70 Chapter Four
vegetation influences the global cycling of atmospheric gases and hence climate.
It is clear that humans have a duty to manage soils judiciously. If soils are lost
through erosion, bad agricultural practice or contamination they no longer fulfil
their fundamental functions. While soils can be destroyed over a period of years
or decades, their regeneration may require hundreds or even thousands of years.4.2 The structure of silicate minerals
Most of the Earth’s crust is composed of silicate minerals, for example feldspars,
and quartz, which crystallized from magma or formed deep in the crust, at high
temperature and pressure during metamorphism. Silicate minerals are com-
pounds principally of silicon (Si) and oxygen (O), combined with other metals.
The basic building block of silicates is the SiO 4 tetrahedron, in which silicon is
situated at the centre of a tetrahedron of four oxygen ions (Fig. 4.2). This arrange-
ment of ions is caused by the attraction—and strength of bonding—between pos-
itively charged and negatively charged ions (see Section 2.3), and the relative size
of the ions—which determines how closely neighbouring ions can approach one
another (Section 4.2.1).4.2.1 Coordination of ions and the radius ratio rule
In crystals where bonding is largely ionic (see Section 2.3.2), the densest possi-
ble packing of equal-sized anions (represented by spheres) is achieved by stacks
of regular planar layers, as shown in Fig. 4.3. Spheres in a single layer have hexag-
onal symmetry, i.e. they are in symmetrical contact with six spheres. The layers
are stacked such that each sphere fits into the depression between three other
spheres in the layer below.O2–Si4+(a) (b)Fig. 4.2Structure of SiO 4 tetrahedron. (a) Silicon and oxygen packing. The shaded silicon
atom lies below the central oxygen atom, but above the three oxygens that lie in a single
plane. (b) SiO 4 tetrahedron with bond length exaggerated.