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 aqueou s (Cu, Na-2)Cl-2 solutions at ambient conditions to investigate biotit e oxidation mechanisms at low temperatures and pressures and at atmosp heric 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 diffract ion, and comprehensive bulk chemical analyses. Even though both biotit es were pre treated with a sodium tetraphenylboron (NaTPB) solution, w hich rapidly exchanges Na for K, only about 50% of the interlayer K wa s exchanged during most of these experiments. As a result, the exchang e 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 experi ments and was nearly independent of the total mass of Cu introduced in to 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 biotit e 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 rat e than the Fe+2 in M(2) sites. Our study suggests that the total amoun t of Fe oxidized was limited by the amount of K exchanged, and that pr eferred oxidation of Fe+2 at M(2) sites relative to M(1) sites was a f unction of the F content of these biotites. Charge transfer from octah edral Fe+2 to the interlayer may be facilitated by deprotonation. In e xchange 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 t o 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 the n the proposed F-OH ordering model could account for the selective dea ctivation of the Fe+2 oxidation mechanism at the M(1) site.