BIOTITE DISSOLUTION UNDER AN ANOXIC CONDITION: IMPLICATION FOR FE BEHAVIOR DURING ANOXIC WEATHERING

Biotite dissolution experiment was carried out under an anoxic condition to elucidate Fe behavior during anoxic weathering. The biotite dissolution was done at 100 degree C for 7 to 120 days in a Teflon reaction vessel that was placed in a glove box where the oxygen concentration was kept less than 1 ppm (about 10(-6) atm of PO2) at room temperature. One atm of PCO2 (at 100 degree C) was introduced into the Teflon vessel during the experiment. The carbon dioxide used for the experiment had an impurity of less than 20 ppm of oxygen that was about 3x10(-5) atm of PO2 at 100 degree C. The PCO2 and PO2 in the vessel were almost equivalent to those before the drastic increase of PO2, i.e., between 3.5 - 2.3 Ga. The pH of the solution was calculated to be 4.6 during the experiment. Similar biotite dissolution experiment was also carried out under an oxic condition, where the pH was 4.7 at 100 degree C. Although the dissolution rates were similar between the anoxic and oxic experiments, the Fe concentrations in solution were quite different: less than 10(-7) mol/L (below quantitative detection-limit) for the oxic experiment and about 5 x 10(-5) mol/L for the anoxic experiment. The molar ratio of Si/Fe in the starting biotite, about 1, suggested precipitation of Fe-bearing mineral(s). Indeed, SEM revealed that the starting biotite grains were covered with clay-like materials for the anoxic experiment. TEM further revealed that the clay-like materials are a sheet silicate with a higher Fe/Mg ratio than that of the starting biotite, and that any apparent Fe(III) oxides did not occur. On the other hand, hematite was formed but not any other secondary minerals for the oxic experiment. The sheet silicate formed for the anoxic experiment is Fe-rich vermiculite or smectite. Our anoxic experiment shows that the secondary vermiculite or smectite contains a large amount of Fe(II) during anoxic weathering. We conclude that part of Fe(II) dissolved from Fe-bearing minerals such as biotite flows out of a profile and the other part is precipitated as Fe-bearing vermiculite or smectite during anoxic weathering. The increase in Fe/Mg ratio in secondary vermiculite or smectite is consistent with the increase in Fe/(Fe+Mg) in chlorite, a major host of Fe in paleosols, towards the top of a weathering profile (Rye and Holland, 2000).