Surface studies of feldspar dissolution using surface replication combined with electron microscopic and spectroscopic techniques

The replica of a microcline cleavage surface was examined before and at various stages of interaction with water and acid solutions at 70°C. For up to 14 weeks in demineralized water the surface as a whole underwent very little change, except some micrometre-sized particles were found on parts of the surface after only one week. Similar particles were found on the actual cleavage surface and on the surfaces of other microcline powders similarly leached at 22°C. These particles were made up of aluminum and silicon with little or no potassium. They were likely formed in some preexisting activated feldspar lattice, either by solid transformation or by local supersaturation and precipitation from solution.Further leaching for 48 hours in a 0.01 mol·L^?1 solution of hydrochloric acid caused only minor pitting of the same cleavage surface, probably due to enhanced dissolution, while contact with a 0.01 mol · L^?1 solution of hydrofluoric acid caused extensive formation of dissolution pits and channels along crystal defects leading to the removal of large portions of the surface. Neither acid appeared to dissolve the newly formed aluminum silicate particles appreciably.Hence during the incongruent dissolution of a feldspar, most of the reactions, dissolution and formation of authigenic Al-silicate phases, occur preferentially along crystal defects. Since the authigenic phases occur as discrete particles occupying only a small fraction of the parent surface their presence will not affect the bulk composition or the overall dissolution rate of the surface.