Effects of different cations in the solution on the weathering of biotite

Knowlege of the mecahnisms of silicate weathering is very important to understand the cycles of elements on the surface of the earth. Biotite is one of the most important rock-forming minerals and vulnerable to weathering. Certain elements such as K and F can be greatly affected by the weathering of biotite. Most studies on the weathering of biotite have been on the weathering processes and weathering rates. However, the effects of different solution compositions including cations in the solution have not been throughly studied. For our study, biotite sample was cut into small particles by scissors and a blender, and finally ground by mortar agate. The biotite with particle size between 55 and 100 μm was used for our weathering experiment. Biotite of 0.1 g was reacted in 50 ml solution of 1 M Na, K, Rb, Cs, Ca, and Mg. The weathering experiment was performed at pH 2 and 4 and at 40 ^oC to increase the reaction rate. Different reaction time, 20, 40, 60, 80, 100, and 120 days were used. From XRD results, the most significant change was oberved for the samples reacted with Na solution. Even with short period of time of 20 days at both pH 2 and 4, small peak at 12.2 Å, which corresponds to (001) peak of hydrobiotite was identified, indicating that the reaction rate of biotite reacted with Na solution was the highest. The other observed change after reaction was increasing (001) peak width of biotite reacted with K and Ca solutions. However, no noticeable peak width change was observed for the biotite reacted with Rb and Cs solutions. Our results indicate that the cations in the solution play important roles in the weathering rate and possbly weathering product of biotite. Rb and Cs have large ion raidii to fit into the frayed edge site of biotite and they probably prevent the release of K from the interlayer while that of Na does not block K due to its small radius. The effects of Ca and Mg are not clear at this stage. More detailed results including the reaction rate with chemical analysis data will be presented at the conference.