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.
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