5 Rock and Sediment Sources

Groundwaters develop their composition by a complex array of geochemical processes that include mineral dissolution and precipitation, sorption, oxidation-reduction reactions, microbial interactions, and ingassing and degassing. It is helpful to have some knowledge of what types of rocks and sediments are likely to be elevated in F content and why.

Chemical analyses of rocks, minerals, and waters are the cornerstone of the field of geochemistry and developed in parallel with the field of chemistry. Compilation and interpretation of numerous analyses led to an understanding of the distribution of chemical elements within the Earth. Early determinations of F in rocks were not reliable because of analytical difficulties until the 1930s and 1940s and only then was F recognized as a not insignificant element in the Earth’s crust (Research Items, 1940; Shepherd, 1940).

Fluorine is most concentrated in three main types of rocks: silicic igneous rocks and volcanic ash; shales and similar shallow ocean sediments; and marine phosphorites (Fleischer et al., 1972). Trends in F geochemistry for rocks were early summarized by Fleischer and Robinson (1963) and Seraphim (1951) who showed that F was more enriched in silicic (or felsic) igneous rocks (granites and rhyolites). Further they found that alkalic rocks, high in Na and K, are the richest in F. Later compilations have been summarized by Cannon and others (1974) as well as Hayes and others (2017). An abbreviated summary from these references is shown in Table 2. For a more detailed breakout of rock types and their F content, see Table G1 in Hayes and others (2017).

Table 2 – Summary of F concentrations (mg/kg except where noted) in major rocks (Cannon et al., 1974^a; Hayes et al., 2017^b) and in Ethiopian obsidians (Nagash et al., 2020^c) in order of highest to lowest F concentration.