Geology Reference
In-Depth Information
these organisms; Si is thus an example of a biolimiting
element. The siliceous hard parts that these organisms
secrete dissolve relatively slowly and therefore acc-
umulate on the ocean floor, eventually to be lithified
into flinty rock called chert , which may often be seen
under the electron microscope to consist of radiolar-
ian debris. (Some cherts, however, are abiogenic
in origin.)
Silicon (Si) is a hard metalloid of intermediate
electronegativity (1.9), with a structure identical to
that of diamond (Figure  7.5). Like the next element
in Group IV, germanium (Ge), it has become very
important as a semiconductor (Chapter  7). For this
use it must be extremely pure (impurities less than 1
part in 10 8 ). High-purity silicon metal is also used in
90% of the world's photovoltaic (electricity-generat-
ing) solar panels, both in crystalline and amorphous
Silicon the element should not be confused (as some-
times happens in the media) with silicone , a class of
synthetic organo-silicon polymers, in which groups
such as CH 3 are attached to − Si − O − Si − O − Si − chains
and networks. Such compounds are widely used as
lubricants and insulators, having greater thermal sta-
bility than equivalent organic polymers.
Silicon is the most abundant of the electropositive
elements in the Earth's crust. It invariably occurs in the
oxidized state (valency 4), as SiO 2 or silicate polymers
(see Table 8.1).
SiO 2 occurs in a variety of structural forms, both
crystalline and amorphous. As well as forming
megascopic crystals, quartz occurs commonly in the
crypto-crystalline form chalcedony, familiar as var-
ieties like agate, jasper, chert and flint. The only truly
amorphous form of silica is opal. Molten high-purity
silica can be drawn into glass fibres a fraction of a
millimetre in diameter, widely used for fibre-optic
Quartz has a low but significant solubility in water
of about 6 ppm at room temperature. SiO 2 solubility
increases markedly with temperature, providing a
geothermometer that can be used to estimate deep
temperatures in hot springs.
Dissolved silica exists in the hydrated form
Si(OH) 4 = H 4 SiO 4 , known as silicic acid (analogous to
carbonic acid but weaker). Except near to ocean-floor
hot springs and lava eruptions, seawater is undersatu-
rated with silica. Diatoms and radiolaria are neverthe-
less able, by extracting SiO 2 from seawater, to secrete
shells of opaline silica. They do so mainly in the upper-
most photic zone of the oceans, where sunlight pro-
motes a high biological productivity. Biogenic
precipitation of silica dramatically reduces the level of
dissolved silica in the surface layer, to the extent that
the available silica actually controls the populations of
Nitrogen and phosphorus
Nitrogen is familiar as the unreactive
diatomic gas N 2 making up the major
part of the atmosphere (Figure 9.3). The
electronegative nitrogen atom has three
vacancies in the valence shell, allowing
three covalent bonds (one σ -bond and
two π -bonds) to be established between the two atoms
in the molecule.
Nitrogen adopts a range of valency states. It forms
three stable gaseous oxides: nitrous oxide (N 2 O), nitric
oxide (NO) and nitrogen dioxide (NO 2 ); 'NO x ' is a con-
venient abbreviation embracing all three. Significant
amounts of NO and NO 2 are produced during com-
bustion of fossil fuels, notably by cars; they contribute
to the formation of the photochemical smog that
degrades air quality in many large cities on hot sum-
mer days (O'Neill, 1998).
In Sn
O 2 2 1 %
N 2 7 8 %
H 2 O < 3%
Ar 0 .9 %
CO 2 0.0 4%
Figure 9.3 The composition (volume percentages) of the
atmosphere at sea-level.
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