Geology Reference
In-Depth Information
Similar behaviour is seen with other
oxy-anion
com-
pounds such as phosphate, nitrate and sulfate. They
are related in a formal sense to corresponding oxy-
acids (carbonic, phosphoric, nitric and sulfuric acids).
Note that OH
-
is another oxy-anion, important in
minerals like brucite [Mg(OH)
2
] and mica.
(a)
(c)
δ
-
δ
+
r
(b)
H ½
δ
+
o
Pure elements, alloys and sulfides
H ½
δ
+
δ
−
Crystals of pure elements, whose atoms have uni-
form electronegativity, will plot at the origin in
Figure 7.8a. This group includes, as we have seen, a
wide range of behaviour, from insulators like dia-
mond and crystalline sulfur, through semiconduc-
tors, to fully conducting metals. These differences can
be appreciated by adding the mean electronegativity
as a third dimension to the diagram shown in
Figure 7.8a, the curving line of which is transformed
into the curved surface shown in Figure 7.8b. Oxide
bonds form a diagonal trend on this surface. At the
foot of this trend lies the O-O bond, one of a group of
non-metal bonds extending along the edge of the sur-
face to phosphorus (P-P). The next element along this
edge, silicon, is a semiconductor. Beyond Si lies a
(shaded) field in which all of the common metals
plot. It extends some distance to the right to include
alloys like brass (Cu-Zn).
The metallic appearance of many sulfides, illus-
trated by the popular reference to pyrite (FeS
2
) as
'fools' gold', reflects their intermediate position
between the oxide and metallic fields in Figure 7.8b.
The dashed line shows that the Fe-S bond is intermed-
iate in character between the Fe-O and Fe-Fe bonds.
The structural reasons for submetallic behaviour in
sulfides are considered in Box 9.8.
Figure 7.9
Dipole interactions. (a) A dipole consists of equal
and opposite charges, a fixed distance apart. (b) The water
molecule as a dipole; δ − signifies a partial negative charge
(equivalent to a fraction of an electron's charge) arising from
the higher electronegativity of the oxygen atom. (c) The
structure of ice. Atoms are shown at 1/10 of the appropriate
size; heavy lines are covalent bonds, thin lines are hydrogen
bonds. Note the difference in O-H and O-H bond lengths.
associations between atoms and molecules that cannot
be explained in these terms.
Various types of weak electrostatic attraction oper-
ate between all molecules, whether or not they possess
overall electric charge. Many molecules, owing to
internal electronegativity differences, are slightly
polarized; and the electrical field associated with such
dipoles
(Figure 7.9a) makes them exert, and be suscep-
tible to, electrostatic forces. A dipole can be attracted to
an ion, or to another dipole. It may, by means of its
electric field, even
induce
an unpolarized molecule to
become a dipole, and thereby attract it. Such dipole
interactions, although much weaker than ionic bond-
ing (an 'ion-ion interaction'), have great mineralogical
significance.
Ion-dipole interactions: hydration
Other types of atomic and
molecular interaction
The shared electron density in an O-H bond is par-
tially concentrated at the oxygen end, owing to the
higher electronegativity of oxygen. This polarization,
symbolized by a partial positive charge ½
δ
+ on each
hydrogen atom and a partial negative charge
δ
− on the
oxygen (Figure 7.9b), results in the water molecule as a
whole acting as a dipole.
In aqueous NaCl solution, each Na
+
cation
is sur-
rounded by a diffuse blanket of water molecules whose
We have seen that ionic, covalent and metallic bonding
form a unified spectrum of chemical interaction
between atoms that possess incomplete valence shells.
These bonding mechanisms, considered separately or
in combination, account for most of the diversity of
appearance, structure and behaviour that we see in
minerals (Figure 7.8). There are nonetheless some
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