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(a)
100
Ca-F
K-O
Na-O
80
Ca-O
Mg-O
60
Al-O
Fe-O
Si-O
40
P-O
H-O
H-O
20
C-O
S-H
N-O
C-H
0
0.0
1. 0
2.0
3.0
Electronegativity difference
(b)
8
0
6
0
4
0
2
0
0
3
.
0
2
.
0
E
le
c
tr
o
n
e
g
a
ti
v
it
y
d
i
ff
e
r
e
n
c
e
1
.
0
0
.0
Figure 7.8
(a) The correlation of percentage ionic character in a bond with the difference in electronegativity, after Pauling.
The approximate status of geologically relevant bonds is shown. (b) Oxide, sulfide and elemental bond types shown on a 3D
figure whose axes are ionicity, mean electronegativity and electronegativity difference. The dashed line links the bonds
formed by iron in oxide (silicate), sulfide and metal states.
same, of course, applies to the halides of these elements,
such as the minerals halite (NaCl) and fluorite (CaF
2
).
(a) Ca
2+
and CO
3
2-
are attracted to each other by ionic
bonds which, being electrostatic, break down when
calcite is dissolved in a polar solvent like water,
releasing separate calcium and carbonate ions
stabilized by electrostatic association with sur-
rounding water molecules (hydration - Box 4.1).
(b) The bonds between C and the three O atoms within
the carbonate ion are largely covalent, and the
anion retains its identity and structure whether in
solution or in crystalline form.
Oxy-anions
Oxygen forms bonds with phosphorus, carbon, sulfur
and nitrogen which are distinctly more covalent than
Si-O (Figure 7.8a). Thus in a carbonate mineral like cal-
cite, for example, we see the different behaviours of
two types of chemical bond:
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