Mineralogical studies on complex zoned tourmaline in diaspore nodules from the Milyang clay deposit,Korea

다국어 초록 (Multilingual Abstract)

Tourmaline occurs as aggregates or disseminatedcrystals in the clay matrix of diaspore nodules from the Milyangclay deposit located in southeastern Korea. The diaspore nodulesoccur along or near the fractured zones developed in the clay deposit.Most tourmalines are dravite to schorl in chemical compositionand show complex chemical zonal textures. Chemical analyses andBSE images confirm that alternating Fe-rich and Mg-rich bandsare the most important components that controlled patterns ofchemical zoning in tourmaline. Mg was enriched in the early stageof crystal growth while Fe was enriched in the late stage, with fluc-tuations of the ratio Fe/Mg. Complex multiple growth zoning andoscilatory zoning in tourmaline imply local disequilibrium of thehydrothermal alteration system that formed diaspore nodules. As theoxygen fugacity increased, tourmaline became more Fe-rich withfluctuation of Fe/Mg activity. Textural observations indicate thattourmaline was unstable with respect to dickite. The aluminum-richminerals associated with diaspore nodule suggest that the growthof tourmaline may be highly favorable with alkali-free composi-tions with trace Na.