundant chamosite dissolving in the
matrix; (d) chamosite coexisting with diaspore and part chamosite was dissolving; (e) acicular goethite filling in the void space in the ore; (f) kaolinite and hematite crystals
filling the void space in the diaspore assemblage; (g) a hematite vein filling the void space in the matrix of the ore; (h) flaky kaolinite and hematite coexisting in the ore; (i)
Crystalline gibbsite filling the void space in the ore; (j) anatase, diaspore and chamosite coexisting in the ore.
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X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
Table 3
Quantitative abundances (mass%) of diaspore, gibbsite and goethite determined by
TG/DTG in the bauxite samples.
Sample
Diaspore
Gibbsite
Goethite
M-01
M-02
M-05
B-01
B-03
L-01
L-03
L-04
X-01
X-03
X-04
X-08
X-14
gxB31
80.4
59.1
57.3
76.9
85.2
60.1
79.9
88.2
81.1
75.8
75.6
66.8
65.1
58.0
2.8
11.6
2.5
2.3
1.2
7.1
–
1.2
2.2
–
4.6
6.9
7.2
–
4.6
15.3
19.6
–
3.3
–
–
–
–
–
–
9.4
–
14.8
–: Not determined.
crystals (Fig. 4h). Most hematite occurs in void spaces as aggregates, veins or single crystals (Fig. 4f, g and h), and a few coexist
with chamosite (Fig. 4g). Two types of goethite were identified
via SEM–EDS. The first mainly coexists with diaspore as short, prismatic and acicular crystals (Fig. 4e), and the second generally appears as aggregates filling in void spaces (Fig. 4c and i). Gibbsite
predominantly occurs as aggregates (Fig. 4i). Anatase commonly
disperses in matrix, coexisting with diaspore and chamosite
(Fig. 4j).
4.4. Thermal decomposition
All analyzed samples show a similar DTA pattern except a
background difference from 20 to 200 °C, and the difference is
attributable to minimal structural changes in mineral lattice
(Garcia-Guineaa et al., 2005). All samples have a remarkable endothermic peak ranging from 519 to 529 °C (Fig. 5) due to diaspore
dehydroxylation. A second distinct endothermic peak from 231
to 267 °C, occurring in most samples (except samples gxB31 and
X-03), represents the breakdown from gibbsite to boehmite
(Bárdossy and Aleva, 1990). Breakdown from boehmite to
a-Al2O3 at 500–550 °C (Smykatz-Kloss, 1974; Paulik and Paulik,
1983; Laskou et al., 2006) is not observed, probably because this
peak is weak and covered by the diaspore endotherm. Another
obvious endothermic peak between 277 and 324 °C is seen in
several samples (B-03, M-01, M-02, M-05, gxB-31, X-08 and
L-01), reflecting the breakdown of goethite (Todor, 1976). In samples X-14 and M-05, a peak around 450 °C is recorded denoting the
chamosite dehydroxylation (Bai et al., 1993). An exothermic peak
at 982 °C is recorded in sample L-04, denoting kaolinite presents.
The DTA endothermic peaks of diaspore in various ore samples
are nearly identical, whereas those of gibbsite and goethite vary
markedly (Fig. 5).
4.5. Chemical properties of major minerals
Chemical compositions of diaspore aggregates are presented in
Table 4. Pure diaspore is composed of 84.98 wt.% Al2O3 and
15.02 wt.% H2O. In our samples, diaspore has a 71.24–84.57%
Al2O3, 0.08–11.76% TiO2, 0.07–3.45% SiO2, 0.01–4.00% FeO, as well
as low levels of CaO, Na2O, K2O, MnO and P2O5. Al2O3 content in
our samples is lower than the expected value, and the difference
is largely made up by FeO, SiO2, and TiO2.
The analyzed goethite varies its Fe2O3 content from 66.61% to
86.86%, Al2O3 from 1.10% to 11.67%, SiO2 from 1.28% to 7.14%,
and TiO2 from 0 to 4.39 wt.% (Table 5). Small amounts of CaO,
Na2O, K2O and MnO were also identified. Fe2O3 vs. Al2O3 and
Fe2O3 vs. Al2O3 were highly negatively correlated (Fig. 6a and b).
5. Discussion
5.1. Genesis of major minerals
5.1.1. Diaspore
Formation of diaspore mainly includes two explanations, i.e.,
metamorphism and supergene crystallization. Most metamorphic
diaspores have a more ordered crystalline structure and a more
pure chemical composition than supergenic diaspore (Hatipoğlu
et al., 2010). Diaspore in western Guangxi bauxite has been
reported to be of supergenic crystallization origin in Permian (Liu
et al., 2010). EPMA analyses suggest that the diaspore in the
studied area has a complex chemical composition. This is due to
the very fine- to fine-grained mixtures of anatase, chamosite ± other Si- and Fe-bearing phases (Löffler and Mader, 2004;
Hatipoğlu et al., 2010). SEM–EDS investigations reveal that the
diaspore shows a small particle size. The consistent endothermic
peak 519–529 °C is relatively lower than the reported data from
490 to 580 °C, also denoting small particle size and low degrees
of crystallinity (Saaifeld, 1958; Todor, 1976). All these features
reflect most diapores are of supergenic origin.
5.1.2. Chamosite
Chamosite is the major clay mineral in the Permian bauxite in
western Guangxi (Zhang, 2011). The coexistence with diaspore
indicates that the chamosite principally precipitated in a reducing
environment in Permian (D’Argenio and Mindszenty, 1995; Temur
and Kansun, 2006). In the following Quaternary weathering process, abundant chamosites were dissolved, as evidenced by BSE
images.
5.1.3. Anatase
Anatase commonly precipitated in a reducing condition in the
formation of the karst bauxite deposit (Özlü, 1983; Zarasvandi
et al., 2008). The anatase occurrence supports it principally precipitated with diaspore under the Permian reducing condition.
5.1.4. Hematite and goethite
Hematite and goethite are the major iron oxide and hydroxide
in the Quaternary bauxite, whereas they are minor or absent in
the Permian bauxite (Zhang, 2011). Occurrences of hematite and
goethite in the Quaternary ores suggest that they formed during
the Quaternary weathering (Anand et al., 1991; Bárdossy and Aleva, 1990). Their formation is resulted from oxidization of Fe2+,
which was released from abundant chamosites (Temur and Kansun, 2006).
EPMA results show that goethite has complex chemical composition. The highly negative correlation between Al2O3 and Fe2O3
suggests Al substitution in goethite (Boulangé et al., 1996; Laskou
et al., 2006). Although Si-associated goethite has been reported in
diverse environments, little evidence supports that Si was incorporated into goethite lattice (Taitel-Goldman et al., 2004; MejíaGómez et al., 2011). The abundant Si presence in goethite may be
a result of fine mineral inclusions and/or precipitation of SiO2 on
the goethite surface (Bischoff, 1969; Taitel-Goldman et al., 2004).
The presence of Ti in goethite is induced by mixture of submicroscopic or fine-grained anatase. As is revealed by EPMA, goethite
precipitated in Quaternary and has a variety of particle size, as contributes to its relatively low and varied endothermic peak.
X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
59
Fig. 5. DTA and TG/DTG curves for the Quaternary bauxite samples from Jingxi and Debao counties.
5.1.5. Kaolinite
Kaolinite is the major clay mineral in the Quaternary bauxite
and a sparse one in the Permian bauxite (Liu et al., 2010; Zhang,
2011). Liu et al. (2010) proposed that kaolinite in the Quaternary
ores is mainly a product of in situ epigenetic silica replacement
of the alumina in diaspore; however, the coexistence of crystalline
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X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
Table 4
Microprobe analyses (wt.%) of diaspore aggregates in bauxite samples.
Sample
Na2O
Al2O3
K2O
CaO
TiO2
SiO2
FeO
MnO
P2 O5
Total
M-02
M-05
0.32
0.38
0.40
0.36
0.85
0.28
0.36
0.37
0.53
0.20
0.48
0.34
–
0.35
0.28
0.34
0.25
0.32
–
–
–
0.23
–
81.43
81.29
78.65
79.87
84.46
84.38
84.57
80.54
80.41
83.27
84.57
82.90
82.25
81.71
80.53
76.76
82.61
77.92
81.76
76.41
83.41
71.43
71.24
–
–
–
0.04
–
–
–
–
0.09
–
–
–
0.20
0.01
–
0.03
0.05
0.02
–
–
–
–
–
0.06
0.16
0.14
–
0.08
–
–
–
0.23
0.06
–
0.17
0.09
0.17
0.19
0.13
0.04
0.15
–
–
–
0.02
0.08
0.11
0.22
1.97
0.43
0.25
0.19
0.35
2.96
2.35
0.08
0.18
0.24
0.63
0.42
1.73
3.52
0.23
5.39
0.13
0.87
0.33
11.76
10.33
1.59
1.46
0.87
0.72
0.93
0.65
0.34
0.71
1.82
0.82
0.85
0.80
1.23
1.39
1.51
2.00
1.02
0.93
2.42
3.45
0.62
0.73
0.07
1.34
2.07
2.38
2.48
0.24
0.03
0.04
0.42
0.20
0.18
0.65
0.81
0.59
0.37
0.77
1.77
0.34
0.71
0.01
4.00
0.43
1.20
0.72
–
–
0.13
0.23
–
–
–
–
0.22
0.16
0.11
–
–
–
–
–
–
–
–
–
–
0.01
0.10
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.86
84.86
85.57
84.55
84.13
86.81
85.53
85.67
85.01
85.85
84.77
85.34
85.34
85.00
84.42
85.00
84.56
84.54
85.45
84.32
84.73
84.78
85.40
84.40
B-03
X-01
X-03
X-04
X-08
X-14
gxB31
–: Below detection limit.
Table 5
Microprobe analyses (wt.%) of goethite from bauxite ore samples.
Sample
Na2O
Fe2O3
Al2O3
TiO2
SiO2
K2O
CaO
MnO
Total
M-05
0.53
0.52
0.49
0.61
0.39
0.44
–
–
0.45
0.60
0.41
–
–
86.86
84.96
80.87
85.03
83.62
81.08
78.65
75.39
76.90
76.31
82.23
81.74
66.61
1.10
6.11
5.84
1.48
2.09
3.28
7.27
7.43
7.82
8.00
4.65
4.91
11.67
0.24
0.28
0.68
0.19
0.57
0.33
0.42
0.12
–
0.60
0.71
1.15
4.39
1.28
2.53
3.14
2.36
2.74
3.65
3.75
6.61
3.99
3.32
2.78
3.08
7.14
–
–
–
–
–
0.17
–
–
0.08
–
–
–
–
0.11
0.21
0.30
0.03
0.16
–
0.33
0.28
0.24
0.21
0.17
–
0.09
–
–
–
–
–
–
0.19
–
0.33
–
–
–
–
90.12
94.61
91.33
89.70
89.57
88.94
90.61
89.84
89.81
89.04
90.94
90.88
89.91
X-01
X-03
X-04
X-08
X-14
–: Below detection limit.
kaolinite with hematite further verified that some kaolinites also
directly precipitated from solution enriched with Al and Si, that
mostly come from chamosite dissolution in weathering.
5.1.6. Gibbsite
Most of the gibbsite in nature was transformed from K-feldspar
and clay minerals during laterization processes, and it is characterized by a small crystal size (Bárdossy and Aleva, 1990). And the
gibbsite with relatively perfect crystals was commonly formed
via precipitation from Al-rich solutions within the bauxite horizon
(Bárdossy and Aleva, 1990). The occurrences of gibbsite suggest
that it arose through precipitation in western Guangxi, and the
Al is a heritance from the dissolved chamosite.
The thermal decomposition process of gibbsite is mainly affected by crystal size (Colombo and Violante, 1996; Kloprogge
et al., 2002; Tabor and Yapp, 2005; Laskou et al., 2006). The gibbsite in the samples was identified mainly through a varied single
endotherm from 231 to 267 °C. This indicates the gibbsite has a
varied particle size, which may be related to the size difference
of filling spaces.
5.2. Formation environments of Permian and Quaternary bauxite
Many previous studies have demonstrated that mineral assemblage in karstic bauxite corresponds to redox conditions (D’Argenio
and Mindszenty, 1995; Mongelli and Acquafredda, 1999; Mongelli,
2002; Temur and Kansun, 2006). The depositional environment in
Permian has been discussed by Liu et al. (2010) and Wang et al.
(2010). However, insufficient understanding of the genesis of major minerals hampered a thorough interpretation. This study clarifies that the diaspore, chamosite and anatase formed in Permian,
while the hematite, goethite, most kaolinite, gibbsite and illite
formed in Quaternary. The thermodynamic data of diaspore (Dangić, 1988; D’Argenio and Mindszenty, 1995), chamosite (Garrels
and Christ, 1965), hematite (Garrels and Christ, 1965), goethite
Fig. 6. Binary diagrams showing the correlations between (a) Fe2O3 and Al2O3 and (b) Fe2O3 and SiO2 in goethite.
X. Liu et al. / Journal of Asian Earth Sciences 52 (2012) 53–62
61
(2) Most diaspores are characterized by a small particle size, a
low crystallinity degree, and mixture of other minerals with
fine-grained characteristics. All these properties indicate a
supergenic origin. Many minerals, e.g., hematite, goethite
and kaolinite show a variety of occurrences. Goethite has
varied crystal sizes and abundant Al substitutions.
(3) Diaspore, chamosite and anatase mainly formed in a mildly
reduced and alkaline environment in the Permian, while
gibbsite, goethite, hematite, kaolinite and illite predominantly formed under the mildly oxidized and varied pH
(from 4.5 to 9.0) conditions in the Quaternary. The chamosite dissolution is thought to provide main materials for
the formation of hematite, goethite, gibbsite and some kaolinites. The reworking of the Permian bauxite in Quaternary
weathering, which has different physicochemical condition
from the Permian depositional environment, resulted in a
complex mineral assemblage in the Quaternary bauxite.
Acknowledgements
Fig. 7. Eh–pH diagram showing the stability fields of minerals in the Jingxi and
Debao Quaternary bauxite and the ore-forming conditions of Permian and
Quaternary bauxite. The dashed and solid lines denote relatively uncertain and
well-recognized boundaries respectively. The direction of the arrows underlying
the words ‘‘Kao’’, ‘‘Goe’’ and ‘‘Hem’’ point to the stability field of the mineral. Areas I
(dark area) and II (yellow area) are the approximate ore-forming environments of
the Quaternary and Permian bauxite respectively. Dia = diaspore, Cha = chamosite,
Goe = goethite, Gib = gibbsite, Hem = hematite, Kao = kaolinite. (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
(Krauskopf, 1967), kaolinite (Dangić, 1985), and gibbsite (Temur
and Kansun, 2006) were compiled to illustrate the formation environments of Permian and Quaternary bauxite (Fig. 7). The results
denote that the formation condition of Permian bauxite was predominantly reduced and alkaline (pH > 7) (Fig. 7). Thus the Permian acid condition proposed by Wang et al. (2010) due to the
existence of churchite was only local or temporal. The mineral
assemblage also denotes that the Quaternary bauxite formed in a
varied pH (4.5–9.0) and mildly oxidized condition (Fig. 7)
(D’Argenio and Mindszenty, 1995; Mongelli and Acquafredda,
1999; Mongelli, 2002). Abundant chamosite dissolution further approves that the Eh value is higher than +0.2 in Quaternary weathering process.
During the transformation from Permian bauxite to Quaternary
bauxite, the chamosite in Permian ores was largely dissolved under
oxidized conditions and abundant Al3+, Si4+ and Fe2+ were released
into solution. Fe2+ was oxidized into Fe3+ in the solutions during
weathering, and it further formed goethite and hematite in situ
or by filling void spaces after short transportation. Abundant Al3+
ions in the solution induced a high Al substitution in goethite.
Some Si4+ may have been leached from the weathering system;
however, most of the Si4+ and Al3+ formed kaolinite in the void
spaces in the ores. The dissolution of pyrite can provide local
mildly acidic conditions, supporting the precipitation of gibbsite
in the void spaces (Temur and Kansun, 2006).
6. Conclusions
(1) Semi-quantitative estimates derived from XRD and TG/DTG
methods suggest that diaspore is the major mineral, and
hematite, kaolinite, anatase, goethite, gibbsite, chamosite,
illite and rutile are minor in the Jingxi and Debao Quaternary
bauxite ores.
We appreciate the valuable and careful comments from the two
reviewers and editor. The helpful discussion with Dr. Ma Hongwei
in the XRD lab in the Indiana University is appreciated. This research is supported by the Key Research Project of the Resource
Exploration Bureau in Guangxi Province, the National Basic Research Program (No. 2009CB421008), the Program for New Century Excellent Talents (Grant No. NCET -10-0752) and the
National Natural Science Foundation (Grant No. 41102048).
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