Retrograde Chlorine-bearing Greenalite from the Iron-formation of Kudremukh, Karnataka, India

摘要

Greenalite occurs locally as tiny acicular bundles at grunerite-calcite and magnetite-calcite boundaries of the BIF of Kudremukh. Chemically it is chlorine en-riched (1.16wt.percent Cl) but close to the theoretical greenalite composition and has the formula (Fe~(2+)_(3.259)Fe~(3+)_(1.537)Mg_(0.379)Mn_(0.041)Ni_(0.009) Ca_(0.008)Na_(0.004)Ti_(0.002) [(OH_(3.778)Cl_(0.222)) (Si_(3.985)Al_(0.015))O_(10)]. It nucleated on syn-or post-tectonic grunerite, consequent upon its development during retrograde metamorphism. Interaction of the locally entrapped traces of brines (or metamorphic fluids) enriched in Cl with grunerite/magnetite in silica deficient sub-system at relatively lower P_(O_2) and under waning metamorphic temperature is advocated for the formation of the mineral.