Geoscience Reference
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which shows the strong attraction of the Zn 2 + cation for the lone electron pair carried
by the oxygen. Water acts as the donor and Zn 2 + is the acceptor. The reaction results
in the liberation of a proton, which justifies the fact that Zn 2 + is referred to as a Lewis
acid and water as a Lewis base. In general, a species donating a pair of electrons is a
Lewis base, whereas a species accepting this pair is a Lewis acid. The variable “hardness”
of Lewis acids, not to be confused with their strength that measures the energy of the
proton bond, helps us to understand the direction of inorganic and organic reactions. Ions
are separated into hard “Class a” (groups I and II of the periodic table plus the lightest
transition elements), soft “Class b” (the upper right of the periodic table), and borderline
(Fe 2 + ,Cu 2 + ,Zn 2 + ,Pb 2 + ) elements as a function of their affinity for lone electron pairs.
Why this additional criterion is important can be illustrated with the following reaction:
CoCl 2 4
2
6
4Cl
+
6H 2 O
Co
(
H 2 O
)
+
(1.4)
Adding the Lewis acid Ca 2 + , which is a harder acceptor than Co 2 + and therefore strongly
binds to water molecules, displaces the reaction to the left and gives the strong blue color
of the chloride to the solution. In contrast, adding Zn 2 + , which is a softer Lewis acid than
Co 2 + , pushes the reaction to the right and shows the pink color of the hydroxide. Such
reactions are essential to account for the role of metals with respect to organic ligands.
Pressure is especially important in the environment of ions. Oxygen is more compress-
ible than smaller ions and, with its cation coordination number increasing, it plays an
essential role. Thus, at depths of more than 660 km, pressure levels in the lower mantle
are such that silicon coordination shifts from fourfold to sixfold thereby promoting the
compact stacking of oxygen atoms. Likewise, under conditions of the lowermost man-
tle, site distortion becomes important and some minerals such as oxides show a transition
from high-spin to low-spin iron. It remains, however, a real challenge to predict with full
accuracy the chemical properties of elements at very great depths.
1.3 States of matter and the atomic environment of elements
Bonds formed by condensed materials are generally more complex than those formed by
gases. In silicates, which are so important to our understanding of geological phenomena,
a small silicon (or possibly aluminum) atom ( Fig. 1.10 ) lies at the center of a tetrahedron
of four oxygen atoms. As in carbon chemistry, SiO 4 tetrahedra may polymerize to varying
degrees by sharing one or more oxygens at their apexes. The Si-O bond is a quite strongly
covalent one. Other elements, such as Mg, Fe, or Na, may be accommodated within the
silicate framework in their ionic forms Mg 2 + ,Fe 2 + ,Na + .
The many crystallized silicate and alumino-silicate structures are classified according to
the pattern formed by their tetrahedra. The most important ones in geology are:
1. Isolated-tetrahedra silicates: the most common minerals in this family are the var-
ious sorts of olivine, such as forsterite Mg 2 SiO 4 , and of garnet, such as pyrope
Mg 3 Al 2 (SiO 4 ) 3 .
 
 
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