Surface area and porosity of primary silicate minerals

Surface area is important in quantifying mineral-water reaction rates. Spec ific surface area (SSA) was measured to investigate controls on this parame ter for several primary silicate minerals (PSM) used to estimate rates of w eathering. The SSA measured by gas adsorption for a given particle size of relatively impurity-free, laboratory-ground samples generally increases in the order: quartz approximate to olivine approximate to albite < oligoclase <approximate to> bytownite < hornblende <approximate to> diopside. Reprodu cibility of BET SSA values range from +/- 70% (SSA < 1000 cm(2)/g) to +/- 5 % (SSA > 4000 cm(2)/g) and values measured with N-2 were observed to be up to 50% larger than values measured with Kr. For laboratory-ground Amelia al bite and San Carlos olivine, SSA can be calculated using log (SSA, cm(2)/g) = b + m log (d), where d = grain diameter (mum), b = 5.2 +/- 0.2 and in = -1.0 +/- 0.1. A similar equation was previously published for laboratory-gr ound quartz. Some other samples showed SSA higher than predicted by these e quations. In some cases, high SSA is attributed to significant second phase particulate content, but for other laboratory-ground samples, high SSA inc reased with observed hysteresis in the adsorption-desorption isotherms. Suc h hysteresis is consistent with the presence of pores with diameters in the range 2 to 50 nm (mesopores). In particular, porosity that contributes to BET-measured SSA is inferred for examples of laboratory-ground diopside, ho rnblende, and all compositions of plagioclase except albite, plus naturally weathered quartz, plagioclase, and potassium feldspar. Previous workers do cumented similar porosity in laboratory-ground potassium feldspar. Surface area measured by gas adsorption may not be appropriate for extrapol ation of interface-limited rates of dissolution of many silicates if intern al surface is present and if it does not dissolve equivalently to external surface. In addition, the large errors associated in measuring SSA of coars e and/or impurity-containing silicates suggest that surface area-normalized kinetics in both field and laboratory systems will be difficult to estimat e precisely. Quantification of the porosity in laboratory-ground and natura lly weathered samples may help to alleviate some of the discrepancy between laboratory- and field-based estimates of weathering rate.