The effect of natural weathering on dissolution rates

Abstract

Surface area measurements and dissolution experiments were performed on a naturally weathered mineral assemblage rich in quartz and feldspar. The surface areas of unfractured (612 μm) and ground samples were measured at different levels of surface detail, using a dissecting microscope, SEM, and the BET-krypton method. Roughness factors of the freshly created surfaces by grinding were 2.1 (SEM at 10,000× magnification) and 4.8 (BET), respectively. Roughness factors of the naturally weathered surfaces were 2.8 (SEM at 10,000×, including effects from etch pitting) and 34 (BET), respectively. These data indicate that (1) a large amount of BET surface area was created during natural weathering, and (2) etch pits did not contribute significantly to the BET surface area created during natural weathering. Comparison with data from the literature further suggests that the surface roughness factor of naturally weathered feldspar grains decreases with decreasing grain diameter. Also, most BET surface area additionally created during natural weathering appeared to be nonreactive. At pH 3 and pH 5 HCl, steady-state dissolution rates of Na, K, Ca, Al, and Si (mol·cm −2 of BET surface·s −1) were determined for the unfractured sample and for four ground samples of increasingly smaller average grain diameter. The data from the ground samples were used to extrapolate the dissolution rate of hypothetical fresh-surface grains to the average diameter of the unfractured, naturally weathered sample material. The results showed that, at 612 μm, freshly created BET surfaces would dissolve approximately one order of magnitude faster than their naturally weathered counterparts. Comparison with literature data suggests that this discrepancy in dissolution rate, at equal diameters of freshly created and naturally weathered grains, increases with increasing grain diameter. Dissolution rates of freshly created feldspar surfaces in the laboratory are frequently up to several orders of magnitude higher than those of naturally weathered feldspar surfaces in actual field situations. The findings from this research partly explain such discrepancies. However, other factors (such as imperfect contact between solution and solids, the presence of organic ligands, etc.) should also be considered in explaining the discrepancies.