Geology Reference
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Mg 2+
Ca 2+
Mg 2+
Simplified symbolism for SiO 4 tetrahedra
Be 2+
AI 3+
Apex up
Apex down
Lower ring
Upper ring
Columns of rings form
hollow cylinders
Figure 8.1 Simplified structures of silicate minerals, with cations drawn approximately to scale. SiO 4 groups are shown
as bare coordination tetrahedra to clarify the structure. (a) Orthosilicate olivine, viewed down the a crystallographic axis;
(b) dimer silicate melilite; (c) ring silicate beryl.
simple crystal structure of olivine (specifically the Mg
end-member forsterite, Mg 2 SiO 4 ) in which Mg 2+ ions
alternate with SiO 4 4− tetrahedra (Figure 8.1a). There is no
direct Si-O bonding between one tetrahedron and its
neighbour, and the cohesion of the crystal as a whole
arises from ionic bonding between Mg 2+ cations and
SiO 4 4− anions: covalently bonded SiO 4 'bricks' are
cemented together by an ionic 'mortar' of Mg 2+ ions.
If cations Mg 2+ are scarce, on the other hand, oxygen
atoms are more likely to bond directly to two silicon
atoms, forming a relatively 'covalent' bridge between
them. Taken to the limit, this can lead to every oxygen
atom being shared between neighbouring tetrahedra,
extending the Si-O bonding to form a three-dimen-
sional network of connected tetrahedra throughout the
crystal. This is the situation we find in the mineral
quartz . Because each oxygen atom is structurally part
of two SiO 4 tetrahedra, half as many oxygen atoms are
needed to fulfil the co-ordination requirements of sil-
icon. Thus quartz, although in a formal sense built of
SiO 4 structural units, has the formula SiO 2 .
The formation of extended Si-O networks in sili-
cates is called polymerization . Whereas familiar organic
polymers consist of chains and rings of carbon atoms
linked directly to each other (-C-C-C-: Chapter  9),
the linkage in silicate polymers is always through oxy-
gen atoms (-Si-O-Si-O-Si-). The degree of Si-O
polymerization in a silicate structure is conveniently
enumerated by the number of non-bridging oxygens
(those linked to only one Si atom) per SiO 4 group. This
number p varies from 4 in olivine (in which SiO 4 tetra-
hedra are linked only indirectly through other cations)
to 0 in quartz. Between these limits, silicate minerals
exhibit a wealth of structural diversity, exceeded in
complexity only by the chemistry of carbon.
Monomer silicates
Pursuing the analogy with organic polymers , olivine
(Figure  8.1a) would be called a monomer , because the
basic unit of the polymer occurs in it uncombined.
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