Geoscience Reference
In-Depth Information
Table 22.1
Ionic radii for major mineral forming elements
Coordination
Ionic
Coordination
Ionic
Ion
Number
Radius
Ion
Number
Radius
Al
3+
IV
0.39
Fe
3+
IV
0.49(HS)
∗
V
0.48
VI
0.55(LS)
VI
0.53
VI
0.65(HS)
Ca
2+
VI
1.00
Mg
2+
IV
0.49
VII
1.07
VI
0.72
VIII
1.12
VIII
0.89
IX
1.18
Fe
2+
IV
0.63(HS)
X
1.28
VI
0.61(LS)
XII
1.35
VI
0.77(HS)
Si
4+
Ti
4+
IV
0.26
V
0.53
VI
0.40
VI
0.61
Na
+
VI
1.02
K
+
VI
1.38
VIII
1.16
VIII
1.51
O
2
−
II
1.35
F
−
II
1.29
III
1.36
III
1.30
IV
1.38
IV
1.31
VI
1.40
VI
1.33
VIII
1.42
Cl
−
VI
1.81
∗
HS, high spin; LS, low spin.
pack the balls together as closely as possible con-
sidering their size and charge. Many crystals are
based on cubic close packing or hexagonal close
packing of the larger ions. The stable packing
and interatomic distances change with temper-
ature, pressure and composition. Most physical
properties are strong functions of interatomic
distances.
Ionic crystal structures, such as oxides and sil-
icates, consist of relatively large ions, usually the
oxygens, in a closest-pack arrangement with
the smaller ions filling some of the interstices.
The large ions arrange themselves so that the
cations do not 'rattle' in the interstices. The 'non-
rattle' requirement of tangency between ions is
another way of saying that ions pack so as to
minimize the potential energy of the crystal. The
so-called
large-ion lithophile (LIL)
or
incompatible ele-
ments
are not essential parts of the crystal struc-
ture but are guest phases that are excluded to
varying degrees upon partial melting.
In high-pressure language, mineral names
such as
spinel, ilmenite, rocksalt
and
perovskite
refer
to structural analogs in silicates rather than to
the minerals themselves. This has become con-
ventional in high-pressure petrology and mineral
physics, but it can be confusing to those trained
in conventional mineralogy with no exposure
to the high-pressure world. To complicate mat-
ters further, high-pressure silicate phases have
been given names;
majorite, ringwoodite,
wadsleyite, akimotoite
and so on.
Interatomic distances in dense silicates
The elastic properties of minerals depend on
interatomic forces and hence on bond type, bond
length and packing. As minerals undergo phase
changes, the ions are rearranged, increasing the
length of some bonds and decreasing others.
For a given coordination the cation--anion dis-
tances are relatively constant. This is the basis
for ionic radius estimates. Cation--anion distances
increase with coordination, as required by pack-
ing considerations. The increases of density and
bulk modulus with pressure are controlled by the
increase in packing efficiency of the oxygen ions.
Table 22.1 gives the ionic radii for the most com-
mon mineral-forming ions.
