A NEW EXPERIMENTAL METHOD FOR DIRECT COMPARISON OF SINGLE-CRYSTAL AND POWDER DISSOLUTION RATES

We have developed an experimental method for studying the dissolution behavior of minerals with the aim of defining and quantifying the relationship between single-crystal and powder dissolution rates. Our method employs a titanium reactor that combines the capabilities of two existing experimental designs (single-crystal and powder flow-through). This new reactor can be used to dissolve a single crystal in the same volume of fluid as a mineral powder. The two samples are reacted simultaneously, leaving no doubt as to whether the two experience the same experimental conditions. We measure dissolution rates for single-crystals using vertical scanning interferometry (VSI). To make measurements absolute, we need a reference surface from which to measure the surface normal retreat of the mineral. A specially designed sample holder that allows us to apply an inert mask (3mm diameter) to the mineral surface provides the necessary reference surface. An important feature of the new titanium reactor is its ability to function over a wide range of temperatures (25-250ºC). This is a requirement when studying minerals with slow dissolution kinetics.

The goal of laboratory mineral dissolution studies is to achieve transferability of rate data between different laboratories and, ultimately, to natural systems. Mineral dissolution rates measured for individual cleavage surfaces of single crystals are typically slower than dissolution rates measured for powders. This discrepancy is a cause for concern and has not been well understood. The purpose of the new experimental method is to identify the cause for the discrepancy by direct observation of a single-crystal surface (by VSI) coupled with powder dissolution rate measurements from the same reactor.