Weathering of Ilmenite from Chavara Deposit and Its Comparison with Manavalakurichi Placer Ilmenite, Southwestern India
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Journal of Asian Earth Sciences 34 (2009) 115–122 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jaes 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 mag- netic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 Â 10À6 m3 kgÀ1, indi- cates 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 com- pared 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 min- eral (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 sam- ples 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 syn- thetic ilmenite. Fig. 1. X-ray patterns for the magnetic fractions of Chavara ilmenite. The mass specific magnetic susceptibility data reveal that frac- tion 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 Â 10À6 m3 kgÀ1 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 consider- able 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 be- comes 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 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) Table 2 Magnetic susceptibility and content of alteration phases in the magnetic fractions have the highest Fe2O3 values of 32.43% and 30.97%.