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The Asurabandha area on the southern side of the main Sukinda Ultramafic Complex (SUC) in the State of Odisha hosts some irregular and sporadic chromitite bodies which are completely detached from the main SUC. The isolated chromitite bodies are highly altered. Chromitite samples from these isolated bodies have been studied using a polarising petrological microscope, scanning electron microscope (SEM), field emission-scanning electron microscope (FE-SEM), X-ray diffraction (XRD) and wavelength dispersive X-ray fluorescence (WD-XRF) spectrometer and the findings are presented here. In this area, we have identified the associated silicate minerals with chromite to be primarily opal-CT and quartz, contrary to the conventional Fe–Mg silicates such as olivine, pyroxene, or serpentine, although in few instances, the Fe–Mg silicates are identified. The original Fe–Mg silicates have almost completely been replaced by the silica polymorphs. Occurrences of opal-CT and quartz have been identified from their prominent X-ray diffraction peaks at d-spacing of 4.10 Å (with a shoulder peak at 4.32 Å) and 3.34 Å, respectively. The phase compositions are also confirmed by SEM-EDAX analyses. The average composition of opal-CT was found to be (in wt.%) SiO2: 95.41, MgO: 1.75 and FeO: 1.74. The presence of MgO and FeO in the composition of opal-CT, along with relict mesh-like micro-structures, indicate its formation by silicification/opalization from serpentines. A low-temperature hydrothermal alteration process driven by Si- and CO2-rich fluids through fractures and fault zones might have caused the removal of Mg and Fe from primary ultramafic silicate minerals and precipitation of opal-CT. This replacement process involved the initial dissolution of serpentine with preferential leaching of the metallic ions (Mg, Fe) and simultaneous precipitation of amorphous silica to form possibly opal-A in the beginning and then to opal-CT which subsequently transforms to microquartz because of the ordering of structure and removal of water through the dissolution–precipitation mechanism. [ABSTRACT FROM AUTHOR]

Extreme wave events of 1000 and 1500 years (radiocarbon ages) have been recently reported in Mahabalipuram region, southeast coast of India. Subsequently, we carried out extensive sedimenttological analysis in regions covering a total lateral coverage of 12 km with a new archeological site as the central portion of the study area. Twelve trenches in shore normal profiles exhibit landward thinning sequences as well as upward fining sequences confirming with the global signatures of extreme wave events. The sediment size ranges from fine-to-medium and moderately well sorted-to-well sorted, and exhibit positive skewness with platykurtic-to-leptokurtic nature. We now propose the abrupt winnowing or back and forth motion including unidirectional transport of these deposited sediments, which results in positive skewness. Textural analyses derived from scanning electron microscope studies (SEM) demonstrate the alteration produced, in the ilmenite mineral with vivid presence of pits and crescents with deformation observed on the surface due to extreme wave activities. This is further confirmed with the predominance of high-density mineral such as magnetite (5.2) and other heavy minerals in these deposits inferred the high-intensity of the reworking process of the beach shelf sediments. [ABSTRACT FROM AUTHOR]

Cretaceous phosphorites from the onshore of Tamil Nadu have been investigated for their origin and compared with those in the offshore. Cretaceous phosphorites occur as light brown to yellowish brown or white nodules in Karai Shale of the Uttatur Group in the onshore Cauvery basin. Nodules exhibit phosphatic nucleus encrusted by a chalky shell of carbonate. The nucleus of the nodules consists of light and dark coloured laminae, phosphate peloids/coated grains and detrital particles interspersed between the laminae. Scanning electron microscope (SEM) studies reveal trapping and binding activity of microbial filaments. A mat structure with linearly arranged microbial filaments and hollow, cell-based coccoid cyanobacterial mat are present. Nodules contain abundant carbonate fluorapatite, followed by minor calcite, quartz and feldspar. The P2O5 content of the phosphorites ranges from 18 to 26%. The CaO/P2O5, Sr and F contents are higher than that of pure carbonate fluorapatite. Concentrations of Si, Al, K, Fe, and Ti are low. We suggest that the nuclei of the nodules represent phosphate clasts related to phosphate stromatolites formed at intertidal conditions. At high energy levels the microbial mats were disintegrated into phosphate clasts, coated with carbonate and then reworked into Karai Shale. On the other hand, Quaternary phosphorites occur as irregular to rounded, grey coloured phosphate clasts at water depths between 180 and 320m on the continental shelf of Tamil Nadu. They exhibit grain-supported texture. Despite Quaternary in age, they also resemble phosphate stromatolites of intertidal origin and reworked as phosphate clasts onto the shelf margin depressions. Benthic microbial mats probably supplied high phosphorus to the sediments. Availability of excess phosphorus seems to be a pre-requisite for the formation of phosphate stromatolites. [ABSTRACT FROM AUTHOR]