299Here, the reduced silicate mineral matrix entrapped the metals reactive enough to be
oxidized (aluminum, calcium, sodium, potassium and magnesium). After loss of volatile
gases, as well as carbon and sulfur via reaction with hydrogen, this silicate mixture of
elements formed most of the Earth's crust. These silicates were of relatively low density
with respect to iron, nickel, and other metals non-reactive to oxygen and thus a residuum
of uncombined iron and nickel sank to the planet's core, leaving a thick mantle consisting
mostly of magnesium and iron silicates above.
Examples of silicate minerals in the crust include those in the pyroxene, amphibole, mica,
and feldspar groups. These minerals occur in clay and various types of rock such as
granite and sandstone.
Silica occurs in minerals consisting of very pure silicon dioxide in different crystalline
forms, quartz, agate amethyst, rock crystal, chalcedony, flint, jasper, and opal. The
crystals have the empirical formula of silicon dioxide, but do not consist of separate silicon
dioxide molecules in the manner of solid carbon dioxide. Rather, silica is structurally a
network-solid consisting of silicon and oxygen in three-dimensional crystals, like diamond.
Less pure silica forms the natural glass obsidian. Biogenic silica occurs in the structure of
diatoms, radiolaria and siliceous sponges.
Silicon is also a principal component of many meteorites, and is a component of tektites,
a silicate mineral of possibly lunar origin, or (if Earth-derived) which has been subjected
to unusual temperatures and pressures, possibly from meteorite strike.
Production
Alloys
Ferrosilicon, an iron-silicon alloy that contains varying ratios of elemental silicon and iron,
accounts for about 80% of the world's production of elemental silicon, with China, the
leading supplier of elemental silicon, providing 4.6 million tons (or 2/3 of the world output)
of silicon, most of which is in the form of ferrosilicon. It is followed by Russia (610,000 t),
Norway (330,000 t), Brazil (240,000 t) and the United States (170,000 t). Ferrosilicon is
primarily used by the steel industry (see below). Aluminum-silicon alloys are heavily used
in the aluminum alloy casting industry, where silicon is the single most important additive
to aluminum to improve its casting properties. Since cast aluminum is widely used in the
automobile industry, this use of silicon is thus the single largest industrial use of
"metallurgical grade" pure silicon (as this purified silicon is added to pure aluminum,
whereas ferrosilicon is never purified before being added to steel).
Metallurgical Grade
Elemental silicon not alloyed with significant quantities of other elements, and usually >
95%, is often referred to loosely as silicon metal. It makes up about 20% of the world total
elemental silicon production, with less than 1 to 2% of total elemental silicon (5–10% of
metallurgical grade silicon) ever purified to higher grades for use in electronics.
Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica
with wood, charcoal, and coal in an electric arc furnace using carbon electrodes. At
temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and
the silicon undergo the chemical reaction SiO 2 + 2 C → Si + 2 CO. Liquid silicon collects
in the bottom of the furnace, which is then drained and cooled.