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Chapter 14: Silicon and its Compounds


Silicon is a non metal element, symbol Si, atomic number 14.

It is the second most common element in Earth's crust (after oxygen) and is 25.7% of the crust by mass. It has many uses, eg, rock and sand, quartz and glass, semiconductors like transistors and integrated circuits, and silicones, used for lubricants, hydraulic fluids, sealants and even thermal heat paste.

Elemental silicon is crystaline and (as a powder) is dark grey with bluish tinge. It has a metallic sheen that increases with the crystal size. It is not as reactive as carbon, which is just above silicon in the periodic table. Like carbon, silicon tries to form four bonds. Silicon is prepared by reacting silica with carbon in electric arc furnaces at temperatures over 1,900 °C:

SiO2 + C → Si + CO2

Silicon is essential in biology, although only trace amounts are required by animals. (This means that babies don't have to eat sand at the beach, although they seem to anyway.) Plant metabolism depends on silicon.

Wikipedia: Silly Putty was originally made by adding boric acid to silicone oil. Now name-brand Silly Putty also contains significant amounts of elemental silicon (silicon binds to the silicone and allows the material to bounce 20% higher).

Aside: Boric acid, B(OH)3, is popularly used among fire jugglers and fire spinners dissolved in methylated spirit to give a bright green flame.

Earth's crust

From Wikipedia, Earth's crust: In all Earth's crust occupies less than 1% of Earth's volume. ... The common rock constituents of the Earth's crust are nearly all oxides. Chlorine, sulphur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. F. W. Clarke has calculated that a little more than 47% of the Earth's crust consists of oxygen. It occurs principally in combination as oxides, of which the chief are silicon, aluminium, iron, calcium, magnesium, potassium and sodium oxides. Silica is a major constituent of the crust occurring as the silicate minerals, which are the most common minerals of igneous and metamorphic rocks. From a computation based on 1672 analyses of all kinds of rocks Clarke arrived at the following as the average percentage composition by mass:

Names Oxide Percent by mass
Silica, silicon dioxide SiO2 59.71
Alumina, aluminium oxide Al2O3 15.41
Lime, calcium oxide CaO 4.90
Magnesia, magnesium oxide MgO 4.36
Sodium oxide Na2O 3.55
Ferrous oxide FeO 3.52
Potassium oxide K2O 2.80
Hematite, ferric oxide, rust, rouge Fe2O3 2.63
Water H2O 1.52
Titanium dioxide, titania TiO2 0.60
Phosphorus pentoxide P2O5 0.22
Total 99.22

All the other constituents occur only in very small quantities, and total less than 1%.

Silicon dioxide (silica) is the most common component of beach sand.

Precious Stones

Many precious stones have silicon in them. A few examples:

Amethyst is silica (quartz, SiO2) with the purple colouring from a complex interaction of iron and aluminium impurities.

Beryl (Be3Al2(SiO3)O6) is colorless in its pure mineral form. Adding chromium (and sometimes vanadium) impurities gives emerald. Manganese impurities gives pink (Mn2+) morganite or red (Mn3+) red beryl. With iron it becomes pale blue (Fe2+) aquamarine, dark blue (Fe2+ and Fe3+) maxixe, or golden yellow (Fe3+) aquamarine chrysolite. The largest known crystal of any mineral in the world is a crystal of beryl from Madagascar, 18 metres long and 3.5 metres in diameter.

Chrysoberyl (BeAl2O4) is not a silicate gem.

Topaz (Al2SiO4(F,OH)2) is colorless and transparent but is usually tinted by impurities; typical topaz is wine, yellow, pale gray or reddish-orange, blue brown. It can also be made white, pale green, blue, gold, pink (rare), reddish-yellow or opaque to transparent/translucent. Naturally occurring Blue Topaz is quite rare. Typically, colorless, gray or pale yellow and blue material is heat treated and irradiated to produce a more desired darker blue.

Garnet is a class of gem with a general formula X3Y2(SiO4)3. The X site is usually occupied by divalent cations (Ca2+, Mg2+, Fe2+) and the Y site by trivalent cations (Al3+, Fe3+, Cr3+) in an octahedral/tetrahedral framework with [SiO4]4- occupying the tetrahedra. One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called tsavorite. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name. (The movie The Ghost and the Darkness was based on real events in Tsavo.)

Error in book: Opal is NOT the most valuable silicate gem. That is simply stupid. Top quality emeralds are even more expensive than diamonds, and I don't think anyone believes opals are more valuable than diamonds. (The most valuable non-silicate gem is possibly cat's eye alexandrite, a form of chrysoberyl that changes colour under different sorts of lighting and has a cat's eye inclusion.) The most valuable form of quartz is citrine according to this page, but this page says amethyst is.


On the periodic table silicon is positioned between the metals and non-metals. It has chemical and electrical properties between the two. It is a semi-conductor, and is by far the most common semi-conductor used for making transistors and integrated circuits. Silicon retains its semiconducting properties at higher temperatures than germanium, another semiconductor and the element just underneath silicon in the periodic table.

An LED is a semi-conductor that uses other semi-conducting elements such as gallium arsenide. To run an LED at a steady brightness (especially from batteries) a constant current is wanted, which can be provided using a circuit with a couple of silicon-based transistors and a couple of resistors. One of the transistors must be able to handle a lot of power, which means a lot of excess heat. To get rid of that heat a heatsink is used, and firmly attached (eg, bolted) to the transistor with silicone thermal compound.

For more information on electronics and electronic components see the Electronics section.