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

T. R. Narayanan Kutty ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

X-ray, infrared spectra and chemical data reveal that the pink mineral from the calc-silicate rocks of Sakarsanahalli, Kolar, is a mixed phase of zoisite and clinozoisite. It is generally cloudy due to fine inclusions of plagioclase. Polymorphic transformation relations could be observed in it. The mixed crystals are found to be stable when the replacement of Al by Fe³⁺ is less than 10 atom per cent. The yellow-green to green mineral, on the other hand, is monophasic epidote, with 30 to 35 atom per cent Al being substituted by Fe³⁺. Hydrothermal experiments show that zoisite-clinozoisite transforms to zoisite-epidote (iron-rich clinozoisite) pair at higher temperature and pressure. However, larger Fe³⁺ activity upsets this transformation relation, where the monoclinic epidote phase alone is stabilised.

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G. V. Anantha Iyer ¹, T. R. Narayanan Kutty ¹

Affiliations
1 Indian Institute of Science, Bangalore, IN

Abstract

Field features and the mineralogical relations of rocks in the Precambrian of South Karnataka indicate progressive increase in intensity of metamorphism from greenschist facies in the north-west through amphibolite facies in the middle to granulite facies in the southern and south-western parts of the State. Chemical analyses of fifty garnets developed in the amphibolite and granulite facies rocks of Karnataka are furnished. The cell edges of these garnets have been determined accurately by X-ray powder diffraction, using the extrapolation technique of Nelson and Riley. The changing pattern of chemical composition and cell edges of garnets enable recognition of successive zones of progressive metamorphism in the amphibolite facies terrain in the Precambrian of South Karnataka.

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M. Ramakrishnan ¹, T. R. Narayanan Kutty ², G. V. Anantha Iyer ²

Affiliations
1 Jayanagar, Bangalore, IN
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

An alteration product of kyanite from Kudinirkatte near Holalkere, Karnataka State, is found to be a hydrous alumino-silicate with a chemical composition corresponding to that of muscovite. Thermal, X-ray diffraction, and infrared spectral data show distinct variation from the known polymorphs of muscovite. On hydrothermal reactions with potassium-rich solution, the alteration product converts to 1M muscovite, and in sodium-rich solution, it converts to a mixture of muscovite and paragonite. When seeded, it converts to 2M muscovite at a higher temperature and pressure. The alteration product of kyanite has a 1M muscovite structure with randomly distributed montmorillonite domains.

 

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M. N. Viswanatha ¹, M. Ramakrishnan ², T. R. Narayanan Kutty ³

Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore 560006, IN
2 63, Geetha Colony, Jayanagar, IV Block, Bangalore 560011, IN
3 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

No Abstract.

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M. Ramakrishnan ¹, M. N. Viswanatha ¹, N. Chayapathi ¹, T. R. Narayanan Kutty ²

Affiliations
1 17, Raja Mahal Vilas Extension, Bangalore-560006, IN
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, IN

Abstract

Anorthosites of Karnataka occur in two distinct tectonic milieu, within a major arcuate zone, namely, (i) as minor differentiates of gabbro in the ultramafic-mafic complexes emplaced in Archaean high-grade supracrustal belts and the 'Dharwar type' greenstone belt of Shimoga; and (ii) as layered concordant sheets, and as cumulates in a layered basic pluton occurring within the gneissic complex of Archaean high grade terrain. Their absence in 'Keewatin type' greenstone belts and the other  'Dharwar type' belts of Bababudan and Chittadurga is probably due to their location away from the arcuate zone in Karnataka Craton. This arcuate zone reflects a progressive vertical zonation of the crust to deeper levels from northwest to southeast.

The anorthositic rocks of mafic-ultramafic complexes are nonlayered and extensively deformed, whereas those of the high grade terrain show layering and cumulus textures. In the transitional zone, the anorthositic rocks of mafic-ultramafic complexes show cumulus textures, thereby indicating a genetic connection between the anorthosites of two tectonic milieu.

Whole-rock chemistry of the anorthosites of Karnataka indicates differentiation trend of iron-poor environment comparable to other Archaean anorthosites. They are enriched in CaO and A1₂O₃ Niggli mg against Cr or Ni shows positive correlation, corresponding to an igneous parentage for the original rock. Similarly, the plot of Niggli c against mg, c against (al-alk) and 100 mg-c-(al-alk) diagrams exhibit a well defined igneous trend. A common differentiation trend is shown by anorthosites of Karnataka, even though they occur in two distinct tectonic milieu. The singularised trends support the view that the ultramafic-mafic complexes are possibly derived by the dismembering of the basal parts of the layered complexes. The Archaean anorthosites of Karnataka, although comparable broadly to lunar anorthosites, are not related to the early history of the Earth-Moon system.

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P. K. Ramam ¹, G. V. Ananta Iyer ², T. R. Narayanan Kutty ², M. V. N. Murthy ³

Affiliations
1 33/4 RT, Barkatpura, Hyderabad-500027, IN
2 Dept. of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560012, IN
3 A/3, Himanshu Bhawan, Sawai Jai Singh Highway, Jaipur-302006, IN

Abstract

The field observation that chrysotile asbestos near Pulivendla is localised along stylolites in the Proterozoic carbonate sediments of the Vempalli Formation (Cuddapah Supergroup) proved crucial and paved the way for a re-thinking on the origin of Pulivendla asbestos. The length of the fibre, nowhere, transgresses the amplitude of the stylolite. Further, the lateral termination of the chrysotile seams invariably coincides with that of the enclosing stylolite. The stylolite development and fibre growth appear to be a sequel to directional pressure. X-ray and infra-red studies of the massive serpentinous zones revealed dioctahedral dewelyite to be the main serpentine mineral. It is suggested that the chemical pre-requisites (e.g. magnesia, silica and hydroxyl molecules) necessary for serpentinisation and pari passu chrysotilisation, were present in the carbonate sediment itself at the loci of stylolitisation and that the intrusive dolerite merely provided the desired thermal energy. The emplacement took place, possibly, when the carbonate sediment was in a semi-consolidated stage. Stylolitisation and chrysotilisation appear to have ceased with the complete consolidation of the enclosing sediment.

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G. V. Anantha Iyer ¹, T. R. Narayanan Kutty ¹

Affiliations
1 Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore, IN

Abstract

Amphiboles from the Precambrian of Karnataka were analysed for major elements by wet chemical methods. Their cell edges were determined by X-ray powder diffraction technique, using Cr K α radiation. The chemical composition of the amphibole is correlatable with the grade of regional metamorphism. Chemical compositions of the Ca-amphiboles from the northern parts show low alkali and Al⁺ characteristic of tremolite-actinolite group corresponding to greenschist facies. The amphiboles of the middle region are common hornblendes with higher alkali and Al⁺, characteristic of amphibolite facies. Amphiboles from the southern parts are aluminous and alkali-rich with higher titanium and Fe³⁺/Fe²⁺ ratios. They coexist with ortho-or clinopyroxenes, suggesting their formation under granulite facies conditions. The restricted occurrence of sodic amphiboles in the Bababudans, and of cummingtonite-grunerite series in Kudurcmukh iron formations is due to their narrow P-T fields of stability and compositional fields. Anthophyllites from certain horizons of the Precambrian may be regarded as products of metasomatism.

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