ARE QUARTZ DISSOLUTION RATES PROPORTIONAL TO B.E.T. SURFACE AREAS?

Abstract

A single quartz powder was dissolved at 200°C and 250°C under far from equilibrium conditions in atmosphere-equilibrated deionized water during a sequential series of experiments performed over one year in a titanium open system mixed flow reactor. Scanning electron microscope (SEM) photomicrographs show that morphological changes of this powder were dominated by grain edge rounding and etch pit formation. Etch pit walls rapidly evolve into unreactive negative crystal faceted forms; etch pit density and diameter are essentially constant during the experiments, indicating that dissolution predominantly deepened rather than widened etch pits. Measured B.E.T. surface areas increase linearly with mass of quartz dissolved to a value 5.6 times greater than that of the initial powder during the course of the experiments. Nevertheless, measured 200°C far from equilibrium mass normalized dissolution rates remained constant within experimental uncertainty. It is concluded that the bulk of the observed increase in B.E.T. surface areas during dissolution consisted of essentially unreactive etch pit walls which contribute negligibly to mineral dissolution. As similar etch pit development is common in natural systems, geometric rather than B.E.T. surface areas may provide a more accurate parameter for estimating dissolution rates.