Metamorphic rocks cover a broad spectrum of lithologies that include metamorphosed igneous rocks, metamorphosed sediments, and hydrothermally altered rocks. Rocks that have been hydrothermally altered are more likely to have higher concentrations of F but might also have large F gradients in different parts of the rock. Many metamorphic rocks contain micas and amphiboles with a hydroxide lattice site that often contains some substituted F. However, if the rock contains high concentrations of F in the hydroxide site, it is also likely to contain some fluorite and fluorapatite which is usually more soluble. Examples of high groundwater F in metamorphic aquifers are scarce, partly because these rock types often do not provide a reliable water supply because of low permeability unless highly fractured. Chae and others (2007) reported high F concentrations in aquifers in South Korea where metamorphic rocks seem to be a greater source rock than other types, with granitoids a close second.