An experimental study of biotite in Na-, F-, and Li-rich pneumatolyto-hydrothermal solutions

Abstract

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 alkalimetasomatism of biotite may provide the necessary components for subsequent reactions. (2) The stability of Li-rich micas is dependent on Mg²⁺ concentration in the medium, which in turn is determined by Mg²⁺ content in the starting biotite. Li-rich micas are favored by metasomatism of only those biotites that are poor in Mg²⁺. (3) Unstability of biotite would favor the incorporation of ore-forming elements contained in it as isomorphous impurities into solutions.