The dissolution of biotite single crystals in dilute HNO 3 at 24°C: Evidence of an anisotropic corrosion process of micas in acidic solutions

Abstract

The aim of this study is to determine the geometrical parameters which influence the dissolution of phyllosilicate crystals. In order to explain the natural transformations of phyllosilicates, like biotite, it is important to have a realistic measure of their reactive surface area. The present investigation is performed on sized biotite single crystals (Bancroft, Ontario). Batch experiments are performed at 24°C and in 0.1 N HNO 3 ( pH = 1.08 ± 0.02) in order to avoid the effect of hydroxide precipitates. Experiments are designed so as to determine the respective contributions of the two types of geometric interfaces between the biotite crystal and solution: 1. (1) external (001) planes of the crystal (surface area [S]), and 2. (2) lateral surface (surface area [SL]). The latter parameter is difficult to measure and the influence of [SL] is accessible in experiments where the mass or the perimeter of the monocrystal is varied while other parameters remain constant. The measure of the concentration of leached cations in solution and the investigations performed on the solid show the preponderant influence of [SL] on the corrosion process. On the external (001) interfaces, dissolution is only localized around defects that have the shape of lines or circles. The dissolution of biotite in 0.1 N HNO 3 is selective. The decreasing order of leaching rates for short runs is Mn > K > Al, Fe> Mg, Ti > Si. Fast ion exchange with H + accounts for the high initial leaching rate of the interlayer cation, K +. The fast initial release of Al with respect to Si is consistent with well established models of the surface chemistry of oxides. For octahedral cations, such understanding is still not possible and we could only present some hypotheses. The alteration process begins with the exfoliation of sheet edges. Then, from these lateral exfoliated sheets around the biotite monocrystal, an alteration front propagates along the sheets. The residual altered layer that develops is zoned, with an external zone composed of amorphous silica. This feature is in agreement with solution analyses, showing that Si is released at the slowest rate. This alteration layer acts as a diffusion barrier that controls the leaching of mobile cations (K, Mg, Fe, Al). This causes the leaching rates of all these elements to come under the control of silicon release in solution.