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 withManavalakurichi 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, Indiab Department of Marine Geology and Geophysics, School of Marine Sciences, Cochin University of Science and Technology, Kochi 682 016, Indiac Department of Marine Geology, Mangalore University, Mangalagangotri 574 199, Indiad INETI, Departamento de Geologia Marinha, Estrada da Portela, Zambujal 2720 Alfragide, Portugal

a r t i c l e i n f o

Article history:Received 22 August 2005Received in revised form 6 February 2006Accepted 21 March 2008

Keywords:ChavaraManavalakurichiIlmeniteAlterationMagnetic fractions

1367-9120/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jseaes.2008.03.005

* Corresponding author.E-mail address: nairajith06@gmail.com (A.G. Nair)

a b s t r a c t

The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have beenstudied for mineralogical and chemical composition to assess the range of their physical and chemicalvariations with weathering. Chavara deposit represents a highly weathered and relatively homogenousconcentration. Significant variation in composition has been documented with alteration. The most mag-netic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 � 10�6 m3 kg�1, indi-cates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of theilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit withan undesirably high Nb2O5 (0.28%), Cr2O3 (0.23%) and Th (149 ppm) contents. The ilmenite from Chavarais compared with that from the nearby Manavalakurichi deposit of similar geological setting and prove-nance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight theadvanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe3+/Fe2+ ratio com-pared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene.

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1. Introduction

The famous Chavara placer deposit along the southwest coast ofIndia (8� 560300 to 9� 802400 N latitude; 76� 2703600 to 76� 3304400 Elongitude) is known for its huge reserves of heavy minerals (127million tonnes; Krishnan et al., 2001), particularly ilmenite ofindustrial grade. In spite of the commercial implications of thedeposit due to its high quality ilmenite and its exploitation fromthe beginning of the 20th century, not many studies have focusedon the alteration patterns of beach ilmenite (Viswanathan, 1957;Gillson, 1959; Ramakrishnan et al., 1997). Studies on the differentmagnetic fractions of beach ilmenite concentrate are useful todelineate the alteration trends and chemical variations of the min-eral (Frost et al., 1986; Suresh Babu et al., 1994) that, in turn, have abearing on the economic value of its deposit.

Magnetic fractionation of ilmenite has proved to be an effectivemethod to study the progressive alteration in a deposit (Subrah-manyam et al., 1982; Frost et al., 1983; Suresh Babu et al., 1994).This approach yields ilmenite fractions belonging to the entirespectrum of alteration ranging from those rich in iron to leucoxen-ised varieties and thus is a suitable method to trace the weathering

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patterns in the mineral. The deposit-to-deposit variations in minorelement chemistry, magnetic susceptibility, mineral phases pres-ent and crystal structure of the mineral are dependent on a hostof factors like the nature of source rocks, intensity of weatheringand age of deposits. The chemical and physical properties deter-mine the quality of the ore and have an important influence onthe choice of techniques for industrial processing. We report herethe qualitative variation in the properties of ilmenite in the Chav-ara (CH) placer deposit consequent to weathering and attempt acomparison of the Chavara ilmenite data with those of ilmenitefrom the adjacent Manavalakurichi (MK) placer deposit (SureshBabu et al., 1994).

2. Materials and methods

Commercial-grade sample of ilmenite of the Chavara (CH) de-posit was obtained from the Indian Rare Earths Ltd. It was repeat-edly washed with water, dried and sieved using a Ro-Tap sieveshaker to obtain the >0.125 mm size fraction. Magnetic fractionsof CH ilmenite crop was separated at successive amperages of0.15, 0.2, 0.25 and so on (i.e., in steps of 0.05 A) using a Frantz iso-dynamic separator (sideward and forward slopes of 15�). The sam-ples were designated CH1, CH2, CH3. . ..CH8, respectively withincreasing separating amperages. The magnetic susceptibility of

Fig. 1. X-ray patterns for the magnetic fractions of Chavara ilmenite.

116 A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122

the different fractions was measured using a Bartington magneticsusceptibility meter (Model MS2B).

Total iron, ferrous iron and titanium dioxide contents weredetermined following standard wet chemical methods (Vogel,1961). Atomic absorption spectrophotometry was used to deter-mine the minor elements following Darby and Tsang (1987). Min-eral phases in the samples were identified using an X-raydiffractometer (Model X’Pert Pro; CuKa, Ni filter). The mineralphases in the samples were estimated by X’Pert High Score Plussoftware. Thermogravimetric analysis was carried out using a Shi-madzu TGA 50H unit with a heating rate of 10 �C/min going up to amaximum temperature of 1000 �C.

3. Results

3.1. Magnetic Susceptibility Data

The weight percentages of the various magnetic fractions andtheir mass specific magnetic susceptibilities are given in Tables 1and 2. The strongly magnetic fraction separated at 0.15 Å formsonly about 4.6% by weight of the bulk sample. About 12% of thebulk sample weight (fractions CH1 and CH2) has a susceptibilityvalue that exceeds the calculated susceptibility value of pure syn-thetic ilmenite.

The mass specific magnetic susceptibility data reveal that frac-tion CH1 has a susceptibility that is much higher than the rest ofthe fractions and the theoretical value for ilmenite. In fact, it isabout 2.3 times that of fraction CH2, which is the closest to thepublished susceptibility value of 1.4 � 10�6 m3 kg�1 for naturalilmenite (Walden et al., 1999).

3.2. XRD data

Fractions CH1, CH2 and CH3 exhibit sharp and prominentilmenite peaks, whereas pseudorutile dominates the rest of frac-

Table 1Weight percentages and elemental ratios of magnetic fractions of Chavara andManavalakurichi ilmenite

Magneticfraction

Amperage Weight (%) Fe3+/Fe2+ Fe/Ti aTi/Ti+Fe

CH bMK CH bMK CH bMK CH bMK

1 0.15 4.57 15.4 2.80 0.94 0.91 0.94 0.52 0.522 0.20 7.28 22.6 2.76 0.65 0.85 1.05 0.54 0.493 0.25 10.47 30.5 1.63 0.36 0.80 1.02 0.56 0.494 0.30 31.72 15.4 2.30 0.54 0.74 0.92 0.57 0.525 0.35 13.9 4 3.08 1.52 0.72 0.87 0.58 0.546 0.40 16.3 8 3.93 3.36 0.66 0.74 0.60 0.577 0.45 9.58 4 6.71 7.74 0.62 0.58 0.62 0.638 >0.45 6.17 9.94 0.49 0.67

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 2Magnetic susceptibility and content of alteration phases in the magnetic fractions

Magneticfraction

Magnetic susceptibility(�10�6 m3 kg�1)

Ilmenite(%)

Pseudorutile(%)

Leucoxene/rutile (%)

CH1 3.20 43 32 10CH2 1.41 44 43 13CH3 0.80 58 32 10CH4 0.64 21 46 33CH5 0.39 20 52 28CH6 0.25 10 60 30CH7 0.16 5 65 30CH8 0.09

tions (Fig. 1). Ilmenite content is noticeably the highest in CH3(Table 2). These fractions possess the highest content of ilmenitephase (43–58%). The pseudorutile contents (32–65%) are consider-able in all fractions with maximum values in fractions CH5–CH7.The rutile phase is marginal in the first three fractions but be-comes significant in the rest. The higher content of the poorlycrystalline, altered phases like pseudorutile and rutile in otherfractions is indicated by the broad and diffused nature of thepeaks. The rutile peaks likely represent leucoxene. This phasehas been identified as essentially microcrystalline rutile (Temple,1966; Frost et al., 1983; Mücke and Chaudhuri, 1991). Presenceof haematite is revealed in CH1.

The cell volume of the magnetic crops of Chavara ilmeniteranges from �313 to �317 Å (Table 3). The length of the c axisranges from about 13.96 to 14.15 Å, whereas the shorter a axislength varies from 5.08 to 5.1 Å. Fig. 2 is a plot of the cell latticevolume (V) against decreasing content of ilmenite phase, an indexof progressive alteration. The cell volume generally decreases withalteration.

3.3. Chemical data

3.3.1. Major elementsThe major elemental distribution (in weight%) of the magnetic

fractions of the Chavara ilmenite sample is given in Fig. 3a–b. Theferrous oxide content ranges from 2.32% to 14.22% and is the high-est for CH3 (14.22%). Ferric oxide dominates over the ferrous com-ponent in all the fractions. The first two fractions (CH1 and CH2)have the highest Fe2O3 values of 32.43% and 30.97%. The total ironoxide content defines maximum values for CH1 (42.87%), CH2(41.68%) and CH3 (39.97%). The TiO2 content significantly exceedsthe theoretical value of 53% for pure ilmenite, and ranges from56.31% (CH1) to 66.45% (CH8). The Fe3+/Fe2+ ratio (Table 1) is gen-erally higher than 2 except in fraction CH3 (1.63).

3.3.2. Minor elementsOf the minor elements studied (Table 3), Al and Si contents

(0.66% and 0.47%) are the highest. The content of Th ranges from42 ppm (CH2) to 254 ppm (CH7) whereas U is negligible. The low-

Table 3Variation of lattice parameters in the magnetic fractions and distribution of minor and trace elements

Magnetic fraction Lattice parameters Elemental composition (minor and trace)

a Å c Å V Å3 Mn (%) Mg (%) Al (%) Si (%) Cr (ppm) V (ppm) Nb (ppm) Ta (ppm) Th (ppm)

CH1 5.087 13.990 313.460 0.23 0.31 0.48 0.45 1060 1103 1148 83 77CH2 5.101 13.956 314.530 0.26 0.29 0.45 0.42 900 953 916 70 42CH3 5.075 14.156 315.750 0.22 0.31 0.37 0.35 960 1079 895 65 48CH4 5.101 14.046 316.560 0.21 0.32 0.45 0.41 960 1028 1148 89 71CH5 5.075 14.133 315.240 0.15 0.29 0.52 0.42 1170 1079 981 80 89aCH6 2.879 4.595 32.975 0.13 0.28 0.62 0.42 1540 1136 3491 112 143aCH7 2.844 4.642 32.524 0.12 0.30 0.66 0.47 1810 1135 1613 94 254

CH8 was not analysed due to uncertainty regarding its purity.a Lattice parameters of pseudorutile phase.

Fig. 2. Variation of lattice volume with alteration as indicated by the decreasingcontent of ilmenite phase.

Table 4Thermogravimetric data for the magnetic fractions of Chavara ilmenite

Amperage Weight loss due to hydroxyls at600 �C (%)

Effective weight gain/loss at1000 �C

CH MK CH MK

0.15 0.69 0.25 0.16 0.600.20 0.32 0.25 0.38 1.450.25 0.42 0.00 3.60 3.000.30 2.00 0.30 �0.07 0.200.35 1.48 1.75 �1.76 �2.300.40 2.40 2.30 �2.02 �2.800.45 4.23 2.80 �4.03 �2.80

>0.45 4.60 �4.20

A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122 117

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 ele-ments in the weakly magnetic fractions (CH6–CH7) are noticeablydifferent from those of fractions CH1–CH5. Accordingly, Al (0.63%),Cr (1563 ppm), Nb (2258 ppm) and Th (149 ppm) register amarked increase, whereas Si (0.44%) and V (1109 ppm) show amore subdued increase in these fractions. In the least magneticfraction analysed, Mn decreases sharply (0.12%), whereas Mg doesso less prominently.

Fig. 3. Distribution of major elements in the magnetic fractions of (a) Chavara (

3.4. TGA data

The thermogravimetric (TG) curves show an initial fall inweight up to around 600 �C (Table 4). The weight loss at this tem-perature 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) (Frostet al., 1983) have been used as indices of progressive weathering

CH) and (b) Manavalakurichi (MK) ilmenite (after Suresh Babu et al., 1994).

118 A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122

undergone by ilmenite (Table 1). These parameters represent oxi-dation and leaching out of iron from the mineral structure. Mostauthors suggest that ilmenite alteration is defined by the oxidationof iron in its primary stage. The subsequent alteration is dominatedby the leaching of iron and oxygen, leading to enrichment of Ti. Theremoval of soluble ferrous ions is also reported (Chernet, 1999). Asthe depletion of ferrous ions defines the alteration of ilmenite in itsprimary stage (Grey and Reid, 1975; White et al., 1994), the higherferrous content is an indicator of the relatively ‘fresh’ and less al-tered state of the grains. Thus fraction CH3 contains the least al-tered ilmenite as shown by its distinctly high FeO content (14.22%).

The ilmenite samples from Chavara and Manavalakurichideposits show a compositional difference; in the latter, the totalFe oxide content is about or more than 41% in the five most mag-netic fractions and is close to the theoretical value of 47%. How-ever, in Chavara, such high total iron oxide content is restrictedto the first three fractions (42.87%, 41.68% and 39.97%, respec-tively). The FeO values are much lower for the magnetic fractionsof Chavara ilmenite than those of corresponding Manavalakurichisamples, indicating the degree of weathering undergone (Fig. 3aand b). This is evidenced by the relatively higher Fe3+/Fe2+ ratiofor Chavara ilmenite (1.6–9.9) in comparison with the Manavalak-urichi ilmenite (0.36–7.74, with the values for MK1–MK4 < 1). Thesame trend is shown by Fe/Ti ratios pointing to the leaching of ironwith alteration. Based on the Ti/(Ti+Fe) ratio, various stages ofalteration undergone by ilmenite (Ferrian Ilmenite, HydratedIlmenite, Pseudorutile and Leucoxene) have been recognised (Frostet al., 1983). Some of the strongly magnetic fractions of the Manav-alakurichi ilmenite (Suresh Babu et al., 1994) are in the ‘FerrianIlmenite’ stage whereas none of the Chavara samples are. The Ti/(Ti + Fe) ratio indicates that the Chavara fractions generally fall inthe fields of ‘Hydrated Ilmenite’ and ‘Pseudorutile’ stages, and ex-tend to that between ‘Pseudorutile’ and the most altered ‘Leucox-ene’ stages (Table 1). This qualitative difference confirms thehigher alteration undergone by the Chavara ilmenite (Nair et al.,1995; Ramakrishnan et al., 1997; Nair et al., 2002). Fig. 4 showsthe advanced weathering undergone by ilmenite grains of Chavara.

The Chavara deposit represents a highly weathered, composi-tionally homogenous unit along its length (Ramakrishnan et al.,1997). The ilmenite composition of Chavara is relatively homoge-nous along the 5 m vertical profile studied (Nair, 2001) althoughthe maximum thickness of the deposit exceeds 15 m. In contrast,

Fig. 4. Crop of ilmenite exhibiting its typical highly altered nature in Chavara de-posit. Ilmenite is replaced mainly by pseudorutile in many grains. Also seen aregrains falling in the entire spectrum of alteration pattern.

the smaller Manavalakurichi deposit exhibits a marked variationin its composition with depth (Nair, 2001).

The variation of ferrous, ferric and the total Fe oxide contents inthe Chavara fractions are given in Fig. 3a. Fractions CH1, CH2 andCH3 have a total Fe oxide content that is the closest to the theoret-ical limit of 47%. In these, ferrous to ferric conversion is at a moreadvanced stage and/or the leaching out of iron has not reached theextent seen in the rest of the fractions, thereby hindering the rela-tive enrichment of ferrous content. The higher content of ilmenite(43%, 44% and 58%) and pseudorutile (32%, 43% and 32%) phases inthese fractions indicate that the grains that constitute these sam-ples mostly occurred in an environment where the formation ofpseudorutile from ilmenite is the dominant chemical process(Fig. 1; Table 2). Such reactions are most favoured in oxidisingand acidic conditions in ground water environment according tothe first stage of the alteration mechanism of ilmenite propoundedby Grey and Reid (1975) and Frost et al. (1983). These fractionscontain the least amount of leucoxene phase. The grains in thesefractions might have been transported by wave activity fromdepths to the near surface zone where leucoxene (secondary rutile)formation is initiated, accounting for only a marginal content ofleucoxene (Fig. 5). In CH1, the rutile could as well have beenformed as described by Frost et al. (1986). They suggest that ilmen-ite exposed to sun over tens of thousands of years would be oxi-dised to form ferrian ilmenite with fine intergrowths of rutile.This would also explain the presence of haematite phase (15%) asdetected in XRD patterns for this fraction (Fig. 1). However, theyare not observed under ore or electron microscopes. Probably theyoccur at a scale below the resolving power of these microscopes. Asimilar phenomenon has been reported elsewhere (Barriga andFyfe, 1998; Kasama et al., 2004). The magnetic susceptibility ofCH1, considerably higher than the observed value for naturalilmenite (1.4 � 10�6 m3 kg�1) is a result of this mineral phase. Infact, it is about 2.3 times that of fraction CH2, which is the closestto the susceptibility value for pure ilmenite (Walden et al., 1999).

Fractions CH6 and CH7 exhibit high percentages of pseudorutileand leucoxene phases (60, 30; 65, 30), whereas ilmenite presenceis minimal (Table 2; Fig. 1). This corresponds to low FeO valuesin chemical data (Fig. 3a). Such features indicate that ilmenitegrains constituting these fractions were transported to near surfaceconditions from ground water environment and deposited therefor fairly long periods. In this acidic and reducing set-up, the dom-inant mineralogical change is leucoxene formation from pseudor-

Fig. 5. Grains with considerable content of ilmenite (I) and pseudorutile (PR). Notethe 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 oxygenfrom the mineral lattice (Grey and Reid, 1975; Frost et al., 1983).The microscopic observations too support the advanced state ofalteration undergone by these fractions (Fig. 6). Leucoxene is alsoreported to form directly from ilmenite in near surface acidic andreducing conditions (Frost et al., 1983). This is characterised byleucoxene and ilmenite/leached ilmenite separated by sharpboundaries (Fig. 7). Yet, other parts of such grains usually showthe presence of intermediate alteration phases indicating leucox-ene is dominantly formed from pseudorutile. Fractions CH4 andCH5 show considerable percentages of leucoxene and pseudorutile,but ilmenite phase form a significant 20% of these grains (Figs. 3and 8). They have a high ferrous content (comparable to those ofCH1 and CH2) in spite of their lower total Fe oxide contents(�37%). They represent grains that have not been subjected toleaching of iron as much as CH6 and CH7 fractions. No inter-growths of haematite or magnetite are observed in the magneticfractions of Chavara ilmenite except in CH1.

Fig. 6. Typical grain from fractions CH6 and CH7 consisting of pseudorutile (PR) andleucoxene (LX) phases. Note the patchy occurrence of relict ilmenite shown byarrows.

Fig. 7. Formation of leucoxene (LX) from ilmenite (I) as a result of discontinuousalteration. Note the sharp boundary between leucoxene and ilmenite phases.

Fig. 8. Grain reflecting the mineralogical phase composition of CH4 and CH5 withdominance of pseudorutile (PR) and leucoxene (LX) with significant content ofilmenite.

In the MK fractions, the higher FeO and total Fe contents(Fig. 3b) are reflected in the dominance of ilmenite peaks in mostof the fractions, i.e., MK1–MK5 (Suresh Babu et al., 1994). The al-tered phase represented by pseudorutile peaks is not documentedin MK2 and MK3 but marginally present in MK1 and MK4. The ru-tile peaks are not significant in the fractions. A comparison basedon the similar elemental distribution pattern is attempted betweenthe CH and MK fractions, taking into consideration the similarprovenance and close geographical proximity (about 110 km apart)of these two deposits. Fraction MK5 shows a similar behaviour inalteration pattern as CH3 in its lower FeO content (FeO < Fe2O3), to-tal 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 comparableto CH5 in the parameters listed above. Fractions MK1–MK4 repre-sent grains with limited alteration where pseudorutile formation isinitiated. Fractions MK7 and CH7 are very similar in the oxidationstate of Fe and Ti contents. However, MK7 constitutes only 4% ofthe total bulk of MK ilmenite, whereas CH7 forms �10% of theCH ilmenite. It could be surmised that MK7 represents the maxi-mum limit of alteration of ilmenite grains in MK. In CH alterationhas proceeded much further as evidenced by further lowering ofiron content (19%) in MK8.

Both the Chavara and Manavalakurichi deposits are similar interms of petrological setting, climate and groundwater conditions(Thampi et al., 1994). Despite this, ilmenite from the two depositsshows compositional heterogeneity. Microscopic and XRD lines ofevidence indicate that Chavara ilmenite is generally in a more ad-vanced stage of alteration when compared to Manavalakurichiilmenite. This might be attributed to the mature state of Chavaraplacers (Nair et al., 1995). In the MK fractions, oxidation of ferrousions is the dominant weathering phenomenon as seen in the strongcorrelation between ferrous and ferric ions (r = �0.96 compared tor = �0.31 for CH), whereas leaching of iron is the prominent pro-cess in the Chavara ilmenite.

Our ongoing investigations on the microprobe analysis ofilmenite and its alteration phases of southwest placers have shedmore light on the elemental variation consequent to alteration.Titanium oxide for instance, shows similar value of 53% in ilmenitephase in both CH and MK ilmenite grains. This is in marked con-trast to TiO2 contents (61% and 56% for CH and MK respectively)of bulk ilmenite in these deposits. Similarly the total iron contentis about 35% in ilmenite phase of both CH and MK has decreased

Fig. 9. Magnified photomicrograph of a fracture in an ilmenite grain. Note the clay-like bodies (C) inside the fracture.

120 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 compositionin its unaltered state (at least of major elements) existing at pres-ent with markedly different chemistry could be explained as a re-sult of the differential duration/intensity of weathering in thesedeposits. The similarity in climate and hinterland geology favoursthe differential duration of alteration undergone as the cause fordiscrepancy in the chemistry of CH and MK ilmenite. This againsupports the contention that CH deposit is mature than MK.

The segregation of products of different alteration environ-ments, based on their magnetic susceptibility, has been discussedby Frost et al. (1986). The results obtained in this work too pointout the differently altered ilmenite grains from different weather-ing settings, forming the various magnetic fractions. Mücke andChaudhuri (1991) argued that ilmenite alteration could be com-plete in the oxidising and acidic conditions itself. However, in theChavara ilmenite at least, we did not find any evidence of goethiteformation as claimed by Mücke and Chaudhuri (1991) during thistype of weathering process.

The discernible difference in trace element contents betweenweakly magnetic fractions (CH6–CH8) and the rest, illustrates thevariation of trace elemental distribution with alteration of ilmenite(Table 3). The structural change undergone during the transforma-tion of the hexagonal ilmenite to the poorly crystalline, pseudohex-agonal, pseudorutile is accompanied by changes in compositionlike the ionic state of iron, the titanium content as well as the con-centrations of minor elements, depending on their compatibility inthe mineral structure. Whereas Mn and Mg are leached out of themineral structure during the progressive alteration of ilmenite(Frost et al., 1983; Anand and Gilkes, 1984; Lener, 1997), Cr, V, Aland Th contents generally increase with alteration (Frost et al.,1983). Uranium contents are negligible. Niobium and Ta valuesdo not show any regular pattern. The value of Si remains more orless constant. Aluminium, Cr and Th concentrations are the highestin CH7 either due to relative concentration with alteration oradsorption from the surrounding soil medium during dissolu-tion–reprecipitation processes, leading to the formation of leucox-ene (Frost et al., 1983). The Th contents in this fraction mightpartially be explained by impurities of monazite that might haveremained in the sample despite purification. The anomalousbehaviour of CH1 in having unexpectedly high contents (consider-ing its less altered nature) of Cr, V, Th and Nb is likely a result ofexsolved haematite present in the grains. The SEM photomicro-graphs of altered grains show phases that appear to be clay miner-als (Fig. 9). This supports the reprecipitation mechanism suggestedby Frost et al. (1983) and could account for, to an extent, the en-hanced contents of Al, Cr and Th in altered products.

Our recent microprobe spot analysis reveals the contrast be-tween trace element chemistry of pure ilmenite phase and thatof bulk mineral grains of CH. Vanadium and Al with contents of0.001% and 0.002%, respectively, in ilmenite phase are enrichedto 100 and 500 times in bulk ilmenite. Chromium and Si with con-centrations of 0.04% and 0.05% in ilmenite phase are augmented bytwo and four times in bulk chemistry.

The total Fe content is positively correlated with magnetic sus-ceptibility (r = 0.72). X-ray and chemical studies (Figs. 1 and 3; Ta-bles 1 and 2) show that fractions CH1 and CH2 are apparently morealtered than CH3, in spite of having higher magnetic susceptibilityand higher Fe/Ti ratio (0.91 and 0.85 for fractions CH1 and CH2,respectively). Moderately altered ilmenite might exhibit enhancedmagnetic susceptibility values than the relatively unaltered frac-tions due to the considerable content of ferric ions in it. As Fe2+

are oxidised to ferric state with alteration, the number of unpairedelectrons increase due to the high spin state of ferric ions resultingin higher magnetic susceptibility (Subrahmanyam et al., 1982). Forfractions CH6–CH8 Fe3+/Fe2+ = 3.93–9.94, magnetic susceptibility

decreases as the total iron content is diminished with leaching.Obviously, the higher ferric to ferrous content enhancing the mag-netic susceptibility becomes relevant only when the total Fe con-tent 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) peaksin the X-ray patterns.

The lattice volume of fraction CH3 (315.75 Å; Table 3) is theclosest 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). Thisis also manifested under the microscope as shrinkage cracks in al-tered grains formed due to the oxidation and leaching of ferric ionsfrom the mineral structure (Temple, 1966; Chaudhuri and Newes-ely, 1990). The a and c values of our samples fall in the range of lat-tice values proposed by Chaudhuri and Newesely (1990). FractionsCH6 and CH7 contain predominantly pseudorutile phase.

The pattern of thermogravimetric curves of ilmenite depends onthe release of water and the oxidation of ferrous ions to the ferricform in the mineral structure with increase in temperature. In theChavara fractions only the least altered CH3 shows any consider-able effective weight gain (3.6%) caused by the oxidation of ferrouscontent in the mineral (Table 4). CH1and CH2 undergoes slightweight gain. The rest of the fractions exhibit varying degrees ofweight 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 weightfall (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 theirnegligible weight loss (0.32–0.69%). Mücke and Chaudhuri (1991)propose that the alteration of ilmenite, particularly in the advancedstages, is characterised by hydrolisation and leaching. In contrast,the four most magnetic fractions of the MK ilmenite register anet increase in weight (Table 4) due to the oxidation of their con-siderable FeO content, complementing the marginal fall in weightdue to loss of structural water. The sharp difference in the TGA pat-terns of the ilmenite fractions of the two deposits underscores therelatively advanced stage of alteration of the Chavara samples.

4.2. Industrial implications

Ilmenite originally formed the bulk of feedstock in the manufac-ture of titanium pigment. Even though titanium slag and synthetic

Table 5Comparison of chemistry of bulk ilmenite ore and its titanium-rich component withthe limits specified for ore grade ilmenite

Elementaloxides

Elemental/size threshold(wt%)

High grade(wt%)

Bulk ilmenite(wt%)

SiO2 1.5 0.93 0.90Al2O3 1.0 1.19 0.77MnO2 1.0 0.21 0.29Cr2O3 0.1 0.23 0.18Nb2O5 0.1 0.28 0.14MgO 1.0 0.49 0.50aU+Th 100 149 54Fe 23 27.50bSize 100–300 lm 93 95

a In ppm.b In lm.

A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122 121

rutile now comprise the chief feedstock for the pigment produc-tion, these are again manufactured from ilmenite. Hence, thechemical and physical qualities of ilmenite ore is important in tita-nium pigment industry, depending on the type of processingroutes adopted. The presence in ilmenite of certain elements likeCr, Al, Mn, Mg and V in undesirable concentrations affects the qual-ity of the pigment by lending colouration to the latter, poses prob-lems during processing like retarding slag formation, or createsdisposal problems (Murthy et al., 1998). Here, we compare thechemical and physical nature of the Chavara ilmenite ore (Table5) with industrial specifications for ilmenite in sulphate and chlo-ride routes as reported by Stanaway (1994) and Taylor et al.(1996). The Chavara ilmenite is suitable for both sulphate and chlo-ride processes except for Cr2O3 (0.18%) and Nb2O5 (0.14%) contentsthat exceed the limits specified for ore grade ilmenite. Thoriumvalues fall within the elemental threshold. About 95 wt% of theilmenite ore is coarser than 100 lm, the desirable limit for chlorideprocess. The Chavara ilmenite has a high TiO2 content (61%). Themagnetic fractions of Chavara ilmenite separated at P0.4 Å(CH6–CH8) constitute a crop of ore containing about 64% TiO2.They have undergone intense weathering and are enriched in al-tered phases as seen from XRD and thermogravimetric data. Theyform about 32% by weight of the bulk ilmenite of Chavara deposit.In the Manavalakurichi ilmenite, the corresponding value is only12% (Table 1). The elemental contents are within the limits speci-fied for ore grade ilmenite (Table 5) by Stanaway (1994) and Tayloret al. (1996), except Cr2O3 (0.23%), Nb2O5 (0.28%) and Th(149 ppm). Geochemical data indicate a 1.5-times enrichment ofTh in the <125 lm fraction (�120 ASTM mesh) when comparedto the >125 lm fraction. Other elements do not show much varia-tion in the two size grades. The <125 lm size fraction forms about13% by weight of the bulk ilmenite of Chavara deposit.

5. Conclusions

Chavara ilmenite deposit represents a highly altered ore withan industrially significant content of TiO2. High-temperatureintergrowths of haematite are detected from XRD and magneticsusceptibility data in grains of the CH1 fraction. The productsof different weathering environments can be separated usingmagnetic methods. The distribution trends of elements show amarked deviation with progressive alteration, reflecting thestructural changes that accompany the ilmenite–pseudorutiletransformation. The lattice volume of ilmenite decreases withprogressive weathering. The quality of ilmenite is suitable forboth chloride and sulphate routes of pigment production exceptfor the undesirably high concentrations of Cr and Nb. The frac-tions separated at P0.4 Å corresponding to a magnetic suscepti-

bility of 0.25 � 10�6 m3 kg�1 constitutes an iron-poor, Ti-rich orecrop, which is about 32% by weight of the bulk ilmenite. How-ever, minor elements like Cr, Nb and Th are present at undesir-ably high levels in this fraction.

Acknowledgements

The authors are grateful to the Directors, Geological Survey ofIndia, Thiruvananthapuram and National Geophysical ResearchInstitute, Hyderabad for extending the AAS and ICP facilities,respectively. The magnetic susceptibility meter used in this studywas obtained from funds provided by the Department of OceanDevelopment, Government of India. AGN thanks the Departmentof Science and Technology, Government of India, for the award ofa Young Scientist Fellowship.

References

Anand, R.R., Gilkes, R.J., 1984. Weathering of ilmenite in a laterite pallid zone. Claysand Clay Minerals 32, 363–374.

Barriga, F.J.A.S., Fyfe, W.S., 1998. Multiphase water-rhyolite interaction and ore fluidgeneration at Aljustrel, Portugal. Mineralum Deposita 33, 188–207.

Chaudhuri, J.N.B., Newesely, H., 1990. Transformation of ilmenite Fe2 TiO3 toleucoxene, TiO2 under the influence of weathering reactions. Indian Journal ofTechnology 28, 13–23.

Darby, D.A., Tsang, Y.W., 1987. Variation in ilmenite element composition withinand among drainage basins: implications for provenance. Journal ofSedimentary Petrology 57, 831–838.

Frost, M.T., Grey, I.E., Harrowfield, I.R., Mason, K., 1983. The dependence of aluminaand silica contents on the extent of alteration of weathered ilmenites fromWestern Australia. Mineralogical Magazine 47, 201–208.

Frost, M.T., Grey, I.E., Harrowfield, I.R., Li, C., 1986. Alteration profiles and impurityelement distribution in magnetic fractions of weathered ilmenite. AmericanMineralogist 71, 167–175.

Gillson, J.L., 1959. Sand deposits of titanium minerals. Mineral Engineering 2, 421–429.

Grey, I.E., Reid, A.F., 1975. The structure of pseudorutile and its role in the naturalalteration of ilmenite. American Mineralogist 60, 898–906.

Kasama, T., McEnroe, S.A., Ozaki, N., Kogure, T., Putnis, A., 2004. Effects ofnanoscale exsolution in haematite–ilmenite on the acquistion of stablenatural remanent magnetization. Earth and Planetary Science Letters 224,461–475.

Krishnan, S., Viswanathan, G., Balachandran, K., 2001. Heavy mineral sand depositsof Kerala. Exploration and Research for Atomic Minerals 13, 111–146.

Lener, E.F., 1997. Mineral chemistry of heavy minerals in the Old Hickory Deposit,Sussex and Dinwiddie Counties, Virginia. Ph. D. thesis submitted to VirginiaPolytechnic Institute and State University, Blacksburg, USA.

Mücke, A., Chaudhuri, J.N.B., 1991. The continuous alteration of ilmenite throughpseudorutile and leucoxene. Ore Geology Reviews 6, 25–44.

Murthy, D.S.R., Gomathy, B., Bose, R., Rangaswamy, R., 1998. A rapid method for thechemical characterization of ilmenites using ICP-AES. Atomic Spectroscopy 19(1), 14–17.

Nair, A.G., 2001. Studies on ilmenite of Chavara and Manavalakurichi deposits,southwest coast of India. Ph.D. thesis submitted to Cochin University of Scienceand Technology, Kochi, India.

Nair, A.G., Damodaran, K.T., Suresh Babu, D.S., 1995. Mineralo-chemical analysis ofilmenites from the river Valliyar and the Manavalakurichi beach, Tamil Nadu.Journal of the Geological Society of India 46, 655–661.

Nair, A.G., Damodaran, K.T., Suresh Babu, D.S., 2002. Chavara deposit: a contributionof Quaternary processes in Kerala. In: Narayana, A.C. (Ed.), Late QuaternaryGeology and Sea Level Changes, Memoir, 49. Geological Society of India,Bangalore, pp. 65–77.

Ramakrishnan, C., Mani, R., Suresh Babu, D.S., 1997. Ilmenite from the Chavaradeposit, India: a critical evaluation. Mineralogical Magazine 1, 233–242.

Roberts, W.L., Rapp, G.R., Weber, J., 1974. Encyclopaedia of Minerals. Van NostrandReinhold Co., New York (p. 693).

Stanaway, K.J., 1994. Overview of titanium dioxide feedstocks. Mining Engineering46, 1367–1370.

Subrahmanyam, N.P., Rao, N.P., Narasimhan, D., Rao, G.V.U., Jaggi, N.K., Rao,K.R.P.M., 1982. Alteration of beach sand ilmenite from Manavalakurichi,Tamil Nadu, India. Journal of the Geological Society of India 23, 168–174.

Suresh Babu, D.S., Thomas, K.A., Mohan Das, P.N., Damodaran, A.D., 1994. Alterationof ilmenite in the Manavalakurichi deposit, India. Clays and Clay Minerals 42(5), 567–571.

Taylor, R.K.A., Scanlon, T.J., Moore, D.E., Reaveley, B.J., 1996. The critical importanceof high quality ilmenite for the TiO2 pigment industry. 12th Industrial MineralsInternational Congress, 61–70.

Temple, A.K., 1966. Alteration of ilmenite. Economic Geology 61, 695–714.Thampi, P.K., Suchindan, G.K., Balasubramonian, G., Vasudevan, V., Ramachandran,

K.K., 1994. Evaluation of beach placers of SW coast of India in terms of REE and

122 A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122

their geochemical significance. Project Report submitted to the Department ofScience and Technology, Government of India, New Delhi, p. 112.

Viswanathan, P., 1957. Studies of Tranvancore beach sands. Indian Mining JournalSpecial Issue – 9957, 1109–1922.

Vogel, A.I., 1961. A Text Book of Quantitative Inorganic Analysis. Longman, London(p. 856).

Walden, J., Oldfield, F., Smith, J.P., 1999. Environmental magnetism: a practicalguide. Technical Guide, vol. 6. Quaternary Research Association, London (pp.53–61).

White, A.F., Peterson, M.L., Hochella, M.F., 1994. Electrochemistry and dissolutionkinetics of magnetite and ilmenite. Geochimica et Cosmochimica Acta 58 (8),1859–1875.