THE DISSOLUTION OF BIOTITE SINGLE-CRYSTALS IN DILUTE HNO3 AT 24-DEGREES-C - EVIDENCE OF AN ANISOTROPIC CORROSION PROCESS OF MICAS IN ACIDICSOLUTIONS

The aim of this study is to determine the geometrical parameters which influence the dissolution of phyllosilicate crystals. In order to exp lain the natural transformations of phyllosilicates, like biotite, it is important to have a realistic measure of their reactive surface are a. The present investigation is performed on sized biotite single crys tals (Bancroft, Ontario). Batch experiments are performed at 24-degree s-C and in 0.1 N HNO3 (pH = 1.08 +/- 0.02) in order to avoid the effec t of hydroxide precipitates. Experiments are designed so as to determi ne the respective contributions of the two types of geometric interfac es between the biotite crystal and solution: (1) external (001) planes of the crystal (surface area [S]), and (2) lateral surface (surface a rea [SL]). The latter parameter is difficult to measure and the influe nce of [SL] is accessible in experiments where the mass or the perimet er 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 influe nce 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 HNO3 is selectiv e. The decreasing order of leaching rates for short runs is Mn > K > A l, Fe > Mg, Ti > Si. Fast ion exchange with H+ accounts for the high i nitial leaching rate of the interlayer cation. K+. The fast initial re lease of Al with respect to Si is consistent with well established mod els of the surface chemistry of oxides. For octahedral cations, such u nderstanding is still not possible and we could only present some hypo theses. The alteration process begins with the exfoliation of sheet ed ges. Then, from these lateral exfoliated sheets around the biotite mon ocrystal, an alteration front propagates along the sheets. The residua l altered layer that develops is zoned, with an external zone composed of amorphous silica. This feature is in agreement with solution analy ses, 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.