The mineralogy and alteration characteristics of beach placer ilmenite from the Manavalakurichi coast were investigated by optical microscopy and electron probe microanalysis. Mineral chemistry by electron probe micro-analysis indicated ;enrichment of TI02, MgO, AI203, Cr203, Si02. K20, V205,. CaO and Na20 in the altered products ofilmenitc:. while there was a loss of iron oxide, maganesc oxide and zinc oxide from the ilmenite grains during weathering leading to leucoxen

Bhaskar Raju, G

Prabhakar, S

Rao, D S

Subba Rao, S

Vijaya Kumar, T V

English

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Journal of Applied GeochemistryVoI.7No.2(2005)pp 195-200ALTERATION CHARACfERISTICS OF THE MANAVALAKURICm BEACHPLACER ILMENITE; TAMILNADUD.S. Rao, T.V.Vijaya Kumar, S. Subba Rao, S. Prabhakar and G. Bhaskar RajuNML Madras Centre. CSIR Madras Complex. Taramani. Chennai-600 J J 3. Tami/nod"Email: nmldsr@yahoo.co.inThe mineralogy and alteration characteristics of beach placer ilmenite from the Manavalakurichi coast were investigatedby optical microscopy and electron probe microanalysis. Mineral chemistry by electron probe micro-analysis indicated ;enrichment ofTI02, MgO, AI203, Cr203, Si0a. K20, V20,. CaO and Na20 in the altered products ofilmenitc:. while therewas a loss of iron oxide, maganesc oxide and zinc oxide from the ilmenite grains during weathering leading to leucoxeneand pseudorutile formation.Keywords: Beach placer, Ilmenite, Alteration, EPMA, Manavalakurichi.1. IntroductionIlmenite (FeTi03), an important and themostabundant ore mineral of titanium, occurs in Indiaalong the coastal beach sands of Orissa, AndhraPradesh, Tamilnadu and Kerala states.Manavalakurichi deposit, Kanyakumari district,Tamilnadu is one such deposit extending over alength of 6 kilometers with an average width of 44meters. The total heavy mineral content of thedeposit is 39% (upto an average depth of 7.5meters). Ilmenite is the predominant mineral witha grade of24% (in raw sand); up to 8% rutile; 0.9%leucoxene; 2% zircon, I% monazite; 3.5%sillimanite and 5.5% garnet (Ali et aI., 2001). TheIndian Rare Earths Limited (IREL), a public sectorundertaking of the Government of India under theadministrative control of the Department ofAtomic Energy, has mined and processed ilmenitealong with other heavy minerals like gamet,monazite, rutile, sillimanite and zircon from·thesesands.The beach placer ilmenite shows significantvariation in the degree of weathering-relatedalteration, which, in turn, is independent of theprovenance and for which it forms the object ofresearch by scientists of different disciplines withtotally different approaches (Sw-esh Babu et aI.,1994, 1996 and Rao et aI., 2002). Several workershave studied ilmenite from across the world toassess the qualitative variation from deposit todeposit (Bailey et at, 1956; Temple, 1966; Wortand Jones, 1980; Mitra et at, 1992), literature onthe alteration of ilmenite is also plentiful (Frost etaI., 1983; Hugo and Cornell, 1991; Suresh Babuand Mohan Das, 1999; Chernet, 1997 and Nimis,1997). Although Suresh babu et aI., (1994) havestudied the alteration of ilmenite from this atea,their study was based on magnetic fractionationof the ilmenite, which is an indirect approach todecipher the alteration characteristics of ilmenite.In the present paper, the authors present detailedcharacteristics of ilmenite from the beach sandsof Manavalakurichi, which is a direct approach,and place a special emphasis on its alteration byoptical microscopy and electron probe micro-analysis.2. Materials and methodsConcentrated ilmenite sample from theManavalakurichi coast was obtained from IREL. Arepresentative sample was prepared from theconcentrate and characterised by opticalmicroscopy, XRD and electron probemicroanalysis (EPMA). The sample was mountedin epoxy resin (cold mounting) and polished usingconventional methods for optical and electronmicroscopic studies. The polished section ofilmenite was examined and analysed with a JEOL,Super Probe JXA-8600 model electronmicroprobe operating with a current setting of2xl0"A and with Standard ProgrammeInternational (SPI) mineral standards, using on-lineZAF correction procedw-es.Individual ilmenitegrains (both altered and unaltered) were selectedfrom the polished specimen for EPMA beforecoating with carbon. Particle size analysis of theilmenite grains was done using CILAS 1180Particle Size Analyser.3. Results and discussionThe Manavalakurichi placer ilmenite occursmostly as sub-rounded to sub-angular grains,marked by numerous surface pits, etch marks!grooves, crescentic pits and mesh like patterns. Theparticle size analysis plot is shown in Fig. I, wherethe mean particle diameter is 169.12 microns (sizeranges from 13 microns to 500 microns).Reflected light microscopy as well as XRD studies(Table 1) suggest that ilmenite is the major phase.Quartz, hematite and rutile were observed underthe microscope rarely. Alteration of ilmenite wasoccasionally observed and seen to proceed alonggrain boundaries and/or fractures giving rise to anamorphous to crypto- or micro-crystalline massresembling leucoxene (Fig. 2). The intensity andmode of alteration also varies from grain to grain.The major and minor elementalcomposition, using electron probe microanalyses,of the ilmenite specimens from theManavalakurichi coast is reported in Table 2. Theilmenite shows good stoichiometry of TiOl(52.193 to 54.151%) and FeO (42.034 to45.885%). Significant amount of V10, (nil to0.306%) is also present. In the analyses of all 4grains of unaltered ilmenite, SiOl and K10 werenot detected. Presence of significant amounts ofmagnesium (0.025 to 2.015%) and manganese(0.096 to 0.902%) indicates that the ilmenite ofManavalakurichi constitutes a solid solution serieswith geikielite and pyrophanite, respectively. As aresult of these solid solutions, the TiOl content ofthe ilmenite is lower or higher than the ideal value(52.65%). The bulk sample analysis (Table 3) iscomparable with that of the ilmenite grain analysisby EPMA. However, Manavalakurichi ilmenitesample shows higher content of ferric iron as wellas titania and low content offerrous iron comparedto the ilmenite sample from Chatrapur (Rao et aI.,2002)Major and minor elemental analyses of thealtered and unaltered portions of two differentilmenites grains were also carried out (Table 2).The analysis of unaltered ilmenite portionscompares well with those in altered ilmenites. Thealtered ilmenite portions are enriched in TiOl(66.906 to 95.243%), MgO (0.012 to 0.945%),AI10, (0.789 to 2.727%), CrlO, (0.178 to0.0.381%), SiOl (0.035 to 2.668%). V10, (0.303to 0.0.555%), CaO (0.001 to 0.005%), K10 (0.001to 0.005%) and NalO (0.195 to 0.581 %), anddepleted in MnO, FeO and ZnO. Similarobservations were made by Grey and Reid (1975),Anand and Gilkes (1984), Suresh Babu et a1.(1994)and Rao et al. (2002). Dissolution and/or oxidationof iron of ilmenite in natural or acidic water leadsto an enrichment of titanium and other elementsin the residuum, which may be the main cause forilmenite alteration (Dimanche and Bartholome,1976). Though ilmenite alteration is neitheruniform nor continuous, the weatheringmechanism is illustrated as a two stage process(Grey and Reid, 1975) or a multistage process(Frost et al., 1983; Hugo and Cornell, 1991).However, the present mineral chemistry suggeststhat the initial step (ferrous-ferric irontransformation stage) is relatively advanced(indicated by a relatively low content of iron orremoval of iron and erihancement of titanium).which is a common low temperature geochemicalprocess. Such a process leads to the incompletealteration of ilmenite and co-existence of ilmenite,pseudorutile and leucoxene. Similarly, the data onsamples 7, 8 and 9 (Table 2) indicate that the endproduct of alteration leads to pseudo-rutile/rutile,which supports the theory of Temple (1966); Greyand Reid (1975); Frostet a1.(1983) and Anand andGilkes (1984).The composition and grade/quality changesnoticed in these ilmenites could only be due tothe exogenic processes undergone by the mineralafter its release from the Precambrianmetasedimentary crystalline rocks. The heavy.mineral assemblage in this sector is suggestive ofprovenance consisting principally of high-grademetamorphic (granulite facies) rocks, principallyof khondalite (garnet-sillimanit-graphite-gneisses), charnokites and granitic gneisses(unclassified crystalline rocks), bordered by thesedimentary rocks exposed along the easterncoastal plains which are the rock types ofTamilnadu (Chandrashekharan and Murugan,200 I). However, the extent of alteration dependson the geological histQry of the deposit and alsoto the weathering environments (Hugo and Cornell,1991; Suresh Babu et aI., 1994). The degree ofalteration of different ilmenite deposits in theworld is quite varied; those of Zululand (SouthAfrica) and Brazil 'being relatively very littlealtered while those of Australia and Florida(U.S.A.) are extensively altered (Hugo and Cornell,1991 and Haseeb et al. 1997). In the present case,the ilmenite from Kerala state suffered maximumalteration whereas that ofTamilnadu state sufferedmoderately and that of Chatrapur coast in Orissastate has undergone least weathering (Suresh Babuet aI., 1996).100•....... 80?f!.'-"u= 60iU>~.~ 40;;§U 200o 100 200 300 400 500Particle size (microns)Fig. I. Particle size analysis plot for the ilmenite from Manvalkuruchi.Table 1. X-ray diffraction data of ilmenitefrom Manavalakurichi.2 a (degrees) d(A) Phase27.759 3.728 Ilmenite38.030 2.745 Ilmenite41.249 2.539 Ilmenite47.200 2.234 Ilmenite57.310 1.865 Ilmenite62.575 1.722 Ilmenite63.963 1.688 Ilmenite66.677 1.627 Ilmenite73.082 1.502 Ilmenite75.208 1.465 Ilmenite·80.688 1.381 IlmeniteFig. 2. Photomicrograph of the ilmenite grains of Manvalakurichi. I is ilmenite and S is ql1..lrtZ.Alteredportion of ilmenite is shown by arrow.Table 2. Electron probe microanalysis data of i1menites and altered i1menites (in wt.%) fromManavalakurichi.Ilmenite Altered ilmenite Altered ilmenite Altered i1menites1 2 Altered Unaltered Altered Unaltered 7 8 9portion portion portion portionTi02 54.151 52.601 94.243 52.193 85.007 53.178 66.906 76.654 91.248FeO 43.404 42.034 NO 45.632 NO 45.885 NO NO NOFe203 NO NO 1.547 NO 6.002 NO 28.195 18.995 3.151AI203 0.251 0.746 0.841 0.452 2.727 0.369 0.789 1.613 1.816MgO 1.003 2.015 0.027 ----- 0.042 0.025 0.945 0.025 0.012MnO 0.902 0.796 0.009 0.863 ----- 0.096 0.127 0.135 0.022ZnO 0.008 ----- ----- 0.055 0.010 0.054 ----- 0.007 0.025Cr203 0.066 0.093 0.250 0.018 0.179 ----- 0.262 0.381 0.178V20S ----- 0.225 0.510 0.290 0.521 0.306 0.303 0.456 0.555Si02 ----- ----- 1.170 ----- 2.668 ----- 0.573 0.390 0.035Na20 ----- ----- 0.581 0.414 0.596 0.350 0.295 0.357 0.195K20 ----- ----- 0.001 ----- 0.005 ----- 0.002 0.001 0.001CaO 0.003 ----- 0.142 ---- 0.136 0.004 0.078 0.111 0.082Total 99.788 98.510 99.321 99.917 97.893 100.267 98.475 99.125 97.320Table 3. Bulk chemical composition ofthe ilmenite sample (wt.%).Compounds ManavalakurichiTi02 55.10FeO 20.30Fe2O, 19.90AI203 0.80Si02 1.50MnO 0.40V20S 0.22CrzO, 0.08MgO 1.00P20, 0.12zrOz 0.06Rare Earths 0.12Total 99.604. Discussion and conclusionsOptical studies indicated the presence ofilmenite, trace amounts of hematite, alteredilmenite (leucoxene) and pseudorutile in theconcentrate sample of the beach placers ofilmenite. The average diameter of these ilmenitegrains is 169 microns. Electron probemicroanalysis data indicate that ilmenite is in solidsolution with pyrophanite and geikielite, and thealteration leads to enrichment ofTi01, MgO, AI103,Cr,o~SiO" K,O, V10" BaO, CaO and Na,O withloss ofFeO, MoO and ZOO.These alterations couldbe due to the exogenic processes that operated onthese ilmenites after their release from the parentrocks. The studies further indicated that thealteration of ilmenite is not uniform.Ilmenite is gradually converted to leucoxeneand pseudorutile in this deposit under oxidisingconditions, leaving enriched residue of TiO, andSiO, along with others, resulting in the formationof porous grains of ilmenite. The pores may remainopen or get filled by a variety of secondary mineralswhich affect their grade/quality. These porousgrains have low density and low magneticsusceptibility (because of change in content ofiron) leading to concentration problems. Thedifferences in physical properties between altered!oxidised ilmenite and fresh/unaltered ilmenite areoften many. The altered or oxidised ores behavedifferently to the beneficiation process comparedto the fresh/unaltered ores. Altered/oxidisedilmenite generally adversely affects the treatmentand the economic viability of any upgradationoperation. Hence, it is necessary to comprehendthe nature of both primary and altered ilmenitesbefore going for any upgradationlbeneficiation andfurther downstream utilisation. It is well knownthat ilmenite is soluble in H,S04 whereas rutile isnot. However, the degree of solubility of ilmenitevaries depending on the extent of alteration.Regardless of the higher Ti01 content in the alteredilmenite, its solubility in H,S04 decreases withincreasing degree of alteration (Sinha, 1979). Inother words, the high ferric iron content of thealtered ilmenite clearly demands a reductionprocess rather than a sulphate process for TiO,production (Chernet, 1999a, b). Hence, the degreeof weathering or alteration can be an indicator ofits economic value. Compositional characterisationby such instrumental techniques not only helps toadopt better methods for industrial processing, butalso facilitates production of different grades ofsynthetic rutile.AcknowledgementsThe authors are indebted to Indian RareEarths Limited, Manavalakurichi and InstituteInstrumentation Center, lIT, Roorkee for providingthe ilmenite sample and Electron Probe facilityrespectively. Thanks are also due to The Director,National Metallurgical Laboratory for his keeninterest in this work and permission to publish thepaper. Wethank the unknown reviewer for valuablesuggestions.ReferencesAli, M.A., Krishnan, S. and Banerjee, D.C., (200 I)Beach and inland heavy mineral sandinvestigations and deposits in India - Anoverview. Exploration and Research forAtomic Minerals, 13, pp.l-21.Anand, R.R. and Gilkes, RJ., (1984). Weatheringof ilmenite in a lateritic Pallid zone. Claysand Clay Minerals, 32, pp.363-374.Bailey, S.W., Cameron, E.N., Spedden, H.R. andWeege, R.J., (1956). 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The dependence of aluminaandsilicacontentson the extent of the alterationof weathered ilmenites from WesternAustralia.Mineral. Mag., 47, pp.201-208.Grey, I.E. and Rtid, A.F., (1975). The structure ofthe pseudorutile and its role in thealteration of ilmenite. Amer. Mineral., 60,pp.898-906.Haseeb, A.S.M., Hug, M.Z. and Kurny, A.S.W.,(1997). Characterization of Bangladeshilmenite and study of its reductionmechanism by X-ray diffraction. Trans.Instil.Min. Metall., Sec.C, 106, pp.39-43.Hugo, V.E. and Cornell, D.H., (1991). AlteredilmenitesinHolocene dWlesfromZululand,South Africa, Petrographic studies formultistage alteration. South Africa Jour.Geol., 94, pp.365-378.Mitra, S., Ahmed, S.S. and Moon, H.S., (1992).Mineralogy and chemistry of the opaqueof Cox's Bazar (Bangladesh) beach sandsand the oxygen fugacity of theirprovenance. Sedimentary Geol., 77,pp.235-247.Nimis, P., (1997). Weathering of ilmenite insouthern Mozambique beach sands. 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Alteration characteristics of manavalakurichi beach placer illmenite, Tamilnadu.

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Alteration characteristics of manavalakurichi beach placer illmenite, Tamilnadu.

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