CRYSTALLO-CHEMICAL CHARACTERISTICS OF CHLORITES FROM THE GREENSTONE BELT OF SOUTH INDIA, AND THEIR GEOTHERMOMETRIC SIGNIFICANCE

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概要

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• Chlorites from Byrapur, Bhaktarhalli and Tagadur mine areas in Nuggihalli schist belt, belonging to the greenstone belt of South India, were investigated for determining the crystallochemical characteristics of these product phases arising from low-grade metamorphism of ultramafic rocks of komatiitic lineage. The analytical equipments used for such investigation were EPMA, ^<57>Fe Mossbauer spectroscopy,XRD and IR. The physicochemical properties of these chlorites are found to lie between aluminian clinochlore and clinochlore varieties. The cell parameters calculated from major XRD peaks reveal the crystallographic parameters, the range of which are: for aluminian-clinochlore, a=5.3027 to 5.3172Å, b=9.2721 to 9.2796Å, c=14.3496 to 14.4078Å, β=97.9840° to 98.085°, V=700.9790 to 701.5324Å for clinochlore, a=5.2815 to 5.3495Å, b=9.1897 to 9.2502, c=14.1634 to 14.2814Å, β=95.200 to 96.554°, V=692.8576 to 695.8528Å and for Cr-clinochlore, a=5.3310 to 5.3190Å, b=9.1303 to 9.2790Å, c=14.3395 to 14.3370Å, β=94.589° to 97.670°, V=695.7210 to 701.2580Å. In all the samples octahedral sites are mostly occupied by Al^<VI>, Fe and Mg. The chemical variation is largely controlled by the isomorphous replacement of Mg^<2+> by Al^<VI> which has effected the Fe^<2+>-Mg^<2+> substitution. The shrinkage resulting from octahedral Al^<3+> substitution is balanced by the expansion effected by Fe-Mg substitution. The octahedral site (M4) occupied by Al^<3+> is smaller than when occupied by Mg^<2+> Fe^<2+>. The initial Al^<3+> occupancies of the two M4 sites change with progressive oxidation and metamorphism. The Mossbauer spectrum of a chlorite sample depicts five symmetric doublets for iron, three of which are assigned to Fe^<2+> at cis and trans positions of talc layer, and in brucite layer; the other two are assigned to Fe^<3+> at trans position and tetrahedral site. The iron distribution over the octahedral site signifies oxidation of Fe^<2+> only at trans position, while at the cis and brucite positions Fe^<2+> remains unaffected. The Mossbauer results of this chlorite suggest the Fe^<+3> occupancy in octahedral sites was the result of oxidation (following chloritisation) and the distribution of other cations in the other two sites remained unaffected. The oxidation of chromite grains would release Cr^<3+> and Al^<3+> from the octahedral sites and these released cations would react with contiguous serpentine (antigorite) to form Cr-clinochlore. The Al^<3+> content of Cr-clinochlore is seen to be distinctly related to the degree of oxidation of the associated chromite-a greater degree of oxidation of chromite is related to a higher Al^<3+> content with a concomitant lowering Mg^<2+> and Si^<4+> contents in the associated Cr-clinochlore. Using the geothermometry of Xie et al. (1997) the temperature of formation of these chlorites (Al^<IV> ranging from 1.398 to 2.540) are found to lie between 263 and 463℃; this wide range of temperature may arise from polymetamorphism experienced by the area.

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著者

• Mitra S

  Jadavpur Univ. Kolkata Ind

• Mitra Saghinath

  Department Of Chemistry Jadavpur University

• BIDYANANDA MAIBAM

  Department of Geological Sciences, Jadavpur University

• Bidyananda Maibam

  Department Of Geological Sciences Jadavpur University

関連論文

• CRYSTALLO-CHEMICAL CHARACTERISTICS OF CHLORITES FROM THE GREENSTONE BELT OF SOUTH INDIA, AND THEIR GEOTHERMOMETRIC SIGNIFICANCE

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