THE INFLUENCE OF STRUCTURAL FLUORINE ON BIOTITE OXIDATION IN COPPER-BEARING, AQUEOUS-SOLUTIONS AT LOW-TEMPERATURES AND PRESSURES

High-F (5.4 wt%) and low-F (0.8 wt%) biotites were reacted with aqueous (Cu, Na-2)Cl-2 solutions at ambient conditions to investigate biotite oxidation mechanisms at low temperatures and pressures and at atmospheric pO(2). The exchange of Cu+2 for interlayer cations increases the rate of biotite oxidation under these conditions. Solid reactants and products were characterized by Mossbauer spectroscopy, X-ray diffraction, and comprehensive bulk chemical analyses. Even though both biotites were pre treated with a sodium tetraphenylboron (NaTPB) solution, which rapidly exchanges Na for K, only about 50% of the interlayer K was exchanged during most of these experiments. As a result, the exchange reactions produced variably expanded phases with d(001) ranging from approximately 10 to 14 Angstrom. Octahedral Fe+2 in samples of high- and low-F biotite was oxidized rapidly during Cu exchange. The degree of Fe+2 oxidation amounted to about 50% of the total Fe in most experiments and was nearly independent of the total mass of Cu introduced into the interlayer which ranged from 2.0 to 9.2 wt% CuO. The Mossbauer spectra also show that the Fe+2 in M(1) octahedra of the high-F biotite was oxidized more slowly than Fe+2 in M(2) sites, whereas in the low-F biotite experiments M(1) Fe+2 was oxidized at a slightly faster rate than the Fe+2 in M(2) sites. Our study suggests that the total amount of Fe oxidized was limited by the amount of K exchanged, and that preferred oxidation of Fe+2 at M(2) sites relative to M(1) sites was a function of the F content of these biotites. Charge transfer from octahedral Fe+2 to the interlayer may be facilitated by deprotonation. In exchange experiments conducted on the F-rich biotite, Fe+2 oxidation at M(1) sites was limited indicating that preferential substitution of F for OH might occur at the M(1) site. We used the Fe-F avoidance law to develop an F-OH ordering model that preferentially distributes F on selected trans positions of Fe-filled M(1) octahedra in Fe- and F-rich biotites. If charge transfer is facilitated by the presence of OH then the proposed F-OH ordering model could account for the selective deactivation of the Fe+2 oxidation mechanism at the M(1) site.