Occurrence

Carbon occurs in native (free) form as graphite. In combined form, it occurs in vast quantities trapped within the earth as coal and crude oil (the fossil fuels), and locked into carbonate rocks such as calcite and magnesite, where it is bound to oxygen and a metal rather than existing as the free element.

Silicon, by contrast, is never found free in nature at all. It occurs mainly as silica -- in the everyday forms of ordinary sand and crystalline quartz -- and, more extensively still, bound up within silicate minerals and clay, the raw materials this unit returns to in detail in Section 2.3.9. Together, silica and the silicate minerals make up the gre …