Geology Reference
In-Depth Information
Table 1
Phillips and van Vechten ionicities (after Levine
1973a
,
b
)
(0.62)
b
CaF
2
0.97
FeO
0.87
SiO in Mg
2
SiO
4
CsCl
0.96
MgO
0.84
SiO
2
0.57
0.96
a
AgCl
Al
2
O
3
0.80
GeO
2
(quartz)
0.51
KCl
0.95
GeO
2
(rutile)
0.73
ClO in NaClO
3
0.49
NaCl
0.94
TiO
2
0.69
PO in AlPO
4
0.48
NaI
0.93
Fe
2
O
3
0.68
InSb
0.33
CaO
0.92
AlO in AlPO
4
0.65
SiC
0.20
LiF
0.91
PbS
0.63
Si
0
ZnS 0.62
BeO 0.62
a
Phillips' (
1970
) value is 0.86 for AgCl by the same method, the difference being that Levine
(
1973a
) has assumed a greater contribution of d-electrons to the valence band; for the other
A
N
B
8-N
compounds listed above, Phillips figures are very close to Levine's.
b
Estimated by analogy with ionicities of 0.63 and 0.58 determined for the SiO bond in mag-
nesian olivine and quartz, respectively, by Tossell (
1977
) using the ionicity scale of Kowalczyk
et al. (
1974
) and observations on X-ray spectra.
fully ionic bonding (Stewart et al.
1980
). Therefore, Stewart et al., defining an
ionicity parameter equal to (net charge)/(formal charge), have concluded that the
Si-O bond in quartz has only 25 % ionic character. However, Pauling (
1980
) has
argued, on the basis of his view of bonding, that a net charge of 1 on the Si
corresponds to 50 % ionic character for the Si-O bond, which would also corre-
spond more nearly with the 57 % given on the dielectric scale (Table
1
), although
still not with the nearly 100 % ionicity deduced by Rosenberg et al. (
1978
) from
observations on Compton scattering. The Si-O bond may have slightly greater
ionicity in the stishovite structure (Kirfel et al.
2001
).
In spite of the difficulty and subjectivity involved in defining ionicity, some
conclusions of a relative nature can be reached from Table
1
that are generally
consistent with the various views and measures of ionicity:
1. The substances listed in the first column are highly ionic and can usually be
treated as purely so in a first approximation.
2. Covalent character is likely to have some influence for substances listed in the
second column, perhaps only minor in the first few cases but more significant in
the last few, although probably not overshadowing the ionic component.
3. The substances in the third column can be expected to be strongly influenced in
their properties by the covalent aspect of the bonding, this being predominant in
the semiconductors.
1.2.2 Crystal Defects
There are many types of defects in mineral crystals, all of which represent some
sort of departure from the perfect periodicity of the ideal crystal. For brief surveys,
