Weathering of Ilmenite from Chavara Deposit and Its Comparison with Manavalakurichi Placer Ilmenite, Southwestern India

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 b 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 article info abstract

Article history: The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been Received 22 August 2005 studied for mineralogical and chemical composition to assess the range of their physical and chemical Received in revised form 6 February 2006 variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous Accepted 21 March 2008 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 Keywords: ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with Chavara an undesirably high Nb O (0.28%), Cr O (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara Manavalakurichi 2 5 2 3 Ilmenite is compared with that from the nearby Manavalakurichi deposit of similar geological setting and prove- Alteration nance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO2 (56.31–66.45%) contents highlight the 3+ 2+ Magnetic fractions advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe /Fe 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 patterns in the mineral. The deposit-to-deposit variations in minor element chemistry, magnetic susceptibility, mineral phases pres- The famous Chavara placer deposit along the southwest coast of ent and crystal structure of the mineral are dependent on a host India (8° 560300 to 9° 802400 N latitude; 76° 2703600 to 76° 3304400 E of factors like the nature of source rocks, intensity of weathering longitude) is known for its huge reserves of heavy minerals (127 and age of deposits. The chemical and physical properties deter- million tonnes; Krishnan et al., 2001), particularly ilmenite of mine the quality of the ore and have an important inﬂuence on industrial grade. In spite of the commercial implications of the the choice of techniques for industrial processing. We report here deposit due to its high quality ilmenite and its exploitation from the qualitative variation in the properties of ilmenite in the Chav- the beginning of the 20th century, not many studies have focused ara (CH) placer deposit consequent to weathering and attempt a on the alteration patterns of beach ilmenite (Viswanathan, 1957; comparison of the Chavara ilmenite data with those of ilmenite Gillson, 1959; Ramakrishnan et al., 1997). Studies on the different from the adjacent Manavalakurichi (MK) placer deposit (Suresh magnetic fractions of beach ilmenite concentrate are useful to Babu et al., 1994). delineate the alteration trends and chemical variations of the mineral (Frost et al., 1986; Suresh Babu et al., 1994) that, in turn, have a 2. Materials and methods bearing on the economic value of its deposit. Magnetic fractionation of ilmenite has proved to be an effective Commercial-grade sample of ilmenite of the Chavara (CH) de- method to study the progressive alteration in a deposit (Subrah- posit was obtained from the Indian Rare Earths Ltd. It was repeat- manyam et al., 1982; Frost et al., 1983; Suresh Babu et al., 1994). edly washed with water, dried and sieved using a Ro-Tap sieve This approach yields ilmenite fractions belonging to the entire shaker to obtain the >0.125 mm size fraction. Magnetic fractions spectrum of alteration ranging from those rich in iron to leucoxen- of CH ilmenite crop was separated at successive amperages of ised varieties and thus is a suitable method to trace the weathering 0.15, 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 samples were designated CH1, CH2, CH3....CH8, respectively with * Corresponding author. E-mail address: [email protected] (A.G. Nair). increasing separating amperages. The magnetic susceptibility of

1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2008.03.005 116 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 deter- mine the minor elements following Darby and Tsang (1987). Min- eral 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 Shi- madzu 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. Fig. 1. X-ray patterns for the magnetic fractions of Chavara 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 tions (Fig. 1). Ilmenite content is noticeably the highest in CH3 published susceptibility value of 1.4 106 m3 kg1 for natural (Table 2). These fractions possess the highest content of ilmenite ilmenite (Walden et al., 1999). phase (43–58%). The pseudorutile contents (32–65%) are considerable in all fractions with maximum values in fractions CH5–CH7. 3.2. XRD data The rutile phase is marginal in the first three fractions but becomes significant in the rest. The higher content of the poorly Fractions CH1, CH2 and CH3 exhibit sharp and prominent crystalline, altered phases like pseudorutile and rutile in other ilmenite peaks, whereas pseudorutile dominates the rest of frac- 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 Table 1 Weight percentages and elemental ratios of magnetic fractions of Chavara and of haematite is revealed in CH1. Manavalakurichi ilmenite The cell volume of the magnetic crops of Chavara ilmenite ranges from 313 to 317 Å (Table 3). The length of the c axis Magnetic Amperage Weight (%) Fe3+/Fe2+ Fe/Ti aTi/Ti+Fe ranges from about 13.96 to 14.15 Å, whereas the shorter a axis fraction b b b b CH MK CH MK CH MK CH MK length varies from 5.08 to 5.1 Å. Fig. 2 is a plot of the cell lattice 1 0.15 4.57 15.4 2.80 0.94 0.91 0.94 0.52 0.52 volume (V) against decreasing content of ilmenite phase, an index 2 0.20 7.28 22.6 2.76 0.65 0.85 1.05 0.54 0.49 of progressive alteration. The cell volume generally decreases with 3 0.25 10.47 30.5 1.63 0.36 0.80 1.02 0.56 0.49 4 0.30 31.72 15.4 2.30 0.54 0.74 0.92 0.57 0.52 alteration. 5 0.35 13.9 4 3.08 1.52 0.72 0.87 0.58 0.54 6 0.40 16.3 8 3.93 3.36 0.66 0.74 0.60 0.57 3.3. Chemical data 7 0.45 9.58 4 6.71 7.74 0.62 0.58 0.62 0.63 8 >0.45 6.17 9.94 0.49 0.67 3.3.1. Major elements a Ti/Ti+Fe (<0.5 – Ferrian Ilmenite; 0.5 to 0.6 – Hydrated Ilmenite; 0.6 to 0.7 – The major elemental distribution (in weight%) of the magnetic Pseudorutile; >0.7 – Leucoxene). fractions of the Chavara ilmenite sample is given in Fig. 3a–b. The b After Suresh Babu et al. (1994). 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) Table 2 Magnetic susceptibility and content of alteration phases in the magnetic fractions have the highest Fe2O3 values of 32.43% and 30.97%. The total iron oxide content defines maximum values for CH1 (42.87%), CH2 Magnetic Magnetic susceptibility Ilmenite Pseudorutile Leucoxene/ (41.68%) and CH3 (39.97%). The TiO content significantly exceeds fraction (106 m3 kg1) (%) (%) rutile (%) 2 the theoretical value of 53% for pure ilmenite, and ranges from CH1 3.20 43 32 10 56.31% (CH1) to 66.45% (CH8). The Fe3+/Fe2+ ratio (Table 1) is gen- CH2 1.41 44 43 13 CH3 0.80 58 32 10 erally higher than 2 except in fraction CH3 (1.63). CH4 0.64 21 46 33 CH5 0.39 20 52 28 3.3.2. Minor elements CH6 0.25 10 60 30 Of the minor elements studied (Table 3), Al and Si contents CH7 0.16 5 65 30 CH8 0.09 (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- A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122 117

Table 3 Variation 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 77 CH2 5.101 13.956 314.530 0.26 0.29 0.45 0.42 900 953 916 70 42 CH3 5.075 14.156 315.750 0.22 0.31 0.37 0.35 960 1079 895 65 48 CH4 5.101 14.046 316.560 0.21 0.32 0.45 0.41 960 1028 1148 89 71 CH5 5.075 14.133 315.240 0.15 0.29 0.52 0.42 1170 1079 981 80 89 aCH6 2.879 4.595 32.975 0.13 0.28 0.62 0.42 1540 1136 3491 112 143 aCH7 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.

Table 4 Thermogravimetric data for the magnetic fractions of Chavara ilmenite

Amperage Weight loss due to hydroxyls at Effective weight gain/loss at 600 °C (%) 1000 °C CH MK CH MK 0.15 0.69 0.25 0.16 0.60 0.20 0.32 0.25 0.38 1.45 0.25 0.42 0.00 3.60 3.00 0.30 2.00 0.30 0.07 0.20 0.35 1.48 1.75 1.76 2.30 0.40 2.40 2.30 2.02 2.80 0.45 4.23 2.80 4.03 2.80 >0.45 4.60 4.20

Fig. 2. Variation of lattice volume with alteration as indicated by the decreasing 3.4. TGA data content of ilmenite phase. The thermogravimetric (TG) curves show an initial fall in weight up to around 600 °C(Table 4). The weight loss at this tem- est values for elements like Nb (895 ppm), Ta (65 ppm), Al (0.37%) perature ranges from 0.32% (CH2) to 4.6% (CH8). Beyond 600 °C, and Si (0.35%) are exhibited by CH3. Average concentrations of ele- the weight increases considerably only for CH3 (3.6%). ments in the weakly magnetic fractions (CH6–CH7) are noticeably different from those of fractions CH1–CH5. Accordingly, Al (0.63%), 4. Discussion Cr (1563 ppm), Nb (2258 ppm) and Th (149 ppm) register a marked increase, whereas Si (0.44%) and V (1109 ppm) show a 4.1. Chemical and mineralogical characteristics more subdued increase in these fractions. In the least magnetic fraction analysed, Mn decreases sharply (0.12%), whereas Mg does Different parameters like Fe3+/Fe2+, Fe/Ti and Ti/(Ti+Fe) (Frost so less prominently. 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). 118 A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122 undergone by ilmenite (Table 1). These parameters represent oxi- the smaller Manavalakurichi deposit exhibits a marked variation dation and leaching out of iron from the mineral structure. Most in its composition with depth (Nair, 2001). authors suggest that ilmenite alteration is defined by the oxidation The variation of ferrous, ferric and the total Fe oxide contents in of iron in its primary stage. The subsequent alteration is dominated the Chavara fractions are given in Fig. 3a. Fractions CH1, CH2 and by the leaching of iron and oxygen, leading to enrichment of Ti. The CH3 have a total Fe oxide content that is the closest to the theoret- removal of soluble ferrous ions is also reported (Chernet, 1999). As ical limit of 47%. In these, ferrous to ferric conversion is at a more the depletion of ferrous ions defines the alteration of ilmenite in its advanced stage and/or the leaching out of iron has not reached the primary stage (Grey and Reid, 1975; White et al., 1994), the higher extent seen in the rest of the fractions, thereby hindering the rela- ferrous content is an indicator of the relatively ‘fresh’ and less al- tive enrichment of ferrous content. The higher content of ilmenite tered state of the grains. Thus fraction CH3 contains the least al- (43%, 44% and 58%) and pseudorutile (32%, 43% and 32%) phases in tered ilmenite as shown by its distinctly high FeO content (14.22%). these fractions indicate that the grains that constitute these sam- The ilmenite samples from Chavara and Manavalakurichi ples mostly occurred in an environment where the formation of deposits show a compositional difference; in the latter, the total pseudorutile from ilmenite is the dominant chemical process Fe oxide content is about or more than 41% in the five most mag- (Fig. 1; Table 2). Such reactions are most favoured in oxidising netic fractions and is close to the theoretical value of 47%. How- and acidic conditions in ground water environment according to ever, in Chavara, such high total iron oxide content is restricted the first stage of the alteration mechanism of ilmenite propounded to the first three fractions (42.87%, 41.68% and 39.97%, respec- by Grey and Reid (1975) and Frost et al. (1983). These fractions tively). The FeO values are much lower for the magnetic fractions contain the least amount of leucoxene phase. The grains in these of Chavara ilmenite than those of corresponding Manavalakurichi fractions might have been transported by wave activity from samples, indicating the degree of weathering undergone (Fig. 3a depths to the near surface zone where leucoxene (secondary rutile) and b). This is evidenced by the relatively higher Fe3+/Fe2+ ratio formation is initiated, accounting for only a marginal content of for Chavara ilmenite (1.6–9.9) in comparison with the Manavalak- leucoxene (Fig. 5). In CH1, the rutile could as well have been urichi ilmenite (0.36–7.74, with the values for MK1–MK4 < 1). The formed as described by Frost et al. (1986). They suggest that ilmen- same trend is shown by Fe/Ti ratios pointing to the leaching of iron ite exposed to sun over tens of thousands of years would be oxi- with alteration. Based on the Ti/(Ti+Fe) ratio, various stages of dised to form ferrian ilmenite with fine intergrowths of rutile. alteration undergone by ilmenite (Ferrian Ilmenite, Hydrated This would also explain the presence of haematite phase (15%) as Ilmenite, Pseudorutile and Leucoxene) have been recognised (Frost detected in XRD patterns for this fraction (Fig. 1). However, they et al., 1983). Some of the strongly magnetic fractions of the Manav- are not observed under ore or electron microscopes. Probably they alakurichi ilmenite (Suresh Babu et al., 1994) are in the ‘Ferrian occur at a scale below the resolving power of these microscopes. A Ilmenite’ stage whereas none of the Chavara samples are. The Ti/ similar phenomenon has been reported elsewhere (Barriga and (Ti + Fe) ratio indicates that the Chavara fractions generally fall in Fyfe, 1998; Kasama et al., 2004). The magnetic susceptibility of the fields of ‘Hydrated Ilmenite’ and ‘Pseudorutile’ stages, and ex- CH1, considerably higher than the observed value for natural tend to that between ‘Pseudorutile’ and the most altered ‘Leucox- ilmenite (1.4 106 m3 kg1) is a result of this mineral phase. In ene’ stages (Table 1). This qualitative difference confirms the fact, it is about 2.3 times that of fraction CH2, which is the closest higher alteration undergone by the Chavara ilmenite (Nair et al., to the susceptibility value for pure ilmenite (Walden et al., 1999). 1995; Ramakrishnan et al., 1997; Nair et al., 2002). Fig. 4 shows Fractions CH6 and CH7 exhibit high percentages of pseudorutile the advanced weathering undergone by ilmenite grains of Chavara. and leucoxene phases (60, 30; 65, 30), whereas ilmenite presence The Chavara deposit represents a highly weathered, composi- is minimal (Table 2; Fig. 1). This corresponds to low FeO values tionally homogenous unit along its length (Ramakrishnan et al., in chemical data (Fig. 3a). Such features indicate that ilmenite 1997). The ilmenite composition of Chavara is relatively homoge- grains constituting these fractions were transported to near surface nous along the 5 m vertical profile studied (Nair, 2001) although conditions from ground water environment and deposited there the maximum thickness of the deposit exceeds 15 m. In contrast, for fairly long periods. In this acidic and reducing set-up, the dominant mineralogical change is leucoxene formation from pseudor-

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 Fig. 5. Grains with considerable content of ilmenite (I) and pseudorutile (PR). Note grains falling in the entire spectrum of alteration pattern. 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.

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 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 state of Fe and Ti contents. However, MK7 constitutes only 4% of arrows. 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 con-

trast to TiO2 contents (61% and 56% for CH and MK respectively) Fig. 7. Formation of leucoxene (LX) from ilmenite (I) as a result of discontinuous of bulk ilmenite in these deposits. Similarly the total iron content alteration. Note the sharp boundary between leucoxene and ilmenite phases. is about 35% in ilmenite phase of both CH and MK has decreased 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 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 com- plete 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. Fig. 9. Magnified photomicrograph of a fracture in an ilmenite grain. Note the clay- The discernible difference in trace element contents between like bodies (C) inside the fracture. 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 transforma- decreases as the total iron content is diminished with leaching. tion of the hexagonal ilmenite to the poorly crystalline, pseudohex- Obviously, the higher ferric to ferrous content enhancing the mag- agonal, pseudorutile is accompanied by changes in composition netic susceptibility becomes relevant only when the total Fe con- like the ionic state of iron, the titanium content as well as the content tends to approach the theoretical value for pure ilmenite. centrations of minor elements, depending on their compatibility in The very low magnetic susceptibility of the fractions separated at the mineral structure. Whereas Mn and Mg are leached out of the >0.45 Å is reflected in the dominance of rutile (leucoxene) peaks mineral structure during the progressive alteration of ilmenite in the X-ray patterns. (Frost et al., 1983; Anand and Gilkes, 1984; Lener, 1997), Cr, V, Al The lattice volume of fraction CH3 (315.75 Å; Table 3) is the and Th contents generally increase with alteration (Frost et al., closest to the theoretical value (315.83 Å; Roberts et al., 1974). 1983). Uranium contents are negligible. Niobium and Ta values Chemical and XRD data label this fraction as the least altered. do not show any regular pattern. The value of Si remains more or The cell volume of ilmenite decreases with weathering (Fig. 2). This less constant. Aluminium, Cr and Th concentrations are the highest is also manifested under the microscope as shrinkage cracks in al- in CH7 either due to relative concentration with alteration or tered grains formed due to the oxidation and leaching of ferric ions adsorption from the surrounding soil medium during dissolu- from the mineral structure (Temple, 1966; Chaudhuri and Newes- tion–reprecipitation processes, leading to the formation of leucox- ely, 1990). The a and c values of our samples fall in the range of lat- ene (Frost et al., 1983). The Th contents in this fraction might tice values proposed by Chaudhuri and Newesely (1990). Fractions partially be explained by impurities of monazite that might have CH6 and CH7 contain predominantly pseudorutile phase. remained in the sample despite purification. The anomalous The pattern of thermogravimetric curves of ilmenite depends on behaviour of CH1 in having unexpectedly high contents (consider- the release of water and the oxidation of ferrous ions to the ferric ing its less altered nature) of Cr, V, Th and Nb is likely a result of form in the mineral structure with increase in temperature. In the exsolved haematite present in the grains. The SEM photomicro- Chavara fractions only the least altered CH3 shows any consider- graphs of altered grains show phases that appear to be clay miner- able effective weight gain (3.6%) caused by the oxidation of ferrous als (Fig. 9). This supports the reprecipitation mechanism suggested content in the mineral (Table 4). CH1and CH2 undergoes slight by Frost et al. (1983) and could account for, to an extent, the en- weight gain. The rest of the fractions exhibit varying degrees of hanced contents of Al, Cr and Th in altered products. weight loss pointing to their low FeO and higher water contents. Our recent microprobe spot analysis reveals the contrast be- The TGA data suggest a strong association of water (hydroxyl ions) tween trace element chemistry of pure ilmenite phase and that with the ilmenite structure (as evidenced by weight loss at 600 °C) of bulk mineral grains of CH. Vanadium and Al with contents of as alteration proceeds. This is very well documented in the weight 0.001% and 0.002%, respectively, in ilmenite phase are enriched fall (2.4–4.6%) due to loss of structural water in fractions CH6–CH8. to 100 and 500 times in bulk ilmenite. Chromium and Si with con- The less altered state of fractions CH1–CH3 is reflected in their centrations of 0.04% and 0.05% in ilmenite phase are augmented by negligible weight loss (0.32–0.69%). Mücke and Chaudhuri (1991) two and four times in bulk chemistry. propose that the alteration of ilmenite, particularly in the advanced The total Fe content is positively correlated with magnetic sus- stages, is characterised by hydrolisation and leaching. In contrast, ceptibility (r = 0.72). X-ray and chemical studies (Figs. 1 and 3; Ta- the four most magnetic fractions of the MK ilmenite register a bles 1 and 2) show that fractions CH1 and CH2 are apparently more net increase in weight (Table 4) due to the oxidation of their con- altered than CH3, in spite of having higher magnetic susceptibility siderable FeO content, complementing the marginal fall in weight and higher Fe/Ti ratio (0.91 and 0.85 for fractions CH1 and CH2, due to loss of structural water. The sharp difference in the TGA pat- respectively). Moderately altered ilmenite might exhibit enhanced terns of the ilmenite fractions of the two deposits underscores the magnetic susceptibility values than the relatively unaltered frac- relatively advanced stage of alteration of the Chavara samples. tions due to the considerable content of ferric ions in it. As Fe2+ are oxidised to ferric state with alteration, the number of unpaired 4.2. Industrial implications electrons increase due to the high spin state of ferric ions resulting in higher magnetic susceptibility (Subrahmanyam et al., 1982). For Ilmenite originally formed the bulk of feedstock in the manufac- fractions CH6–CH8 Fe3+/Fe2+ = 3.93–9.94, magnetic susceptibility ture of titanium pigment. Even though titanium slag and synthetic A.G. Nair et al. / Journal of Asian Earth Sciences 34 (2009) 115–122 121

Table 5 bility of 0.25 106 m3 kg1 constitutes an iron-poor, Ti-rich ore Comparison of chemistry of bulk ilmenite ore and its titanium-rich component with crop, which is about 32% by weight of the bulk ilmenite. How- the limits speciﬁed for ore grade ilmenite ever, minor elements like Cr, Nb and Th are present at undesir- Elemental Elemental/size threshold High grade Bulk ilmenite ably high levels in this fraction. oxides (wt%) (wt%) (wt%)

SiO2 1.5 0.93 0.90 Acknowledgements Al2O3 1.0 1.19 0.77 MnO 1.0 0.21 0.29 2 The authors are grateful to the Directors, Geological Survey of Cr2O3 0.1 0.23 0.18 India, Thiruvananthapuram and National Geophysical Research Nb2O5 0.1 0.28 0.14 MgO 1.0 0.49 0.50 Institute, Hyderabad for extending the AAS and ICP facilities, aU+Th 100 149 54 respectively. The magnetic susceptibility meter used in this study Fe 23 27.50 was obtained from funds provided by the Department of Ocean bSize 100–300 lm9395 Development, Government of India. AGN thanks the Department a In ppm. of Science and Technology, Government of India, for the award of b In lm. a Young Scientist Fellowship.

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