Abstract
The dissolution rates of plagioclase in acids are very sensitive to the mineral composition. At pH = 2.0 and 25 °C, rates range from ≈ 1 × 10−15 feldspar-mol/cm2/s for albite to greater than 1 × 10−12, feldspar-mol/cm2/s for anorthite. At pH = 3.0, the respective rates range from ≈ 3 × 10−16 feldspar-mol/cm2/s to ≈ 7 × 10−14 feldspar-mol/cm2/s. The relationship between mineral composition and dissolution rate, however, is not simple. In order to better define such a relation at pH = 2, rates were measured on eight plagioclase minerals and compared to existing data.
In general, the dissolution rates of a given plagioclase mineral estimated by different researchers generally agree to within a factor of two to five. The uncertainty is much larger for Ca-rich minerals such as bytownite and anorthite, and reasons for the disagreement are unclear. Even with this uncertainty, however, it is apparent that dissolution rates vary nonlinearly with composition and that the rates for the Ca-rich minerals vary more with composition than Na-rich minerals.
