Geoscience Reference
In-Depth Information
100
network
forming
elements
P
high field-
strength
elements
Si
10
Ta , N b
Al (IV)
Ti
Hf
Fe
3+
(IV)
Zr
large-ion
lithophile
elements
U
Al (VI)
Fe
3+
(VI)
Th
Yb
5+
4+
Ni
Ce
Mg
Fe
2+
Ca
Sr
3+
Ba
Eu
2+
2+
1
Na
1+
K
Rb
Cs
0.1
0.0
0.5
1.0
1.5
2.0
Ionic radius
Figure 1.11
Plot of the electrostatic potential (charge/ionic radius) vs. ionic radius for different metallic ions.
Ionic radii are given in Å (10
−
10
m). The curves of constant charge (1
) are also shown. The
roman numerals refer to coordination numbers. This plot is useful for the identification of groups
of elements with coherent geochemical behavior: network-forming elements, high-field-strength
elements (HFSE), and large-ion lithophile elements (LILE).
+
to 5
+
The ratio of the charge to the radius of an ion defines its electrostatic potential, i.e. its
ability to modify the electrostatic field in which adjacent ions are bathing. This potential
defines a large number of the properties associated with each element, such as its tendency
to attract bond electrons (electronegativity) or to form compounds such as oxo-anions and
hydrates. Plotting this potential vs. the ionic radius itself, which is a measure of the ion's
ability to fit specific sites in minerals, is particularly informative of the geochemical prop-
accommodated with difficulty by the main minerals of the mantle, except for K-feldspars,
and so are concentrated in the continental crust. They are known as large-ion lithophile
elements (LILE). Small ions carrying strong charges (Zr, Nb, Th, U) develop intense elec-
trostatic fields (high-field-strength elements, HFSE), hence they do not readily substitute
for the major elements in ordinary minerals. Relative positions in the periodic table remain,
however, essential, and elements in the same column share common geochemical proper-
ties, as is the case of the alkali metals (Na, K, Cs), alkaline-earth metals (Mg, Ca, Sr),
halogens (F, Cl, Br), or transition elements of the same row.
