Journal of Asian Earth Sciences 34 (2009) 115–122
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Journal of Asian Earth Sciences
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Weathering of ilmenite from Chavara deposit and its comparison with
Manavalakurichi placer ilmenite, southwestern India
Ajith G. Nair a,*, D.S. Suresh Babu a, K.T. Damodaran b, R. Shankar c, C.N. Prabhu d
a
Centre for Earth Science Studies, PB No. 7250, Akkulam, Thuruvikkal P.O., Thiruvananthapuram 695 031, India
Department of Marine Geology and Geophysics, School of Marine Sciences, Cochin University of Science and Technology, Kochi 682 016, India
c
Department of Marine Geology, Mangalore University, Mangalagangotri 574 199, India
d
INETI, Departamento de Geologia Marinha, Estrada da Portela, Zambujal 2720 Alfragide, Portugal
b
a r t i c l e
i n f o
Article history:
Received 22 August 2005
Received in revised form 6 February 2006
Accepted 21 March 2008
Keywords:
Chavara
Manavalakurichi
Ilmenite
Alteration
Magnetic fractions
a b s t r a c t
The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been
studied for mineralogical and chemical composition to assess the range of their physical and chemical
variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous
concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 106 m3 kg1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the
ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with
an undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara
is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the
advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/
leucoxene.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The famous Chavara placer deposit along the southwest coast of
India (8° 560 300 to 9° 80 2400 N latitude; 76° 270 3600 to 76° 330 4400 E
longitude) is known for its huge reserves of heavy minerals (127
million tonnes; Krishnan et al., 2001), particularly ilmenite of
industrial grade. In spite of the commercial implications of the
deposit due to its high quality ilmenite and its exploitation from
the beginning of the 20th century, not many studies have focused
on the alteration patterns of beach ilmenite (Viswanathan, 1957;
Gillson, 1959; Ramakrishnan et al., 1997). Studies on the different
magnetic fractions of beach ilmenite concentrate are useful to
delineate the alteration trends and chemical variations of the mineral (Frost et al., 1986; Suresh Babu et al., 1994) that, in turn, have a
bearing on the economic value of its deposit.
Magnetic fractionation of ilmenite has proved to be an effective
method to study the progressive alteration in a deposit (Subrahmanyam et al., 1982; Frost et al., 1983; Suresh Babu et al., 1994).
This approach yields ilmenite fractions belonging to the entire
spectrum of alteration ranging from those rich in iron to leucoxenised varieties and thus is a suitable method to trace the weathering
* Corresponding author.
E-mail address: [email protected] (A.G. Nair).
1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2008.03.005
patterns in the mineral. The deposit-to-deposit variations in minor
element chemistry, magnetic susceptibility, mineral phases present and crystal structure of the mineral are dependent on a host
of factors like the nature of source rocks, intensity of weathering
and age of deposits. The chemical and physical properties determine the quality of the ore and have an important influence on
the choice of techniques for industrial processing. We report here
the qualitative variation in the properties of ilmenite in the Chavara (CH) placer deposit consequent to weathering and attempt a
comparison of the Chavara ilmenite data with those of ilmenite
from the adjacent Manavalakurichi (MK) placer deposit (Suresh
Babu et al., 1994).
2. Materials and methods
Commercial-grade sample of ilmenite of the Chavara (CH) deposit was obtained from the Indian Rare Earths Ltd. It was repeatedly washed with water, dried and sieved using a Ro-Tap sieve
shaker to obtain the >0.125 mm size fraction. Magnetic fractions
of CH ilmenite crop was separated at successive amperages of
0.15, 0.2, 0.25 and so on (i.e., in steps of 0.05 A) using a Frantz isodynamic separator (sideward and forward slopes of 15°). The samples were designated CH1, CH2, CH3. . ..CH8, respectively with
increasing separating amperages. The magnetic susceptibility of
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A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
the different fractions was measured using a Bartington magnetic
susceptibility meter (Model MS2B).
Total iron, ferrous iron and titanium dioxide contents were
determined following standard wet chemical methods (Vogel,
1961). Atomic absorption spectrophotometry was used to determine the minor elements following Darby and Tsang (1987). Mineral phases in the samples were identified using an X-ray
diffractometer (Model X’Pert Pro; CuKa, Ni filter). The mineral
phases in the samples were estimated by X’Pert High Score Plus
software. Thermogravimetric analysis was carried out using a Shimadzu TGA 50H unit with a heating rate of 10 °C/min going up to a
maximum temperature of 1000 °C.
3. Results
3.1. Magnetic Susceptibility Data
The weight percentages of the various magnetic fractions and
their mass specific magnetic susceptibilities are given in Tables 1
and 2. The strongly magnetic fraction separated at 0.15 Å forms
only about 4.6% by weight of the bulk sample. About 12% of the
bulk sample weight (fractions CH1 and CH2) has a susceptibility
value that exceeds the calculated susceptibility value of pure synthetic ilmenite.
The mass specific magnetic susceptibility data reveal that fraction CH1 has a susceptibility that is much higher than the rest of
the fractions and the theoretical value for ilmenite. In fact, it is
about 2.3 times that of fraction CH2, which is the closest to the
published susceptibility value of 1.4 106 m3 kg1 for natural
ilmenite (Walden et al., 1999).
3.2. XRD data
Fractions CH1, CH2 and CH3 exhibit sharp and prominent
ilmenite peaks, whereas pseudorutile dominates the rest of frac-
Table 1
Weight percentages and elemental ratios of magnetic fractions of Chavara and
Manavalakurichi ilmenite
Magnetic
fraction
Amperage
1
2
3
4
5
6
7
8
0.15
0.20
0.25
0.30
0.35
0.40
0.45
>0.45
Weight (%)
Fe3+/Fe2+
CH
b
CH
b
4.57
7.28
10.47
31.72
13.9
16.3
9.58
6.17
15.4
22.6
30.5
15.4
4
8
4
2.80
2.76
1.63
2.30
3.08
3.93
6.71
9.94
0.94
0.65
0.36
0.54
1.52
3.36
7.74
MK
a
Fe/Ti
MK
Ti/Ti+Fe
CH
b
CH
b
0.91
0.85
0.80
0.74
0.72
0.66
0.62
0.49
0.94
1.05
1.02
0.92
0.87
0.74
0.58
0.52
0.54
0.56
0.57
0.58
0.60
0.62
0.67
0.52
0.49
0.49
0.52
0.54
0.57
0.63
MK
MK
a
Ti/Ti+Fe (<0.5 – Ferrian Ilmenite; 0.5 to 0.6 – Hydrated Ilmenite; 0.6 to 0.7 –
Pseudorutile; >0.7 – Leucoxene).
b
After Suresh Babu et al. (1994).
Table 2
Magnetic susceptibility and content of alteration phases in the magnetic fractions
Magnetic
fraction
Magnetic susceptibility
(106 m3 kg1)
Ilmenite
(%)
Pseudorutile
(%)
Leucoxene/
rutile (%)
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH8
3.20
1.41
0.80
0.64
0.39
0.25
0.16
0.09
43
44
58
21
20
10
5
32
43
32
46
52
60
65
10
13
10
33
28
30
30
Fig. 1. X-ray patterns for the magnetic fractions of Chavara ilmenite.
tions (Fig. 1). Ilmenite content is noticeably the highest in CH3
(Table 2). These fractions possess the highest content of ilmenite
phase (43–58%). The pseudorutile contents (32–65%) are considerable in all fractions with maximum values in fractions CH5–CH7.
The rutile phase is marginal in the first three fractions but becomes significant in the rest. The higher content of the poorly
crystalline, altered phases like pseudorutile and rutile in other
fractions is indicated by the broad and diffused nature of the
peaks. The rutile peaks likely represent leucoxene. This phase
has been identified as essentially microcrystalline rutile (Temple,
1966; Frost et al., 1983; Mücke and Chaudhuri, 1991). Presence
of haematite is revealed in CH1.
The cell volume of the magnetic crops of Chavara ilmenite
ranges from 313 to 317 Å (Table 3). The length of the c axis
ranges from about 13.96 to 14.15 Å, whereas the shorter a axis
length varies from 5.08 to 5.1 Å. Fig. 2 is a plot of the cell lattice
volume (V) against decreasing content of ilmenite phase, an index
of progressive alteration. The cell volume generally decreases with
alteration.
3.3. Chemical data
3.3.1. Major elements
The major elemental distribution (in weight%) of the magnetic
fractions of the Chavara ilmenite sample is given in Fig. 3a–b. The
ferrous oxide content ranges from 2.32% to 14.22% and is the highest for CH3 (14.22%). Ferric oxide dominates over the ferrous component in all the fractions. The first two fractions (CH1 and CH2)
have the highest Fe2O3 values of 32.43% and 30.97%. The total iron
oxide content defines maximum values for CH1 (42.87%), CH2
(41.68%) and CH3 (39.97%). The TiO2 content significantly exceeds
the theoretical value of 53% for pure ilmenite, and ranges from
56.31% (CH1) to 66.45% (CH8). The Fe3+/Fe2+ ratio (Table 1) is generally higher than 2 except in fraction CH3 (1.63).
3.3.2. Minor elements
Of the minor elements studied (Table 3), Al and Si contents
(0.66% and 0.47%) are the highest. The content of Th ranges from
42 ppm (CH2) to 254 ppm (CH7) whereas U is negligible. The low-
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A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
Table 3
Variation of lattice parameters in the magnetic fractions and distribution of minor and trace elements
Magnetic fraction
CH1
CH2
CH3
CH4
CH5
a
CH6
a
CH7
Lattice parameters
Elemental composition (minor and trace)
3
aÅ
cÅ
VÅ
5.087
5.101
5.075
5.101
5.075
2.879
2.844
13.990
13.956
14.156
14.046
14.133
4.595
4.642
313.460
314.530
315.750
316.560
315.240
32.975
32.524
Mn (%)
Mg (%)
Al (%)
Si (%)
Cr (ppm)
V (ppm)
Nb (ppm)
Ta (ppm)
Th (ppm)
0.23
0.26
0.22
0.21
0.15
0.13
0.12
0.31
0.29
0.31
0.32
0.29
0.28
0.30
0.48
0.45
0.37
0.45
0.52
0.62
0.66
0.45
0.42
0.35
0.41
0.42
0.42
0.47
1060
900
960
960
1170
1540
1810
1103
953
1079
1028
1079
1136
1135
1148
916
895
1148
981
3491
1613
83
70
65
89
80
112
94
77
42
48
71
89
143
254
CH8 was not analysed due to uncertainty regarding its purity.
a
Lattice parameters of pseudorutile phase.
Table 4
Thermogravimetric data for the magnetic fractions of Chavara ilmenite
Amperage
0.15
0.20
0.25
0.30
0.35
0.40
0.45
>0.45
Fig. 2. Variation of lattice volume with alteration as indicated by the decreasing
content of ilmenite phase.
est values for elements like Nb (895 ppm), Ta (65 ppm), Al (0.37%)
and Si (0.35%) are exhibited by CH3. Average concentrations of elements in the weakly magnetic fractions (CH6–CH7) are noticeably
different from those of fractions CH1–CH5. Accordingly, Al (0.63%),
Cr (1563 ppm), Nb (2258 ppm) and Th (149 ppm) register a
marked increase, whereas Si (0.44%) and V (1109 ppm) show a
more subdued increase in these fractions. In the least magnetic
fraction analysed, Mn decreases sharply (0.12%), whereas Mg does
so less prominently.
Weight loss due to hydroxyls at
600 °C (%)
Effective weight gain/loss at
1000 °C
CH
MK
CH
MK
0.69
0.32
0.42
2.00
1.48
2.40
4.23
4.60
0.25
0.25
0.00
0.30
1.75
2.30
2.80
0.16
0.38
3.60
0.07
1.76
2.02
4.03
4.20
0.60
1.45
3.00
0.20
2.30
2.80
2.80
3.4. TGA data
The thermogravimetric (TG) curves show an initial fall in
weight up to around 600 °C (Table 4). The weight loss at this temperature ranges from 0.32% (CH2) to 4.6% (CH8). Beyond 600 °C,
the weight increases considerably only for CH3 (3.6%).
4. Discussion
4.1. Chemical and mineralogical characteristics
Different parameters like Fe3+/Fe2+, Fe/Ti and Ti/(Ti+Fe) (Frost
et al., 1983) have been used as indices of progressive weathering
Fig. 3. Distribution of major elements in the magnetic fractions of (a) Chavara (CH) and (b) Manavalakurichi (MK) ilmenite (after Suresh Babu et al., 1994).
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A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
undergone by ilmenite (Table 1). These parameters represent oxidation and leaching out of iron from the mineral structure. Most
authors suggest that ilmenite alteration is defined by the oxidation
of iron in its primary stage. The subsequent alteration is dominated
by the leaching of iron and oxygen, leading to enrichment of Ti. The
removal of soluble ferrous ions is also reported (Chernet, 1999). As
the depletion of ferrous ions defines the alteration of ilmenite in its
primary stage (Grey and Reid, 1975; White et al., 1994), the higher
ferrous content is an indicator of the relatively ‘fresh’ and less altered state of the grains. Thus fraction CH3 contains the least altered ilmenite as shown by its distinctly high FeO content (14.22%).
The ilmenite samples from Chavara and Manavalakurichi
deposits show a compositional difference; in the latter, the total
Fe oxide content is about or more than 41% in the five most magnetic fractions and is close to the theoretical value of 47%. However, in Chavara, such high total iron oxide content is restricted
to the first three fractions (42.87%, 41.68% and 39.97%, respectively). The FeO values are much lower for the magnetic fractions
of Chavara ilmenite than those of corresponding Manavalakurichi
samples, indicating the degree of weathering undergone (Fig. 3a
and b). This is evidenced by the relatively higher Fe3+/Fe2+ ratio
for Chavara ilmenite (1.6–9.9) in comparison with the Manavalakurichi ilmenite (0.36–7.74, with the values for MK1–MK4 < 1). The
same trend is shown by Fe/Ti ratios pointing to the leaching of iron
with alteration. Based on the Ti/(Ti+Fe) ratio, various stages of
alteration undergone by ilmenite (Ferrian Ilmenite, Hydrated
Ilmenite, Pseudorutile and Leucoxene) have been recognised (Frost
et al., 1983). Some of the strongly magnetic fractions of the Manavalakurichi ilmenite (Suresh Babu et al., 1994) are in the ‘Ferrian
Ilmenite’ stage whereas none of the Chavara samples are. The Ti/
(Ti + Fe) ratio indicates that the Chavara fractions generally fall in
the fields of ‘Hydrated Ilmenite’ and ‘Pseudorutile’ stages, and extend to that between ‘Pseudorutile’ and the most altered ‘Leucoxene’ stages (Table 1). This qualitative difference confirms the
higher alteration undergone by the Chavara ilmenite (Nair et al.,
1995; Ramakrishnan et al., 1997; Nair et al., 2002). Fig. 4 shows
the advanced weathering undergone by ilmenite grains of Chavara.
The Chavara deposit represents a highly weathered, compositionally homogenous unit along its length (Ramakrishnan et al.,
1997). The ilmenite composition of Chavara is relatively homogenous along the 5 m vertical profile studied (Nair, 2001) although
the maximum thickness of the deposit exceeds 15 m. In contrast,
Fig. 4. Crop of ilmenite exhibiting its typical highly altered nature in Chavara deposit. Ilmenite is replaced mainly by pseudorutile in many grains. Also seen are
grains falling in the entire spectrum of alteration pattern.
the smaller Manavalakurichi deposit exhibits a marked variation
in its composition with depth (Nair, 2001).
The variation of ferrous, ferric and the total Fe oxide contents in
the Chavara fractions are given in Fig. 3a. Fractions CH1, CH2 and
CH3 have a total Fe oxide content that is the closest to the theoretical limit of 47%. In these, ferrous to ferric conversion is at a more
advanced stage and/or the leaching out of iron has not reached the
extent seen in the rest of the fractions, thereby hindering the relative enrichment of ferrous content. The higher content of ilmenite
(43%, 44% and 58%) and pseudorutile (32%, 43% and 32%) phases in
these fractions indicate that the grains that constitute these samples mostly occurred in an environment where the formation of
pseudorutile from ilmenite is the dominant chemical process
(Fig. 1; Table 2). Such reactions are most favoured in oxidising
and acidic conditions in ground water environment according to
the first stage of the alteration mechanism of ilmenite propounded
by Grey and Reid (1975) and Frost et al. (1983). These fractions
contain the least amount of leucoxene phase. The grains in these
fractions might have been transported by wave activity from
depths to the near surface zone where leucoxene (secondary rutile)
formation is initiated, accounting for only a marginal content of
leucoxene (Fig. 5). In CH1, the rutile could as well have been
formed as described by Frost et al. (1986). They suggest that ilmenite exposed to sun over tens of thousands of years would be oxidised to form ferrian ilmenite with fine intergrowths of rutile.
This would also explain the presence of haematite phase (15%) as
detected in XRD patterns for this fraction (Fig. 1). However, they
are not observed under ore or electron microscopes. Probably they
occur at a scale below the resolving power of these microscopes. A
similar phenomenon has been reported elsewhere (Barriga and
Fyfe, 1998; Kasama et al., 2004). The magnetic susceptibility of
CH1, considerably higher than the observed value for natural
ilmenite (1.4 106 m3 kg1) is a result of this mineral phase. In
fact, it is about 2.3 times that of fraction CH2, which is the closest
to the susceptibility value for pure ilmenite (Walden et al., 1999).
Fractions CH6 and CH7 exhibit high percentages of pseudorutile
and leucoxene phases (60, 30; 65, 30), whereas ilmenite presence
is minimal (Table 2; Fig. 1). This corresponds to low FeO values
in chemical data (Fig. 3a). Such features indicate that ilmenite
grains constituting these fractions were transported to near surface
conditions from ground water environment and deposited there
for fairly long periods. In this acidic and reducing set-up, the dominant mineralogical change is leucoxene formation from pseudor-
Fig. 5. Grains with considerable content of ilmenite (I) and pseudorutile (PR). Note
the selective formation of pseudorutile from ilmenite.
A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
119
utile (Fig. 6), as a result of leaching out of ferric iron and oxygen
from the mineral lattice (Grey and Reid, 1975; Frost et al., 1983).
The microscopic observations too support the advanced state of
alteration undergone by these fractions (Fig. 6). Leucoxene is also
reported to form directly from ilmenite in near surface acidic and
reducing conditions (Frost et al., 1983). This is characterised by
leucoxene and ilmenite/leached ilmenite separated by sharp
boundaries (Fig. 7). Yet, other parts of such grains usually show
the presence of intermediate alteration phases indicating leucoxene is dominantly formed from pseudorutile. Fractions CH4 and
CH5 show considerable percentages of leucoxene and pseudorutile,
but ilmenite phase form a significant 20% of these grains (Figs. 3
and 8). They have a high ferrous content (comparable to those of
CH1 and CH2) in spite of their lower total Fe oxide contents
(37%). They represent grains that have not been subjected to
leaching of iron as much as CH6 and CH7 fractions. No intergrowths of haematite or magnetite are observed in the magnetic
fractions of Chavara ilmenite except in CH1.
Fig. 8. Grain reflecting the mineralogical phase composition of CH4 and CH5 with
dominance of pseudorutile (PR) and leucoxene (LX) with significant content of
ilmenite.
Fig. 6. Typical grain from fractions CH6 and CH7 consisting of pseudorutile (PR) and
leucoxene (LX) phases. Note the patchy occurrence of relict ilmenite shown by
arrows.
Fig. 7. Formation of leucoxene (LX) from ilmenite (I) as a result of discontinuous
alteration. Note the sharp boundary between leucoxene and ilmenite phases.
In the MK fractions, the higher FeO and total Fe contents
(Fig. 3b) are reflected in the dominance of ilmenite peaks in most
of the fractions, i.e., MK1–MK5 (Suresh Babu et al., 1994). The altered phase represented by pseudorutile peaks is not documented
in MK2 and MK3 but marginally present in MK1 and MK4. The rutile peaks are not significant in the fractions. A comparison based
on the similar elemental distribution pattern is attempted between
the CH and MK fractions, taking into consideration the similar
provenance and close geographical proximity (about 110 km apart)
of these two deposits. Fraction MK5 shows a similar behaviour in
alteration pattern as CH3 in its lower FeO content (FeO < Fe2O3), total iron content close to 40%, similar ferric–ferrous ratio (1.5, 1.6)
and TiO2 values (Fig. 3a,b and Table 1). Fraction MK6 is comparable
to CH5 in the parameters listed above. Fractions MK1–MK4 represent grains with limited alteration where pseudorutile formation is
initiated. Fractions MK7 and CH7 are very similar in the oxidation
state of Fe and Ti contents. However, MK7 constitutes only 4% of
the total bulk of MK ilmenite, whereas CH7 forms 10% of the
CH ilmenite. It could be surmised that MK7 represents the maximum limit of alteration of ilmenite grains in MK. In CH alteration
has proceeded much further as evidenced by further lowering of
iron content (19%) in MK8.
Both the Chavara and Manavalakurichi deposits are similar in
terms of petrological setting, climate and groundwater conditions
(Thampi et al., 1994). Despite this, ilmenite from the two deposits
shows compositional heterogeneity. Microscopic and XRD lines of
evidence indicate that Chavara ilmenite is generally in a more advanced stage of alteration when compared to Manavalakurichi
ilmenite. This might be attributed to the mature state of Chavara
placers (Nair et al., 1995). In the MK fractions, oxidation of ferrous
ions is the dominant weathering phenomenon as seen in the strong
correlation between ferrous and ferric ions (r = 0.96 compared to
r = 0.31 for CH), whereas leaching of iron is the prominent process in the Chavara ilmenite.
Our ongoing investigations on the microprobe analysis of
ilmenite and its alteration phases of southwest placers have shed
more light on the elemental variation consequent to alteration.
Titanium oxide for instance, shows similar value of 53% in ilmenite
phase in both CH and MK ilmenite grains. This is in marked contrast to TiO2 contents (61% and 56% for CH and MK respectively)
of bulk ilmenite in these deposits. Similarly the total iron content
is about 35% in ilmenite phase of both CH and MK has decreased
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A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
to 28% and 32%, respectively, in the bulk ilmenite composition.
Thus ilmenite of two deposits having similar initial composition
in its unaltered state (at least of major elements) existing at present with markedly different chemistry could be explained as a result of the differential duration/intensity of weathering in these
deposits. The similarity in climate and hinterland geology favours
the differential duration of alteration undergone as the cause for
discrepancy in the chemistry of CH and MK ilmenite. This again
supports the contention that CH deposit is mature than MK.
The segregation of products of different alteration environments, based on their magnetic susceptibility, has been discussed
by Frost et al. (1986). The results obtained in this work too point
out the differently altered ilmenite grains from different weathering settings, forming the various magnetic fractions. Mücke and
Chaudhuri (1991) argued that ilmenite alteration could be complete in the oxidising and acidic conditions itself. However, in the
Chavara ilmenite at least, we did not find any evidence of goethite
formation as claimed by Mücke and Chaudhuri (1991) during this
type of weathering process.
The discernible difference in trace element contents between
weakly magnetic fractions (CH6–CH8) and the rest, illustrates the
variation of trace elemental distribution with alteration of ilmenite
(Table 3). The structural change undergone during the transformation of the hexagonal ilmenite to the poorly crystalline, pseudohexagonal, pseudorutile is accompanied by changes in composition
like the ionic state of iron, the titanium content as well as the concentrations of minor elements, depending on their compatibility in
the mineral structure. Whereas Mn and Mg are leached out of the
mineral structure during the progressive alteration of ilmenite
(Frost et al., 1983; Anand and Gilkes, 1984; Lener, 1997), Cr, V, Al
and Th contents generally increase with alteration (Frost et al.,
1983). Uranium contents are negligible. Niobium and Ta values
do not show any regular pattern. The value of Si remains more or
less constant. Aluminium, Cr and Th concentrations are the highest
in CH7 either due to relative concentration with alteration or
adsorption from the surrounding soil medium during dissolution–reprecipitation processes, leading to the formation of leucoxene (Frost et al., 1983). The Th contents in this fraction might
partially be explained by impurities of monazite that might have
remained in the sample despite purification. The anomalous
behaviour of CH1 in having unexpectedly high contents (considering its less altered nature) of Cr, V, Th and Nb is likely a result of
exsolved haematite present in the grains. The SEM photomicrographs of altered grains show phases that appear to be clay minerals (Fig. 9). This supports the reprecipitation mechanism suggested
by Frost et al. (1983) and could account for, to an extent, the enhanced contents of Al, Cr and Th in altered products.
Our recent microprobe spot analysis reveals the contrast between trace element chemistry of pure ilmenite phase and that
of bulk mineral grains of CH. Vanadium and Al with contents of
0.001% and 0.002%, respectively, in ilmenite phase are enriched
to 100 and 500 times in bulk ilmenite. Chromium and Si with concentrations of 0.04% and 0.05% in ilmenite phase are augmented by
two and four times in bulk chemistry.
The total Fe content is positively correlated with magnetic susceptibility (r = 0.72). X-ray and chemical studies (Figs. 1 and 3; Tables 1 and 2) show that fractions CH1 and CH2 are apparently more
altered than CH3, in spite of having higher magnetic susceptibility
and higher Fe/Ti ratio (0.91 and 0.85 for fractions CH1 and CH2,
respectively). Moderately altered ilmenite might exhibit enhanced
magnetic susceptibility values than the relatively unaltered fractions due to the considerable content of ferric ions in it. As Fe2+
are oxidised to ferric state with alteration, the number of unpaired
electrons increase due to the high spin state of ferric ions resulting
in higher magnetic susceptibility (Subrahmanyam et al., 1982). For
fractions CH6–CH8 Fe3+/Fe2+ = 3.93–9.94, magnetic susceptibility
Fig. 9. Magnified photomicrograph of a fracture in an ilmenite grain. Note the claylike bodies (C) inside the fracture.
decreases as the total iron content is diminished with leaching.
Obviously, the higher ferric to ferrous content enhancing the magnetic susceptibility becomes relevant only when the total Fe content tends to approach the theoretical value for pure ilmenite.
The very low magnetic susceptibility of the fractions separated at
>0.45 Å is reflected in the dominance of rutile (leucoxene) peaks
in the X-ray patterns.
The lattice volume of fraction CH3 (315.75 Å; Table 3) is the
closest to the theoretical value (315.83 Å; Roberts et al., 1974).
Chemical and XRD data label this fraction as the least altered.
The cell volume of ilmenite decreases with weathering (Fig. 2). This
is also manifested under the microscope as shrinkage cracks in altered grains formed due to the oxidation and leaching of ferric ions
from the mineral structure (Temple, 1966; Chaudhuri and Newesely, 1990). The a and c values of our samples fall in the range of lattice values proposed by Chaudhuri and Newesely (1990). Fractions
CH6 and CH7 contain predominantly pseudorutile phase.
The pattern of thermogravimetric curves of ilmenite depends on
the release of water and the oxidation of ferrous ions to the ferric
form in the mineral structure with increase in temperature. In the
Chavara fractions only the least altered CH3 shows any considerable effective weight gain (3.6%) caused by the oxidation of ferrous
content in the mineral (Table 4). CH1and CH2 undergoes slight
weight gain. The rest of the fractions exhibit varying degrees of
weight loss pointing to their low FeO and higher water contents.
The TGA data suggest a strong association of water (hydroxyl ions)
with the ilmenite structure (as evidenced by weight loss at 600 °C)
as alteration proceeds. This is very well documented in the weight
fall (2.4–4.6%) due to loss of structural water in fractions CH6–CH8.
The less altered state of fractions CH1–CH3 is reflected in their
negligible weight loss (0.32–0.69%). Mücke and Chaudhuri (1991)
propose that the alteration of ilmenite, particularly in the advanced
stages, is characterised by hydrolisation and leaching. In contrast,
the four most magnetic fractions of the MK ilmenite register a
net increase in weight (Table 4) due to the oxidation of their considerable FeO content, complementing the marginal fall in weight
due to loss of structural water. The sharp difference in the TGA patterns of the ilmenite fractions of the two deposits underscores the
relatively advanced stage of alteration of the Chavara samples.
4.2. Industrial implications
Ilmenite originally formed the bulk of feedstock in the manufacture of titanium pigment. Even though titanium slag and synthetic
A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122
Table 5
Comparison of chemistry of bulk ilmenite ore and its titanium-rich component with
the limits specified for ore grade ilmenite
Elemental
oxides
Elemental/size threshold
(wt%)
High grade
(wt%)
Bulk ilmenite
(wt%)
SiO2
Al2O3
MnO2
Cr2O3
Nb2O5
MgO
a
U+Th
Fe
1.5
1.0
1.0
0.1
0.1
1.0
100
0.93
1.19
0.21
0.23
0.28
0.49
149
23
0.90
0.77
0.29
0.18
0.14
0.50
54
27.50
b
100–300 lm
93
95
Size
a
b
In ppm.
In lm.
121
bility of 0.25 106 m3 kg1 constitutes an iron-poor, Ti-rich ore
crop, which is about 32% by weight of the bulk ilmenite. However, minor elements like Cr, Nb and Th are present at undesirably high levels in this fraction.
Acknowledgements
The authors are grateful to the Directors, Geological Survey of
India, Thiruvananthapuram and National Geophysical Research
Institute, Hyderabad for extending the AAS and ICP facilities,
respectively. The magnetic susceptibility meter used in this study
was obtained from funds provided by the Department of Ocean
Development, Government of India. AGN thanks the Department
of Science and Technology, Government of India, for the award of
a Young Scientist Fellowship.
References
rutile now comprise the chief feedstock for the pigment production, these are again manufactured from ilmenite. Hence, the
chemical and physical qualities of ilmenite ore is important in titanium pigment industry, depending on the type of processing
routes adopted. The presence in ilmenite of certain elements like
Cr, Al, Mn, Mg and V in undesirable concentrations affects the quality of the pigment by lending colouration to the latter, poses problems during processing like retarding slag formation, or creates
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