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

抄録

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, ⁵⁷Fe Mössbauer 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 5.3172 Å, b=9.2721 to 9.2796 Å, c=14.3496 to 14.4078 Å, β=97.984° to 98.085°, V=700.9790 to 701.5324 ų; for clinochlore, a=5.2815 to 5.3495 Å, b=9.1897 to 9.2502, c=14.1634 to 14.2814 Å, β=95.200 to 96.554°, V=692.8576 to 695.8528 ų; and for Cr-clinochlore, a=5.3310 to 5.3190 Å, b=9.1303 to 9.2790Å, c=14.3395 to 14.3370 Å, β=94.589° to 97.670°, V=695.7210 to 701.2580 ų.
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 Mg²⁺by Al^VI which has effected the Fe²⁺-Mg²⁺ substitution. The shrinkage resulting from octahedral Al³⁺ substitution is balanced by the expansion effected by Fe-Mg substitution. The octahedral site (M4) occupied by Al³⁺ is smaller than when occupied by Mg²⁺ or Fe²⁺. The initial Al3+ occupancies of the two M4 sites change with progressive oxidation and metamorphism.
The Mössbauer spectrum of a chlorite sample depicts five symmetric doublets for iron, three of which are assigned to Fe²⁺ 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 Fe²⁺ only at trans position, while at the cis and brucite positions Fe²⁺ remains unaffected. The Mössbauer results of this chlorite suggest the Fe³⁺ 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 Cr³⁺ and Al³⁺ from theoctahedral sites and these released cations would react with contiguous serpentine (antigorite) to form Cr-clinochlore. The Al³⁺ 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 Al³⁺ content with a concomitant lowering Mg²⁺ and Si⁴⁺ contents in the associated Cr-clinochlore. Using the geothermometry of Xie et al.(1997) the temperature of formation of these chlorites (Al^IVranging from 1.398 to 2.540) are found to lie between 263 and 463°C; this wide range of temperature may arise from polymetamorphism experienced by the area.