Geology Reference
In-Depth Information
(a)
(b)
Mg
2+
Ca
2+
Mg
2+
(c)
Key
Simplified symbolism for SiO
4
tetrahedra
Be
2+
AI
3+
Apex up
Apex down
Lower ring
Upper ring
Columns of rings form
hollow cylinders
O
Si
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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