Occurrence The group 16 elements

  Figure 1.1 illustrates the relative abundances of the group 16 elements in the Earth’s crust. Dioxygen makes up 21% of the Earth’s atmosphere, and 47% of the Earth’s crust is composed of O-containing compounds, e.g. water, limestone, silica, silicates, bauxite and haematite. It is a component of innumerable compounds and is essential to life, being converted to CO₂ during respiration. Native sulfur occurs in deposits around volcanoes and hot springs, and sulfur-containing minerals include iron pyrites ( fool’s gold, FeS₂), galena (PbS), sphalerite or zinc blende (ZnS), cinnabar (HgS), realgar (As₄S₄), orpiment (As₂S₃), stibnite (Sb₂S₃), molybdenite (MoS₂) and chalcocite (Cu₂S). Selenium and tellurium are relatively rare.

Fig. 1.1 Relative abundances of the group 16 elements (excluding Po) in the Earth’s crust. The data are plotted on a logarithmic scale. The units of abundance are parts per billion (1 billion =10⁹). Polonium is omitted because its abundance is only 3 ×10^-7 ppb, giving a negative number on the log scale.

Selenium occurs in only a few minerals, while Te is usually combined with other metals, e.g. in sylvanite (AgAuTe₄).

Fig. 1.2 Production of sulfur in the US from 1970 to 2001; note the increasing importance of recovery methods which have now replaced the Frasch process as a source of sulfur in the US. [Data: US Geological Survey.]