5.2 Fluorine in Sedimentary Rocks

Considerable amounts of F are fixed in marine phosphorite deposits, the primary ore of commercial phosphorus, because francolite, a carbonate-fluorapatite, is the dominant phosphorus ore mineral. Fluoride contents are typically 2 to 4 by weight percent in phosphorites. These deposits were formed in shallow seas of high biological productivity such as inland seas, continental shelves, and areas where upwelling of deep nutrient-rich ocean water circulates to shallow depths (Föllmi, 1996; Piper and Perkins, 2014). These deposits are of many different ages and are found in several places in the world, notably in North Africa from Western Sahara and Morocco east to Egypt and Jordan (“Mediterranean phosphorites”), eastern seaboard of the USA from North Carolina to Florida, western USA (Phosphoria Formation), and in Australia, Russia, China, and Mongolia. Groundwater in these areas could potentially be contaminated from F caused by mineral processing activities or from natural processes.

A study on artificial recharge and storage of groundwater encountered release of excessive fluoride from the dissolution of carbonate-fluorapatite based on mineralogical characterization and dissolution measurements for a sandstone aquifer in Western Australia (Schafer et al., 2018). Phosphorites may be closely associated with limestones and may be interstratified with clays and shales.

Microcrystalline fluorite has also been found to form in lake sediments at ambient temperatures (Sheppard and Gude, 1969; Sheppard and Gude, 1980; Sheppard and Mumpton, 1984).

Shales and claystones develop in similar environments to phosphorites and often contain some phosphorus mineralization as an important source of F. Sandstones and non-phosphatic limestones are among the lowest in F concentration and are usually good aquifers for drinking water supplies.