Abstract
Ilmenite from the Chhatrapur coast, Odisha, India, was studied using optical microscope, X-ray diffraction, particle size analysis, and electron microprobe to decipher their micromorphology, texture(s), and elemental composition. The micromorphological features by electron microscope indicate that weathering processes such as mechanical and chemical, affected the placer heavy mineral ilmenite. These detrital ilmenites contain TiO2 in the range of 50.25% to 55.41% and FeO 42.72% to 49.99% in addition to Al2O3, MgO, MnO, CaO, Na2O, Cr2O3, NiO, ZnO, ZrO2, V2O5, and HfO2 (0 to 0.034%). Ti/(Ti + Fe) ratio in the ilmenite varied from 0.413 to 0.5, which indicates the effect of weathering/oxidation confirming microscopic observations. All the results revealed that these ilmenite grains were derived from the gneissic/granitic, basic and high grade metamorphic rocks, belonging to the Eastern Ghats Group of the Precambrian complex of coastal Orissa.
1. Introduction
Ilmenite (FeTiO3), an important and the most abundant ore mineral of titanium, occurs in India along the coastal beach sands of Odisha, Andhra Pradesh, Tamil Nadu, and Kerala states. One important occurrence in Odisha is in the coastal stretch over a strike length of 18 kms (covering a total area of 26 Km2) between Gopalpur in the south and confluence of the Rushikulya river with Bay of Bengal at Ganjam in the north [1–6]. The Indian Rare Earths Limited (IREL), a public sector undertaking of the Department of Atomic Energy, Government of India, is mining and processing ilmenite along with other heavy minerals like garnet, monazite, rutile, sillimanite, and zircon from these sands since 1984, at its Chhatrapur (Matikhalo) plant. Several researchers have studied the ilmenites from across the world to assess the provenance from the geochemistry of ilmenites [7–11]. However, no in depth study has been undertaken on the variation in the chemical composition from grain to grain for the Chhatrapur beach placer ilmenites, aside from some preliminary investigations [12, 13] and the work on surface microtextures [14]. In view of this, the authors present here a detailed mineral geochemistry (by EPMA) for characterizing the ilmenites from the beach sands of Chhatrapur area, Ganjam district, Odisha. The results have been interpreted based on the present study for not only the understanding of the provenance of the ilmenites in this region but also their effects in processing and/or utilization.
2. Geology of the Area
The coastal area of Odisha (from Gopalpur to the Mahanadi delta) runs in a NE–SW direction, nearly perpendicular to the strike of the Eastern Ghats rocks. The Chhatrapur beach placer deposit (84°54′′22′–85°3′′48′ N Lat.: 19°15′′–19°26′′36′′ E Long) is located in the Ganjam District of Orissa State on the southeastern sea coast of India. The details of the location map are described elsewhere [6]. The Orissa coast, in general, and the Chhatrapur coast, in particular, are essentially alluvial, devoid of any rocky exposures, and consist of unsorted fine to medium grained, rounded to subrounded, and moderate to well sorted sand [15], mixed to varying degrees with heavy ilmenite, sillimanite, zircon, monazite, garnet, rutile, and pyroxenes as well as amphiboles and light (quartz, feldspar) minerals. The coast is characterised by extensive formation of dunes [4] that have around 20% of the heavy minerals [2]. These dunes are separated by low-lying areas and interdunal valleys. Regionally, the area forms a part of the Precambrian (belonging to the Eastern Ghats) complex and includes upper Gondwana laterites, Tertiary sediments, and Quaternary beach placers. The Eastern Ghats mobile belt is one of the oldest groups of rocks in the Indian Peninsula [16]. The rocks of this belt are mainly consisting of charnockites, khondalites, granites, granodiorites, and unclassified granulites. The belt shows a fairly consistent trend (NE–SW) for over 1000 kms from Prakasam District of Andhra Pradesh to the southeastern edge of the Talcher coalfield of Odisha. The Eastern Ghats around Chhatrapur in the Ganjam District form detached hill ranges that are mainly composed of charnockite, khondalite group, granites, granodiorites, and unclassified granulites. The charnockite rocks are silica rich, with orthopyroxene (hypersthene)-bearing granulitic composition whereas khondalite is metasedimentary and contains a variety of minerals such as sillimanite + garnet ± graphite ± spinel ± cordierite and hypersthene, in addition to quartz and K-feldspar (orthoclase) [17, 18]. Associated with these and also included in the khondalite group are the quartzites/garnetiferous quartzites, liptinites, and calc-silicate rocks. More or less, these Eastern Ghats rock types form a banded assemblage, metamorphosed under granulite facies conditions, and are permeated by quartz-feldspar neosomes [4]. Based on the mode of occurrence of different lithounits along with their structural analysis Rao et al. [4] published a stratigraphic succession of the area. This area lies in a semiarid, subtropical climate. The main drainage system of this area is the river Rushikulya (that flows southeasterly), which originates from the highlands of Eastern Ghat group of rocks and debouches into the sea at Ganjam. Of course, many streams and streamlets (Bahuda and Ghoda Hoda) which originate from the nearby coastal hills also join the sea near by the Chhatrapur deposit. These streams are ephemeral in nature and are the major suppliers of the sediments to this region.
3. Materials and Methods
Concentrated ilmenite sample from Chhatrapur coast was obtained from the Indian Rare Earths Limited Company, Chhatrapur. The sample was characterized by optical microscopy, X-ray diffraction (XRD), particle size analysis, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The samples were mounted in epoxy resin (cold mounting) and polished using conventional methods for optical and electron microscopic studies. The polished section of ilmenites was then examined and analysed by a JEOL, Super Probe JXA-8600 model electron microprobe operating along with a current setting of 2 × 10−8 A, and using Standard Programme International (SPI) mineral standards and online ZAF correction procedures. Particle size analysis of the sample was carried out using laser diffraction analyzer (CILAS-1180 Particle Size Analyzer, France make).
4. Results
4.1. XRD and Particle Size Analysis
The bulk ilmenite sample was ground to below 45 microns and subjected to X-ray diffraction. The X-ray diffraction of the ilmenite sample (Figure 1) indicates predominantly ilmenite (JCPDS number 29-0733) and minor quantities of pseudorutile (JCPDS number 29-1494). Similar observation was also made by Sasikumar et al., [19] for the ilmenite of this area. The variation of particle size and particle size distribution is shown in Figure 2. The particle size of the sample varied between 0.04 microns to 600 microns. The particle size of the sample varies between 100 and 500 microns and 80% of the ilmenite is around 200 microns. The mean particle size is 175.14 microns. Ilmenite in this deposit is fine to medium grained and moderately well sorted to well sorted and shows unimodal distribution and fine to coarsely skewed. Based on the particle size and perfection of roundness it can be stated that the ilmenite from this locality is texturally matured.
4.2. Optical Microscopy
Reflected light microscopic studies of this ilmenite sample indicate that ilmenite is the major phase with minor amounts of hematite. The quantity (volume percentage) of hematite in the sample is too low for which hematite peaks were not observed in the X-ray diffraction pattern. The ilmenite occurs mostly as subrounded to subangular grains marked by numerous surface pits, etch marks/grooves, crescentic pits, and mesh-like patterns. Sometimes ilmenite contains exsolved laths, streaks, and irregular bodies of hematite. Similarly, hematite also contains exsolved bodies of ilmenite which may be laths, fine streaks, and as patches. Ilmenite-hematite intergrowth also gives rise to emulsion texture and seriate texture. The alteration of the grains has occasionally resulted in an amorphous to cryptocrystalline to microcrystalline mass resembling leucoxene (Figure 3) and pseudorutile. Leucoxene and anatase occur as patches along the margins and fractures of ilmenite which is due to alteration of ilmenite. The alteration characteristics of the ilmenite from this area were studied in detail in [4, 20]. Rao et al. [4] concluded that the alteration leads to enrichment of TiO2, MgO, Al2O3, Cr2O3, SiO2, K2O, V2O5, BaO, CaO, and Na2O with loss of FeO, MnO, and ZnO. They further advocated that the alteration products could be due to the exogenic processes that operated on these ilmenites after their release from the parent rocks of the Eastern Ghats complex.
4.3. Scanning Electron Microscopy
Micromorphological studies of ilmenite from the study area by SEM depict the development of a number of microfeatures on the ilmenite grains. The ilmenite grains of this area exhibit subrounded to rounded shape (Figures 4(b), 4(d), and 5(a)) along with impact “V” marks and deep pits are seen resulting from mechanical collision and later from solution activity (Figures 4(c), 5(b), and 5(d)). Mechanical features like V-shaped pits suggest that grains are formed by grain to grain collision in an aquatic environment [21]. Even the crescentic structures, pits as well as sets of grooves (Figures 5(c) and 6(c)) oriented either in same different directions or in different directions might have developed by the effects of solution activity. Undulatory wavy surfaces formed due to solution effect and removal of blocks were also observed on these ilmenite grains (Figures 4(c) and 5(d)). The present study of micromorphological features by electron microscope establishes the fact that two types of weathering processes such as mechanical and chemical, affected the placer heavy mineral ilmenite that operated during their transportation as well as after deposition.
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4.4. Mineral Chemistry
The mineral chemistry of ilmenite was determined by EPMA spot analysis. The major and minor elemental composition analyses of the ilmenite from this deposit are given in Table 1 along with the structural formulae and end member compositions. The TiO2 content of the ilmenite from this deposit varies from 50.25% to 55.411% which is comparatively lower or higher than the theoretical ilmenite 52.75% [22]. Higher TiO2 may be due to leaching of other cations. Lower TiO2 could be due to the presence of hematite exsolved with ilmenite. Sukumaran and Nambiar [7] reported 50% to 56% of TiO2 from the ilmenites of Ratnagiri coast of Maharashtra which is well comparable with the ilmenite of Chhatrapur coast. The minor variations could be due to the source rock basis. The FeO content of the ilmenite from this deposit varies from 42.719% to 49.987%. The large range of FeO in the ilmenite could be due to hematite exsolved with in ilmenite. Ilmenite from the Honnavar beach is having 50 to 56.33% TiO2 and 41 to 46.89% FeO [8]. The result presented here is broadly in agreement with those of Hegde et al. [8]. The MnO content of ilmenite in this deposit varies from 0.125% to 0.579% while the MgO content varies from 0.069 to 1.357%. The presence of significant amounts of manganese and magnesium indicates that the ilmenite of Chhatrapur constitutes a solid solution series with pyrophanite and geikielite, respectively [23]. As a result of these solid solutions, the TiO2 content of the ilmenite is higher or lower than the ideal value. Significant amounts of V2O5 (0.247% to 0.306%), Al2O3 (0 to 0.087%), Cr2O3 (0 to 0.089%), NiO (0 to 0.051), ZnO (0 to 0.233%), CaO (0 to 0.061%), and Na2O (0 to 0.19%) were also detected in these ilmenite grains. K2O and SiO2 were analysed but not detected in any of these grains. However, these grains contain trace amounts of ZrO2 (0 to 0.036%) and HfO2 (0 to 0.034%) in their crystal lattice which is reported here for the first time.
5. Discussion and Conclusions
The chemical composition (particularly TiO2 concentration) of the ilmenites can be compared with that of the igneous and metamorphic ilmenites to identify the possible source rocks [10, 11, 24]. The igneous ilmenites show a wide variation of TiO2 concentrations (42%–52%) whereas the ilmenite compositions derived from metamorphic sources have a mean value of about 51% TiO2. These Chhatrapur ilmenites have titanium dioxide concentration ranging from 50.25% to 55.41% and are well comparable to the metamorphic ilmenites. The TiO2 concentration of the present ilmenite suggests that the ilmenites were formed in a high grade metamorphic environment such as granulite facies metamorphic rocks. The granulite facies metamorphic rocks include charnockites, khondalites (quartzites/garnetiferous quartzites, quartz-granulites, quartz-garnet-sillimanite-graphite-schist, liptinites, and calc-silicate rocks), granites, pegmatites, granodiorites, and unclassified granulites. As mentioned earlier the geology of the catchment area of Rushikulya river and Chhatrapur is dominantly constituted of the Eastern Ghat group of rock types. Hence, the Eastern Ghat group of rocks appears to be the major source for the ilmenite. Outcrops of igneous intrusive rocks in the Eastern Ghat group also suggest that the ilmenites were formed in a high grade metamorphic environment [25].
The wide ranges of the V2O5 (0.247% to 0.306%), Cr2O3 (0 to 0.089%), and NiO (0 to 0.051) content indicates that the ilmenites from this area represent an admixture of multiple of source rock types. Elevated abundances of these siderophile elements in the Chhatrapur ilmenite suggest that basic rocks like pyroxene granulites along with metasedimentary rocks like khondalite and charnockite which are present in abundance in the Eastern Ghats complex of Orissa are also the source rocks.
The present study on beach sand ilmenite of Chhatrapur coast is restricted only on a concentrate sample drawn from IREL, and hence it could be assumed to be a mixture of recent and older sediments as well as from a near and distant source based on microscopic, electron microscopic, and available literature. Mineralogical studies by optical as well as electron microscope on the ilmenite from this deposit revealed that these grains heave been subjected to chemical weathering giving rise to altered rims. Chhatrapur ilmenite has undergone least weathering compared to the ilmenite from beach sands of Kerala and Tamil Nadu [26]. Sasikumar et al. [27] have reported that the ilmenite of Chhatrapur coast has 90% ilmenite and 10% altered ilmenite. Ti/(Ti + Fe) ratio has also been considered as an important parameter for estimation of alteration of ilmenites. The common process involved in the alteration of ilmenite is hydration and conversion of ilmenite into pseudorutile that has Ti/(Ti + Fe) ratio between 0.5 to 0.6 [8]. This is due to the leaching of iron. The observed ratio of Ti/(Ti + Fe) for ilmenite of Chhatrapur coast (0.413 to 0.500) is comparable to that of the ilmenites from the Honnavar coast [8].
From microscopic as well as from electron microprobe studies, it can be inferred that the ilmenite from the Chhatrapur coast fall under three different types. These are as follows.(a)Unaltered ilmenite: ilmenites without any sign of alteration. They are fresh and compact (Figures 4(b), 4(c), and 5(c)).(b)Moderately altered ilmenite: ilmenite grains which show signatures of alteration along grain boundaries or fracture planes (Figures 3, 5(a), 5(b), and 5(d)).(c)Highly altered ilmenite: ilmenite grain is completely affected by alteration leading to leucoxene (Figure 3) and forming porous structure (Figures 6(c) and 6(d)) often associated with foreign materials along the fractures and grooves (Figures 5(b), 6(b), and 6(c)). Sasikumar et al., [27] have reported that the ilmenite of Chhatrapur coast has 90% ilmenite and 10% altered ilmenite.
From the above studies it can be concluded that the Eastern Ghats group of rocks consists of khondalite suite of rocks (consisting of garnetiferous quartz-sillimanite schists, gneisses, garnetiferous quartzites, quartz granulites, calc-silicate rocks, and quartz- sillimanite-graphite schists), charnockite suite of rocks (tonalitic and granodioritic varieties in composition as well as pyroxene granulites), and leptynite besides intrusive granites, pegmatites, quartz veins, and other metasediments which appears to be the major source of the Chhatrapur beach placer for the heavy mineral assemblage in general and ilmenite in particular.
6. Implications in Metallurgical Processing
The Ilmenites are the major heavy mineral of the total heavy mineral assemblage at Chhatrapur coast. The physical and chemical properties of the ilmenites vary drastically with respect to their alteration. Considering the magnetic, conducting, and density properties used during separation of heavy minerals one from the other, it is obvious that the altered as well as unaltered ilmenites behave differently. Unaltered ilmenite has a higher specific gravity and a better electrostatic and magnetic response than the iron poorer and titanium richer altered ilmenite. Coexistence of unaltered-altered ilmenite and presence of leucoxene in the sample need to be identified and quantified under microscope as they have direct bearing on separation. Since the degree of weathering or alteration can be an indicator of its economic value, compositional characterisation by such integrated instrumental techniques would not only help to adopt better methods for industrial processing but also facilitate the production of different grades of synthetic rutile.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors are indebted to Indian Rare Earths Limited, Chhatrapur, Institute Instrumentation Centre of IIT, Roorkee, for providing the sample, and EPMA, respectively. Thanks are also due for the permission of the Director of CSIR-IMMT, Bhubaneswar, to publish this paper.