MINERAL DURABILITY

In 1938, Samuel Goldich proposed a “mineral-stability series in weathering” that has been featured in many generations of physical geology textbooks. The growing body of mineral dissolution rate data allows us to investigate the geochemical basis for this diagram. We might expect that mineral durability is related to dissolution rate and use the mineral lifetime model to try to quantify Goldich’s diagram. This approach predicts that the lifetime of 1 mm diameter mineral grains in pH 5.5 water ranges from 9.7 million years for quartz to 141 years for forsterite and the model predicts the lifetimes for all of the minerals on the diagram in the correct order. This encourages us to think that mineral durability is simply a function of dissolution rate. However, if we extend this approach to the more than 50 minerals for which dissolution rate data are available, we find that the predicted lifetimes of some minerals are very short relative to their observed durability (e.g. apatite and calcite). This is because natural waters are often nearly saturated with respect to these minerals so that they dissolve much more slowly than predicted by the lifetime model. Indeed, more than 50% of groundwater samples are supersaturated with respect to quartz and kaolinite so that these phases are as likely to grow as to dissolve in the weathering environment. This means that understanding the durability of minerals in the weathering environment requires consideration of both equilibrium solubility and dissolution kinetics.