Etch pits naturally weathered

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

Figure 3 Scanning electron photomicrographs of feldspar surfaces in various stages of weathering, a) Fresh surface, b) Incipient formation of shallow almond-shaped etch pits, c) Moderate development of prismatic etch pits, d) Extensive penetration of prismatic etch pits into feldspar interiors. Photographs b-d are from naturally weathered materials. All photomicrographs by Alan S. Pooley and the author.

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Figure 6 Etch pit distrihutions (PSD) of naturally weathered microcline and hornblende in soils from Illinois, USA (after Brantley et al., 1993).

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