MICROPORE SURFACE AREA VARIATION WITH GRAIN SIZE IN UNWEATHERED ALKALI FELDSPARS: IMPLICATIONS FOR SURFACE ROUGHNESS AND DISSOLUTION STUDIES - 1. QUANTIFICATION OF MINERALOGICAL AND SURFACE AREA CHANGES AND CALCULATION OF PRIMARY SILICATE REACTION RATES

Abstract

The BET and micropore surface areas of different size fractions of four different unweathered alkali feldspars were calculated using gas adsorption data. BET surface area decreased with increasing grain size. Micropore surface area accounted for 12-84 % of BET surface area. For two of the feldspars, Eifel sanidine and KB14, micropore surface area increased with decreasing grain size; with regard to the other two, Perth perthite and Keystone microcline, surface area varied randomly with grain size. Micropores have diameters =< 2 nm, which is on the same scale as alkali feldspar unit-cell dimensions. As there are no channels in the alkali feldspar crystal structure, it was concluded that the measured micropore surface area was due to cracks generated during the grinding involved in sample preparation. Values for the surface roughness of the grains were calculated assuming that micropore surface area equaled internal surface area. Values varied between 1 and 7 for the different grain size fractions of Perth perthite, Keystone microcline, and Eifel sanidine. For KB14 surface roughness varied between 7 and 24. It is possible that the dissolution of feldspars proceeds at different rates down cracks and at the outer surface of grains. If this is the case then the present data suggest that care must be taken when comparing surface area normalised dissolution rates obtained from chemically similar samples which have been ground for different periods of time with different degrees of force.