Mechanism and kinetics of mineral weathering under acid conditions

This study deals with the relationships between crystal structure, grain diameter, surface morphology and dissolution kinetics for feldspar and quartz under acid conditions.Intensively ground samples from large, naturally weathered mineral fragments are frequently used in dissolution studies. The surface area of such samples, estimated from their gas adsorption isotherm (BET method), is normally implied to be all freshly created by grinding. This study revealed that: (1) during natural weathering, micropores (diameters ≈2 nm) develop in feldspar but not in quartz grains; (2) the micropores account for virtually all BET surface area of naturally weathered feldspar grains; and (3) due to the micropores, grinding of large, naturally weathered feldspar fragments is highly ineffective in creating samples with only freshly ground BET surface area.By assuming all BET surface area of ground feldspar samples to be freshly created, experimental dissolution data have been explained from dissolution rates essentially independent of the grain diameter. For ground feldspar samples this study revealed that: (1) the dissolution rate of the freshly created BET surfaces is essentially proportional to the grain diameter; and (2) the dissolution rate of the naturally weathered BET surfaces, still present after grinding, is most likely independent of the grain diameter. Moreover, the dissolution rate, normalized to BET surface area, of unfractured, naturally weathered feldspar grains was essentially independent of the grain diameter. These findings can be explained if: (1) the average density of dissolution sites on freshly created feldspar surfaces is approximately proportional to the grain diameter; (2) micropores develop at dissolution sites during natural weathering; and (3) the BET surface area of the micropore "walls" (i.e. the area perpendicular to the grain surface) is essentially non-reactive.Thermodynamical considerations and Monte Carlo simulations showed that: (1) the formation of micropores in feldspar but not in quartz grains during natural weathering can be explained from enhanced dissolution at crystal defects; and (2) the BET surface area of micropore "walls" from enhanced dissolution at crystal defects is essentially non-reactive. A kinetic model is developed, showing for feldspar that the non-reactivity of the micropore "walls" helps to explain the discrepancy, reported in the literature, between laboratory and field dissolution rates.