THE EFFECT OF NATURAL WEATHERING ON DISSOLUTION RATES

Surface area measurements and dissolution experiments were performed o n a naturally weathered mineral assemblage rich in quartz and feldspar . The surface areas of unfractured (612 mu m) and ground samples were measured at different levels of surface detail, using a dissecting mic roscope, SEM, and the BET-krypton method. Roughness factors of the fre shly created surfaces by grinding were 2.1 (SEM at 10,000X magnificati on) and 4.8 (BET), respectively. Roughness factors of the naturally we athered surfaces were 2.8 (SEM at 10,000X, including effects from etch pitting) and 34 (BET), respectively. These data indicate that (1) a l arge 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 natu rally weathered feldspar grains decreases with decreasing grain diamet er. Also, most BET surface area additionally created during natural we athering appeared to be nonreactive. At pH 3 and pH 5 HCl, steady-stat e dissolution rates of Na, K, Ca, Al, and Si (mel cm(-2) of BET surfac e s(-1)) were determined for the unfractured sample and for four groun d samples of increasingly smaller average grain diameter. The data fro m the ground samples were used to extrapolate the dissolution rate of hypothetical fresh-surface grains to the average diameter of the unfra ctured, naturally weathered sample material. The results showed that, at 612 mu m, freshly created BET surfaces would dissolve approximately one order of magnitude faster than their naturally weathered counterp arts. Comparison with literature data suggests that this discrepancy i n dissolution rate, at equal diameters of freshly created and naturall y weathered grains, increases with increasing grain diameter. Dissolut ion rates of freshly created feldspar surfaces in the laboratory are f requently up to several orders of magnitude higher than those of natur ally weathered feldspar surfaces in actual field situations. The findi ngs from this research partly explain such discrepancies. However, oth er factors (such as imperfect contact between solution and solids, the presence of organic ligands, etc.) should also be considered in expla ining the discrepancies.