Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

No Abstract.

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Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

No Abstract.

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Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

The chromite-bearing ultramafic pluton in Sukinda, Orissa, was emplaced in two horizons in the Iron Ore Series rocks which were subjected to shearing deformation and hydrothermal activity, followed by silicification. These processes in most cases completely transformed the early silicates to chlorites and produced, in some cases, compositional zoning in the periphery of chromite grains. The chromite grains and samples from different horizons show variations in composition, reflectivity and latticeparameter. Eight analyses of separated chromite samples are presented along with spot microprobe analyses for Cr, Mg, Fe and Al; these reveal not only marked compositional differences among discrete grains but also show distinct compositional zoning in individual grains.

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Sachinath Mitra ¹, Syed Samsuddin Ahmed ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta 700 032, IN

Abstract

The beach and coastal dune sands of Cox's Bazar constitute the most important source of metallic minerals in Bangladesh. The sands are fine, well-sorted, negatively skewed and leptokurtic in their grain-size distributions. Plots of different size parameters suggest that (a) the finer the grain-size the better is the sorting; (b) the sediments are more negatively skewed as the grain-size decreases; and (c) sorting improves as the skewness changes from nearly symmetrical to negative. The dune sands are finer, better sorted and more negatively skewed than beach sands. The percentage of heavy minerals increases from 2 φ (250 μ m) to 3.5 φ (88 μ m) fraction. The light fraction consists predominantly of quartz (about 90 %), while amphibole, ilmenite, garnet, epidote, mica, pyroxene, magnetite. kyanite, tourmaline, staurolite, zircon, rutile and monazite constitute the heavy minerals. The distribution of the heavies in the dunes and beaches depend on their respective shape. The equant ones are predominant in the former than in the latter. The predominance of unstable minerals and their shapes suggest that the major source of the sediment may be in the easterly highlands of Chittagong-Arakan hills.

Keywords

Sedimentology, Beach and Dune Sands, Cox's Bazar, Bangladesh.

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Sachinath Mitra ¹

Affiliations
1 Jadavpur University, Calcutta 700 032, IN

Abstract

No Abstract.

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Samsuddin Ahmed ¹, Taraknath Pal ¹, Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta - 700 032, IN

Abstract

About 52% of the samples of Cox's Bazar beach contain six textural types of hematite-ilmenite exsolutions,the commonest (∼83%) being the seriate type. Hemo-ilmenite with greater ilmenite contnet exsolves in a process of continuous exsolution mechanism, whereas more hematiterich phases (very common) exsolve discontinuously. The hemo-ilmenites (IIm₄₅ Hem₅₅ to IIm₇₅ Hem₂₅) are ferrimagnetic in nature. Modal analysis has indicated the percentages of pure ilmenite (exsolved + unexsolved type) and hematite (exsolved, both host and guest) to be 73.74% and 26.26% respectively. This result also corroborates with X-ray, Mossbauer and chemical analyses.

Keywords

Cox's Bazar Beach Sand, Bangladesh, Exsolution Textures, Mossbauer Spectroscopy, X - ray Diffraction, Petrography.

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Tapan Pal ¹, Hi-Soo Moon ², Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta- 700032, IN
2 Department of Geology, Yonsei University, Seoul, KR

Abstract

⁵⁷Fe Mossbauer spectroscopic investigation of natural chromites from two chromite deposits of India (Sukinda and Byrapur) documents partly inverse spinel structure arising out of oxidation. The spectral fitting was based on allowing a disordering distribution of Fe²⁺ and Fe³⁺ ions at tetrahedral (A) and octahedral (B) sites. Mossbauer investigation of the samples taken from the physico-chemically distinct two horizons of Sukinda viz. brown are and grey ores, and Byrapur area revealed three types of iron ion distribution as: Fe²⁺(A), Fe³⁺(A) and Fe²⁺(B) (GC-group); (b) Fe³⁺(A1), Fe³⁺(A2) and Fe²⁺(B) (BC-2 group), and (c) Fe³⁺(A1), Fe³⁺(A2) and Fe³⁺(B) (BC-1 group).

The distribution pattern of iron cations at A and B sites was linked to the degree of oxidation. The stages of oxidation could be modelled from normal to inverse fonn. A model suggesting 'electron localisation' at the B-sites makes the intermediate stage. Iron site occupancy determined by Mossbauer spectroscopy of the presently studied samples indicates that these fall under three groups of oxidation stages. An early stage of oxidation is shown by samples of group GC, intermediate stage by group BC-2 and final stage by BC-1 group of chromite samples. The imprint of progressive oxidation manifested by Fe cation site occupancy has been correlated with the Fe²⁺/Σ:Fe ratios, obtained for each group of samples.

Keywords

Chromites, Mineralogy, Sukinda, Orissa, Byrapur, Karnataka, Mossbauer Spectroscopy.

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Sachinath Mitra ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700 032, IN

Abstract

A low Al, low Ca enstatite from within an orthopyroxenite dyke rock in the Sukinda chromite field was physico-chemically investigated by use of ⁵⁷Fe Mossbauer spectroscopy XRF and XRD. The parameters of the fitted ⁵⁷Fe Mossbauer spectra show the presence of Fe³⁺ in the octahedral M1 site of pyroxene structure.

The room temperature Mossbauer results show three doublets corresponding to Fe²⁺ (Ml), Fe²⁺ (M2) and Fe³⁺ (M1) sites. The QS values in two samples are found to be 2.80 and 2.96 mm/s for Fe²⁺ , (M1), and 1.89 and 2.08 mm/s for Fe²⁺ (M2) doublets. In both samples Fe³⁺-(M1) gives IS values 0.38 and 0.56 mm/s and QS values 0.48 and 0.49 mm/s. From their dependence on X^Fe21_M¹and X^Fe21_M2 distributions the bond distances of tetrahedral and octahedral sites have been measured.

In the present case Fe³⁺ in the orthopyroxene structure is presumed to have been formed primarily at the stage of its crystallisation from a melt having requisitely high fO₂. The temperature recorded by the Sukinda orthopyroxenile body has been estimatcd as 800°C from the distribution of Fe²⁺ - Mg over Ml and M2 sites. From the Fe³⁺/Fe²⁺ ratio the oxygen fugacity has been calculated as log fO₂ = -6.189, which suggests that crystallisation of Sukinda orthopyroxene took place in the lower crustal regions rather than in the mantle.

Keywords

Mineralogy, Pyroxenes, Sukinda, Orissa.

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Sachinath Mitra ¹, Asok Kumar Samanta ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700 032, IN

Abstract

The presence of Fe³⁺ at both tetrahedral (A) and octahedral (B) sites (-45% of total iron). from Mossbauer spectroscopic data of Sittampundi chromite, strongly suggests that high fO₂(10^-7.30 to 10^-7.34) conditions were prevalent during its formation. This Cr-spinel was formed with enough of Ti and Al (at its B-site) in its initial structure, attained while in the primary melt at high P-T field (>10.6 kbar, >950°C). The high fO₂ in the crystallizing environment did not allow iron to remain as "free" Fe²⁺ ion to be stabilized into ilmenite structure. In chromite lattice, Ti⁴⁺ gets stabilized at Bsite only by coupled occupancy with bivalent cations, strongly with Fe²⁺. Inadequacy of Fe²⁺ content to charge-balance Ti⁴⁺ at B-site due to further oxidation of iron to Fe³⁺ caused an expulsion of Ti⁴⁺ from the chromite lattice. The released free Ti⁴⁺ ion readily combines with oxygen alone to form rutile (or its ilk) which comes out as acicular exsolution bodies within chromite, often following its crystallographic planes [along (111)].

Keywords

Chromite, Mineral Chemistry, Rutile Exsolution, Sittampundi Complex, Tamil Nadu.

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Sachinath Mitra ¹, Syed Samsuddin Ahmed ²

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700 032, IN
2 Department of Geology and Mining, Rajshahi University, Bangladesh, BD

Abstract

Ti-poor magnetites from Cox's Bazar beach are seen to be remarkably homogeneous, chemically and optically, containingless than 2% uIvospinel. Mg, Ti, Cr, A1 and V, though in insignificant amounts, are found to be responsible in contributing to the reduction in hyperfine field of the Mossbauer spectrum, which showed linewidths of 0.18 mm/s at B-site and 0.26mm/s at A-site. Certain fraction of these magnetites manifests authigenic growth, presumably in a locally reduced condition. Intergrowth of ilmenite with titanomagnetites possibly resulted from oxidation of ulvospinel-magnetite and a regular partitioning of Cr, V, AI, Mg and Mn observed between coexisting titanomagnetite and ilmenite suggests a chemical equilibrium. The temperature and oxygen fugacity of coexisting titanomagnetite-ilmenite phases as calculated ranged from 625-850°C and 10^-15 to 10^-21 respectively. These temperatures and oxygen fugacities equilibrated in a T-fO₂, field very near to FMQ (Fayalite-Magnetite-Quartz) buffer curve, lending support to its genesis in the upper crust, more being of igneous sources rather than metamorphic.

Keywords

Mineralogy, Geochemistry, Geothermometry, Magnetite, Beach Sands, Bangladesh.

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