An Experimental Study of Biotite in Na-,F-,and Li-rich Pneumatolyto-hydrothermai Solutions

Methodology is described in this paper for investigating biotite reactions in pneumatolyto-hydrothermal solutions rich in Na,F and Li by using tow buffer systems,NNO and CFG.The results show:(1)Under the experimental conditions biotite is unstable and different new stable phases will be formed,depending upon the fugacity of oxygen.Na-rich minerals,such as aegirine and albite,will be produced at low oxygen fugacity. while Li-rich micas are found stable at high oxygen fugacity.This agrees with the field observation that albitization occurs in general at a lower position than that of Li-rich micas,suggesting that alkali- metasomatism of biotite may provide the necessary components for subsequent reactions.(2)The stability of Li-rich micas is dependent on Mg~(2+) concentration in the medium,which in turn is determined by Mg~(2+) content in the starting biotite.Li-rich micas are favored by metasomatism of only those biotites that are poor in Mg~(2+).(3)Unstability of biotite would favor the incorporation of ore-forming elements contained in it as isomorphous impurities into solutions.