Geoscience Reference
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Fig. 5.1
The schematic demonstration of Si tetrahedron in minerals
If one tetrahedron shares its one vertex with that of another tetrahedron, they
form twins. If tetrahedrons share two vertices, they form a chain. Three and more
tetrahedrons have the capacity to form a plane where each vertex is shared by neigh-
boring tetrahedrons. This characteristic sharing of vertices leads to a great variation
of individual lattices. The second type of geometric confi guration is the octahedron
with eight faces, or walls. Using Greek terminology, we replaced tetra by octa
which means eight. We can obtain this eight-sided confi guration when we join the
base of a classical form of an Egyptian pyramid to another Egyptian pyramid with
its top vertex placed to the opposite side of the vertex of the fi rst pyramid. Of course,
the size of these Egyptian pyramids is on the microscale of atoms. Octahedrons
have the capacity to share their vertices with neighboring octahedrons or tetrahe-
drons as we have shown for tetrahedrons. The vertices of geometric confi gurations
are occupied by spheres that are the models of big anions - mainly oxygen O. In
some instances, a sphere representing the hydroxyl anion OH occupies the vertices.
In the center of the tetrahedron or octahedron, smaller spheres of a cation are
located. In the center of the tetrahedron, silicon Si is located in the cavity between
the anions, while for the octahedron it is either Al or Fe and Mg.
Silicates are held together by ionic and covalent bonds that occur roughly in
equal proportions. Let us now explain the models in more detail. A silicate tetrahe-
dron is formed by a triangle of three O spheres above which is a fourth O sphere
seated on the shallow triangular depression of those three O spheres. The cavity
below the fourth O sphere is fi lled by a Si sphere carrying four positive charges,
while the O spheres on the four vertices carry a total of eight negative charges. This
lack of balance between positive and negative charges accounts for the linkage with
neighboring confi gurations. Individual tetrahedral confi gurations bind themselves
into bow tie, ring, chain, double chain, and sheet silicates; see Figs. 5.2 , 5.3 , and 5.4 .
Octahedral confi gurations are usually seated on the top vertices of a tetrahedral
base. With the exception of Na + or K + , a cation such as Al 3+ , Mg 3+ , Fe 3+ , Fe 2+ , or Ca 2+
is again in the center of each octahedron (Fig. 5.5 ). Let us denote them all as the
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