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V. Swamy ¹, G. V. Anantha Iyer ², A. G. Menon ³

Affiliations
1 Department of Inorganic and Physical Chemistry, I. I. Sc., Bangalore - 560 012, IN
2 J. N. Centre for Advanced Scientific Research, I. I. Sc., Campus, Bangalore - 560 012, IN
3 Mass Spectrometry Laboratory, I. I. Sc., Bangalore - 560 012, IN

Abstract

This paper presents a numerical simulation of the well-documented, fluid-controlled Kabbal and Ponmudi type gneiss-charnockite transformations in southern India using a free energy minimization method. The computations have considered all the major solid phases and important fluid species in the rock - C-O-H and rock - C-O-H-N systems. Appropriate activity-composition relations for the solid solutions and equations of state for the fluids have been included in order to evaluate the mineral-fluid equilibria attending the incipient charnockite development in the gneisses. The C-O-H fluid speciation pattern in both the Kabbal and Ponmudi type systems indicates that CO₂ and H₂O make up the bulk of the fluid phase with CO, CH₄ H₂ and O₂ as minor constituents. In the graphite-buffered Ponmudi-system, the abundance of CO, CH₄ and H₂ is orders of magnitude higher than that in the graphite-free Kabbal system. Simulation with C-O-H -N fluids of varying composition demonstrates the complementary role of CO₂ and N₂ as rather inert dilutants of H₂O in the fluid phase. The simulation, carried out on available whole-rock data, has demonstrated the dependence of the transfonnation X(H₂O) on P, T, and phase and chemical composition of the precursor gneiss.

Keywords

Charnockitization, Thermodynamic Modelling, High-Grade Metamorphism, Mineral-Fluid Equilibria, Granulites.

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V. Swamy ¹, A. G. Menon ², G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemisty, Indian Institute of Science, Bangalore-560 012, IN
2 Instrumentation and Services Unit, Indian Institute of Science, Bangalore-560 012, IN

Abstract

High-temperature reactions (Ca 900°C) involving albite, K-feldspar or plagioclase and K, Ba-or K, Sr chlorides were experimentally studied. These experiments reveal that the reaction between K-exchanged albite, potash feldspar, or plagioclase and Ba-chloride/Ba-K chloride results in the formation of celsian by the breakdown of the starting feldspar structure above 800°C. Sr-feldspar does not form under similar conditions. A size-effect of the large M-site cation appears to be responsible for the formation of celsian. The reaction between K-feldspar and barium chloride may be used as a method for synthesizing celsian.

Keywords

Albite, K-Feldspar, Plagioclase, Celsian, Igneous Petrology.

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V. Swamy ¹, G. V. Anantha Iyer ², A. G. Menon ³

Affiliations
1 Division of Minerals, CSIRO, Clayton South,. Victoria 3169, AU
2 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560 012, IN
3 Department of Instrumentation, Indian Institute of Science, Bangalore - 560 012, IN

Abstract

The structural state of K-feldspars in the quartzofeldspathic gneisses, charnockites, metapelites and pegmatites from the southern Karnataka, northern Tamil Nadu and southern Kerala high-grade regions of southern India has been characterized using petrographic and powder X-ray diffraction methods. The observed distribution pattern of structural state with a preponderance of disordered K-feldspar polymorphs in granulites compared to the ordered microclines in the amphibolite facies rocks is interpreted to reflect principally the varying H₂O contents in the metamorphic-metasomatic fluids across metamorphic grade. The K-feldspars in the pegmatites of granitic derivation and in a pegmatite of inferred metamorphic origin also point to the important role of aqueous fluids in their structural state.

Keywords

X-ray Diffraction, Amphibolite Facies, Granulites, Metamorphic-Metasomatic Fluids.

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