Occurrence of the group 17 elements

Figure 1.1 shows the relative abundances of the group 17 elements in the Earth’s crust and in seawater. The major natural sources of fluorine are the minerals fluorspar ( fluorite, CaF₂), cryolite (Na₃[AlF₆]) and fluorapatite, (Ca₅F(PO₄)₃), although the importance of cryolite lies in its being an aluminium ore. Sources of chlorine are closely linked to those of Na and K rock salt (NaCl), sylvite (KCl) and carnallite (KCl.MgCl₂.6H₂O). Seawater is one source of Br₂ (Figure 1.1), but significantly higher concentrations of Br^- are present in salt lakes and natural brine wells .

   The natural abundance of iodine is less than that of the lighter halogens; it occurs as iodide ion in seawater and is taken up by seaweed, from which it may be extracted. Impure Chile saltpeter (caliche) contains up to 1% sodium iodate and this has become an important source of I₂; brines associated with oil and salt wells are of increasing importance.

Fig. 1.1 Relative abundances of the halogens (excluding astatine) in the Earth’s crust and seawater. The data are plotted on a logarithmic scale. The units of abundance are parts per billion (1 billion =10⁹).