Geoscience Reference
In-Depth Information
3
4
Triangular
(e.g. CO
3
, BO
3
)
Tetrahedral
(e.g. SiO
4
, AlO
4
)
6
12
Octahedral
Dodecahedral
(e.g. K
+
)
(e.g. Fe
2+
, Mg
2+
, Ca
2+
)
Figure 1.6
The main ion coordination systems in naturally occurring minerals: triangular (three closest
neighbors), tetrahedral (4), octahedral (6), and dodecahedral (12) coordination.
The energy stored in silicates as chemical bonds depends on the nature of the cations
and the crystal sites accommodating them. Depending on their ionic radius, cations may
occupy sites of varying size and the number of oxygen neighbors with which they bond
(coordination number) increases with the size of the site. Carbon and boron atoms combine
with oxygen in a triangular arrangement (i.e. threefold coordination,
Fig. 1.6
), while silicon
and aluminum atoms combine with oxygen to form a tetrahedron (fourfold coordination).
However, medium-sized cations, such as Fe
2
+
,Mg
2
+
,orCa
2
+
, will take up vacant octa-
hedral sites (sixfold coordination) between SiO
4
tetrahedra while the biggest ions, such
as K
+
or OH
−
hydroxyl anions, require the most spacious sites, normally of twelvefold
coordination.
1.2 Chemical bonding
Atoms and ions combine to form matter in its solid, liquid, or gaseous states. The
importance of occupation of the outer electron shell can be illustrated by comparing the
interaction between two atoms of helium, where two electrons occupy orbital 1s, with
the interaction between two hydrogen atoms, each with a single electron only. When the
two helium atoms come close together and their electron clouds interpenetrate, one atom's
electrons cannot be accommodated by the orbital of the other as this would infringe the
Pauli exclusion principle. They must therefore jump to the 2s orbital, at a cost in energy that
penalizes the formation of such bonds. Two hydrogen atoms, however, can lend one another
