Major Elements from Minor Minerals: Garnet and Epidote in Watershed Calcium Budgets.

In many carbonate-free soils, silicate-mineral weathering is the dominant source of Ca.  Major (common, widespread, and abundant) rock-forming silicate minerals that release dissolved Ca upon weathering include most varieties of plagioclase feldspar, clinopyroxene, and clinoamphibole.  In numerous studies of forested watershed solute budgets, simple inverse modeling of solute mass-balance using stoichiometries of major minerals, sometimes augmented by invoking one or more minor minerals, yields ratios of silicate-mineral dissolution rates that are broadly similar to rate-ratios determined in internally consistent laboratory experiments. 

Minor minerals such as garnet have been shown to be locally important in supplying Ca.  The bulk garnet/plagioclase weathering-rate ratio in a forested Southern Appalachian watershed at the Coweeta Hydrologic Laboratory was previously determined to be 0.86 (4.1 on a mineral-surface-area normalized basis).  The comparable ratio from laboratory experiments is 1.05.  However, this early work did not account for other possible silicate-mineral sources of Ca.  More recent work identified another even less abundant Ca-silicate, the epidote-group mineral allanite, to be an important additional mineral source of Ca.  Reapportioning Ca among the larger suite of source minerals results in a reduction of the calculated garnet weathering rate, such that bulk garnet/plagioclase weathering-rate ratio for the same forested Coweeta watershed is 0.41 and the mineral-surface-area normalized garnet/plagioclase dissolution-rate ratio is 1.9, much closer to the ratio of 1.05 from laboratory experiments.  Accounting for the Ca contribution of a very minor silicate (allanite) modifies the inferred weathering rates of some other more abundant Ca-silicate minerals in a manner that suggests greater conformity of laboratory and field garnet/plagioclase weathering-rate ratios than previously recognized.  Explicit incorporation of other Ca-bearing mineral groups (e.g., phosphates such as apatite) can be expected to reveal new relationships among Ca-source minerals and their influence on Ca balances.