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Optical microscopy, X-ray diffractometry, micro CT studies and mineral chemistry by SEM-EDS has been carried out on cordierite rich rock fragments from Bandarguha and Orabahala areas of Kalahandi district, Odisha. Macroscopically, cordierite grains exhibit distinct pleochroism in the shades of violet blue to light blue and colourless and are of gem quality (iolite). Biotite is the dominant associated mineral and Mg-Fe distribution coefficient (KD) between coexisting cordierite and biotite varies from 1.71 to 2.76. The cordierite is low cordierite (distortion index, Δ=0.257). Mineral analyses indicate that cordierite is magnesian rich (XMg=0.82-0.88) with limited Fe2+ substitution for Mg2+ and dichroism is due to presence of part of iron in ferric form. Cordierite is formed during metamorphism by reactions: (a) orthopyrox ene+sillimanite+quartz=cordierite+K-feldspar+water and (b) orthopyroxene+sillimanite=cordierite+corundum.
Keywords: Cordierite; Pleochroism; Pinitisation; Distortion index; Mg-Fe exchange; Odisha
Cordierite mineral is predominantly a product of metamorphism. It was regarded high temperature-low pressure mineral typically developed in thermal aureoles (hornfels), as a major constituents of high grade regionally metamorphosed rocks of pelitic composition (shales and other argillaceous rocks), and as constituent of aluminium rich xenoliths in igneous rocks. Cordierite in small quantities occurs in magmatic rocks as well as in granite, pegmatites, gabbro, rhyolite and andesite. Cordierite is an index mineral for the anatectic origin of the rocks or contamination by Al-rich contact rocks (e.g., clay-rich sediments). In the regionally metamorphosed rocks, granulites and gneisses, cordierite coexists with other Mg-Fe mineral phases such as garnet, biotite, orthopyroxene, spinel, or staurolite as well as with other more aluminium bearing phases like sillimanite, sapphirine and corundum.
When transparent and of high clarity and pleochroic in shades of violet blue to blue, yellow and colourless, cordierite is used as a gemstone which is known as "iolite" in the gem and jewellery industry. India is an important market for iolite gem trading and iolite is extensively used in jewellery industry. Iolite occurrences have been reported from high grade metamorphic rocks of Eastern Ghat Mobile Belt of the states of Odisha and Andhra Pradesh and in metamorphic rocks of Karnataka and Tamil Nadu . However, gem quality iolite deposits are found in few localities in Kalahandi district, Odisha which are: Orabahala (19°48'13"N/83°04'12"E), Dedar (19°47’30”N/83°02’30”E) and Bandarguha (19°48’26”N/83°02’30”E) (Figure 1a) [2-4]. The evolution of Eastern Ghat Mobile Belt occurring in eastern India (mainly in the states of Odisha and Andhra Pradesh) has been described by many authors [5-7]. The granulite facies Eastern Ghat Mobile Belt has been divided into four longitudinal zones which from east to west are: western charnockite zone, western khondalite zone, central migmatite-charnockite zone and eastern khondalite zone (Figure 1b). Samples from the present study fall in the leptynite and quartz-feldpathic lithounits of western khondalite zone, Kalahandi district, Odisha (Figure 1a).
Figure 1a: Geological map around Bandarguha area . 1: Quartz feldspathic gneisses and mafic granulites; 2: Mafic granulites and amphibolites with enclaves of cordierite-phlogopite rocks; 3: Mafic granulite, charnockite gneiss, garnet plagioclase dominated Fe-Al rich rocks and amphibole dominated Mg-Al rich rocks.
Figure 1b: Geological extension and different lithounits of Eastern Ghat Mobile Belt, India . Bandarguha location is shown in this figure.
During 2012, mining operation for iolite was being carried out from Bandarguha area. The mining area represents a flat terrain with gentle sloping topography. The cordierite schists and gneisses occur as discontinuous lensoid bodies within colluvial zone represented by highly weathered quartzo-feldspathic gneisses (leptynite). The major rock type is cordierite (iolite)-biotite schists (Figure 2), the total reserve of schist is about 8 million tonne and the plant capacity was 3.6 tonne cordierite (iolite) per year. Natural large gem quality cordierite crystals (iolite) are also found in the cordierite–phlogopite dominated Mg-Al rich rocks near Orabahala [2,3].
Semi-precious pure iolite samples and cordierite (iolite) rich rock fragments were collected by hand picking from the mining dumps of the cordierite mines at Bandarguha and from cordierite gneiss outcrops of Orabahala areas, Kalahandi district. The size of the samples ranges between 40 to 3 mm. In the laboratory the samples were cleaned with water in ultrasonic bath to remove loosely adhering soil particles. Thin and polished sections of the rocks and mineral fragments were prepared by adopting standard section preparation techniques. The thin sections were studied under petrographic microscope (model: DM2500P; M/s. Leica Microsystem Gmbh, Wetzler, Germany). The polished blocks were carbon coated and semi-quantitative analysis of the mineral phases was done by Energy Dispersive Spectrometer (M/s. Bruker AXS Microanalysis, Berlin, Germany) attached to scanning electron microscope (SEM; model: EVO-50; M/s. Zeiss, Germany). The pure cordierite fragments were powdered and heated to 900°C for one hour to determine the water content. Pure cordierite samples were handpicked and finely powdered in an agate mortar. The powdered sample was taken for X-ray diffraction analysis using an XRD unit (Philips PW 3710) with Cukα1 radiation operating at voltage of 40 kV, current of 20 mA and scan speed of 1/8 degree 20 per minute. 3D characterization of small cordierite sample was performed with X-ray microcomputer tomography (microCT) (M/s. Bruker, Belgium; model: Sky Scan 2211) in combination with 3D analysis using Avizo 9.0 software.
The coarse cordierite bearing fragments are translucent to almost opaque and cordierite grains are clustered with each other (Figure 3a). The coarse cordierite bearing fragments/rocks mostly consist of cordierite and biotite and small amounts of quartz, feldspars and fine friable clay minerals along grain boundaries and fractures (Figures 3 b and 3c). Under naked eye, thin Iolite fragments exhibit strong to weak pleochroism that varies from colourless to violet blue to blue, greyish blue to light gray, and rarely with yellowish hue (Figures 3d-3f). It has duller somewhat greasy and vitreous lustre. Biotite flakes and sheets are dark brown in colour, translucent and vitreous and quartz grains are granular.
In thin section under plane polarised light, cordierite grains are colourless to very pale blue mostly with no pleochrosim, though grain mounts in R.I. liquid (1.53) are distinctly pleochroic. It occurs as idioblastic, subidioblastic to highly irregular porphyroblastic grains (Figures 4a, 4b and 4k). Under crossed nicols, cordierite grains are colourless to slightly blue. Twinning is common in the cordierite grains which are mostly lamellar and polysynthetic types; twin lamellae are mostly discontinuous and wedge out in the form of a tip of a knife or pen (Figures 4c and 4d). Some of the twins are abruptly stepped, irregularly distributed, at times coalesces with each other forming patchy twinning (Figure 4e). Some grains show two sets of twin lamellae which occur in 60o orientations (Figure 4f). Complex or multiple twinning features in cordierite are shown in Figures 4g and 4h. Cordierite showing simple and polysynthetic types of twinning are formed at a lower temperature growth condition . Cordierite is at places altered to pinite, a finegrained greenish or yellowish aggregate of chlorite, muscovite, serpentine, iron oxides and other silicates. Extensive alterations often lead to pinitized pseudomorph after cordierite (Figure 4i). The alteration is mostly prominent along cracks and grain margins (Figure 4j). The sericite bearing component is formed by the action of K-bearing solution on cordierite.
Biotite is abundant, occurs as weakly oriented tabular and flattened platy crystals along the grain boundaries of cordierite (Figures 4b and 4k) and as irregular massive bodies (Figures 4l and 4m). Biotite is brown, red-brown, brown-green and green in colour and strongly to weakly pleochroic in nature. The greenish biotite is secondary while the red brown biotite is primary in origin . Corundum varies widely in shape and size from small euhedral to large subhedral grain (Figure 4n). Sillimanite occurs as bladed and acicular crystals up to 3 mm in length usually associated with masses of biotite and cordierite (Figure 4o). Sillimanite also occurs as fibrous masses (fibrolite) within cordierite grains (Figure 4p). Minute zircon, monazite, rutile, and pyrite grains are sometimes noted as inclusions within cordierite (Figure 4q). Perthite occurring as vein is also found in cordierite bearing fragments (Figure 4r). Cordierite grains often contain inclusions of fine quartz grains which occur as trails (Figure 4s). In cordierte fragments, orthopyroxene is rare and occurs as small stout grain.
X-ray diffraction studies
The powder X-ray diffractogram of pure cordierite sample between 29 to 31 degree 2θ using CuKα1 radiations is given in Figure 5. The distortion index (Δ), calculated for this sample using the 2θ positions at (511), (421) and (131) planes, provided a value of 0.257 which is close to the distortion index of 0.25 reported by Majumder and Mathew . This distortion index computes the deviation of the cell structure from a geometrically hexagonal unit and cordierite sample of the study area is assigned as low cordierite with ordered structure. The interpretation assumes that cordierites with large distortion indices typically occur in regionally metamorphosed terrain characterized by slow heating and cooling .
X-ray micro CT studies
X-ray microCT is a powerful non-destructive probing technique for 3D visualisation of the internal structure and composition of a wide range of objects. X-ray microCT images of cordierite sample in three different slices in 2D sectional view are presented in Figures 6a-6c. Within the cordierite matrix, the included grains/mineral phases are unevenly distributed. Light grey mineral phase (Figures 6a and 6b) occurring as small platy and acicular grains are possibly biotite and altered minerals. The rounded and subrounded grains are possibly zircon, monazite, iron oxides and pinite. The cordierite grain is pore free.
The semi-quantitative analyses of the minerals (cordierite, biotite, plagioclase feldspar, potash feldspar, corundum and orthopyroxene) are given in Table 1. The loss on ignition percent of pure cordierite sample was determined to be 1.85% which indicates presence of volatile matter (H2O and CO2) in the crystal structure. Cordierite shows restricted range of aluminium contents corresponding to 3.945 to 4.073 apfu (atoms per formula unit). The no. of silicon atoms (apfu) is close to five and a small part of silicon is possibly substituted by Al. The cordierite grains are magnesian rich and iron poor (Mg- 1.734–1.940; Fe- 0.273-0.380 apfu), Mg/Fe and Mg/(Mg+Fe) ratios varies between 4.63-7.11 and 0.82-0.88 respectively (Table 1). The data indicate that within cordierite structure replacement of Mg by Fe2+ is restricted which corroborates the views of earlier workers [3,10]. Biotite in the cordierite fragments/rocks has high TiO2 (2.28-3.32%), high MgO (12.86-14.72%) and Al2O3 (16.65- 18.88%) (Table 1). The Mg/Fe and Mg/Mg+Fe ratios in biotite varies from 1.85-2.72 and 0.65-0.73 respectively. The Mg/Fe distribution coefficient between coexisting cordierite and biotite (KD) varies from 1.71 to 2.76 and is characteristic of low to high grade metamorphic rocks . The plagioclase feldspar is oligoclase in composition. The orthopyroxene has Mg/Mg+Fe ratio of 0.67 and the Al2O3 content 5.93 %.
|Number of ions on the basis of|
Table 1: Chemical analyses (wt.%) by SEM-EDS of different minerals. [Biotite (5 and 6) coexisting with cordierite (1 and 2) respectively; K-felds: K-feldspar in perthite, Plag-Plagioclase feldspar in perthite, OPX-Orthopyroxene, Crn-Corundum].
(1) Cordierite grains are Mg rich with XMg values ranging between 0.82-0.88.
(2) Mineral chemistry of gem quality cordierite samples by SEMEDS studies indicated the presence of a small amount of Fe in the crystal structure. A part of iron in the cordierite structure is possibly present in the form of Fe3+ that facilitate charge transfer process resulting in blue to violet pleochroism, hence, rendering these stones as gem quality cordierite (iolite) .
(3) Several reactions have been proposed for cordierite crystallisation in the metamorphic rocks . Biotite, orthopyroxene, sillimanite, corundum, feldspars and quartz have been identified in the cordierite samples which corroborate the observations of [2,3,10]. One of the possible reactions for cordierite formation is: orthopyroxene+sil limanite+quartz=cordierite+K-feldspar+water. Further orthopyroxene can react with sillimanite to form cordierite and corundum as per reaction- orthopyroxene+sillimanite=cordierite+corundum.
The authors thank the Director, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar for kind permission to publish this paper. We are also grateful to Mr. M.K. Senapati, Deputy Director (Retd.), Directorate of Geology, Govt. of Odisha and Mr. Mund at Bhawanipatna for help in field work and valuable discussions.