Geology Reference
In-Depth Information
at 24.6 K and argon at 84 K). These low melting tem-
peratures signify a force so weak that even the very
slight thermal vibrations experienced just above these
low temperatures are sufficient to overcome it (Box 7.7).
• Ion-dipole and dipole-dipole interactions, although
much weaker (Table 7.7.1), are nonetheless impor-
tant in determining the properties of water and ice
(Figure 7.9), the softness and sheet structure of
graphite (Figure 7.4.1), and the capacity of even
inert gases
like Ar to form crystals at very low
temperatures.
Review
Many properties of minerals and everyday materials
(hardness, atomic structure, electrical conductivity,
solubility, lustre, etc) are readily explained in terms of
the chemical bonds that hold them together on the
atomic scale:
Further reading
Atkins, P., Overton, T., Rourke, J.,
et al
. (2010)
Inorganic
Chemistry
, 5th edn. Oxford: Oxford University Press.
Barrett, J. (2001)
Structure and Bonding
. Cambridge: Royal
Society of Chemistry.
Henderson, P. (1982)
Inorganic Geochemistry
. Oxford:
Pergamon.
• Ionic bonds - involving the donation and accept-
ance of electrons between atoms - form between
elements of
contrasted electronegativity
. Ionic com-
pounds exist in the solid and molten states but not
as gases. Their structures are dictated by the packing-
together of spherical ions of various sizes, and can
be predicted from the radius ratio of cations to
anions (Table 7.1).
• Covalent bonds - involving the sharing of electrons
between atoms - form between elements of the
same
or similar electronegativity
. The structures of simple
molecules (e.g. H
2
O - Figure 7.5c) and extended
structures (e.g. diamond - Figure 7.5d) can be pre-
dicted from the geometry of atomic - particularly
hybrid
-orbitals (Figures 7.4, 7.5 and 7.4.1).
• Metallic bonds form between elements of similar
electronegativity
when the mean value is low
(Figure 7.8b). Metallic properties derive from the
availability of vacant
conduction bands
of electron
energy levels that extend throughout the crystal
(Figure 7.6). Some sulfides share attributes of met-
als, such as lustre and measurable electrical conduc-
tivity (Figure 7.8b).
• The Si-O bond in silicate minerals is 50:50
ionic:covalent (Figure 7.8). For this reason it plays an
important part in determining the architecture of
silicate crystals and melts, as described in Chapter 8.
Exercises
7.1
Predict the co-ordination numbers of the following
ions in a silicate crystal (Box 7.2):
Si Al
4
+
,
3
+
,
Ti
4
+
,
Fe
3
+
,
Fe Mg Ca Na K
2
+
,
2
+
,
2
+
,
+
,
+
7.2
Explain the difference in ionic radius (Box 7.2)
between Fe
2+
and Fe
3+
and between Eu
2+
and
Eu
3+
. Calculate the ionic potentials of the ions.
What effect does the oxidation state have on the
covalency of bonding of these elements?
7.3
Identify the type of bonding between the follow-
ing pairs of atoms in the solid state. How does it
influence the properties of the elements at room
temperature? (Use Figure 6.3. Holmium (Ho) is a
lanthanide metal.)
He He Ho Ho Ge Ge
-
-
-
7.4
Discuss the bonding in the following minerals:
KClsylvite TiO utile MoSmolybdenite
NiAs niccolite
(
)
,
(
)
,
(
)
,
2
2
CaSO anhydrite
CaSO HOgypsum
(
)
,
(
)
,
4
.
2
(
)
.
4
2
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