Silicon is the second most abundant element on earth, comprising approximately
20% of the earth’s crust. Pure silicon almost immediately forms a layer of SiO2
on its surface when exposed to air, so it exists in nature mostly in SiO2-based
minerals such as quartzite or in silicates such as mica, feldspars, and zeolites.
The raw material for photovoltaics and other semiconductors could be common
sand, but it is usually naturally purified, high-quality silica or quartz (SiO2) from
mines. The first processing step to convert silica into crystalline silicon is an
energy-intensive process in which SiO2 is reduced to an impure Si using carbon
in an arc furnace. It is then transformed to a liquid trichlorosilane (SiHCl3),