Fluorine [F, 9]

Fluorine is an element of the halogen group 17 in the periodic table of elements. Its total content in the upper crust of Earth is estimated at up to 625 mg/kg. In igneous rocks its concentrations vary between 300 and 1200 mg/kg, being the highest in mafic rocks. It is likely to accumulate in argillaceous sediments and in black shales. A high proportion of Fluorine goes to the aluminum industry and to steel industries, mainly as hydrogen fluoride and hydrofluoric acid. It is also greatly used in glass, ceramic, and plastic production. Concentrations of fluorine in the atmosphere are highly variable, due to its differentiated sources: marine aerosols, volcanic eruptions, and industrial emissions, especially from coal burning. Phytoavailability of fluorine is relatively low. It is taken up by plants passively by roots, and is apparently easily transported in plants.