Geology Reference
In-Depth Information
8
SILICATE CRYSTALS AND MELTS
Silicate polymers
The majority of rock-forming minerals are
silicates
,
compounds in which metals are combined with silicon
and oxygen. In this chapter we consider how the
chemical structure of these compounds, particularly
the nature of the bonding, determines the familiar
morphological and physical properties of crystalline
silicate minerals and the physical properties of silicate
melts (Box 8.3). A minimal knowledge of crystallog-
raphy will be needed.
The relative covalency of the Si-O bond (Figure 7.8)
gives silicon a fundamental structural role as the prin-
cipal
network-forming
element in silicate crystals and
melts, establishing the structural skeleton upon which
their properties depend. In this respect silicon (with
phosphorus and to some extent aluminium) contrasts
with the more ionic constituents of silicates like Mg
2+
and K
+
, which are
network-modifying
elements that
influence structure only because they restrict the way
in which Si-O networks are stacked together.
Whether silicates are examined from the ionic or
covalent viewpoint, the behaviour of silicon is the
same: it lies at the centre of a tetrahedral group of four
oxygen atoms or ions. In structural terms, the
SiO
4
tet-
rahedron
(Figure 8.1) is the basic building brick from
which all silicate crystals and melts are constructed.
But the silicon atom itself can satisfy only half of the
bonding capacity of its four oxygen neighbours (four
bonds out of a total of eight). How are the remaining
oxygen bonds used?
If, when the silicate crystallizes, there is a high con-
centration of an electropositive element like magnesium
(which forms a
basic oxide
- Box 8.1), relatively ionic
bonds are likely to be established between each 'tetra-
hedral' oxygen and nearby Mg
2+
ions, the SiO
4
group
acquiring in the process an overall negative charge
(totalling a nominal 4−). This leads to the chemically
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