AN INTERFEROMETRY INVESTIGATION OF PHLOGOPITE DISSOLUTION: IMPLICATIONS FOR CRYSTAL DISSOLUTION WITHOUT THE FORMATION OF ETCH PITS

Direct observations of dissolution at clay mineral surfaces are not common because of their small size and structural complexity. However, it may be possible to use mica minerals as an analog for clay mineral dissolution. In the case of phlogopite, we know that the formation of etch pits at the (001) cleavage surface under acidic conditions is not common (e.g., Rufe and Hochella, 1999). The question we wish to answer is whether etch pits can develop at (hk0) faces, or if these minerals dissolve without etch pit formation. If etch pits do not form, then these minerals must dissolve by a mechanism that is distinctly different from other rock-forming minerals (e.g., carbonates, feldspars, hornblende, etc.).

In a new model of crystal dissolution, Lasaga and Lüttge (2001) suggest that bulk dissolution occur by trains of steps moving across the mineral surface. These so-called dissolution stepwaves are generated at the outskirts of developing etch pits. Therefore, etch pits serve as the nucleation centers for migrating stepwaves. The question now becomes whether this model can be applied to the dissolution of mica minerals, and therefore by analogy to clay minerals as well.

We have measured absolute dissolution rates in HCl (pH 3) of the (001) and (hk0) faces of a near endmember phlogopite with a vertical scanning interferometer at 25, 50, and 80° C. This technique allows direct observation of dissolution by quantifying surface normal retreat or advance of oriented phlogopite surfaces. The changes in surface topography are measured relative to a reference surface, and do not require an external measurement or estimate of surface area. Our preliminary results suggest that etch pit formation, although important in carbonate and feldspar dissolution, is not a significant process on phlogopite (hk0) faces.