The Chemistry of the Solid Earth - Principles of Environmental Chemistry - page 238

5.2 MINERAL COMPONENTS OF SOIL
5.2.1 Inputs
The nature of the parent material is the most important factor influen-
cing the mineral components of a soil. In particular, the textural
(particle size) properties and inherent fertility are directly affected by
the types of rocks and minerals found in the parent material.
Parent material can be consolidated rock (igneous, sedimentary, or
metamorphic) or unconsolidated superficial deposits that have been
transported by some agency, such as wind, ice, or water. Igneous rocks
are formed from molten magma from the earth's crust and are the
ultimate source of all rocks. Extruded igneous rock is formed when
molten magma appears at the surface and cools quickly, resulting in
small grain size (e.g. basalt). Intruded igneous rock cools slowly on its
way to the surface and results in large grain size (e.g. granite). Sedimen-
tary rocks are formed from the weathering products of igneous,
metamorphic, or older sedimentary rocks. Metamorphic rocks are
formed by changes in igneous or sedimentary rocks owing to high
temperature and/or high pressure, which usually has the effect of making
the rock more resistant to weathering. Unconsolidated materials are
surface deposits of partially weathered rock that have been transported
and deposited in various ways. The most important groups of such
materials, from which much of the world's major soils are formed, are:
glacial deposits, resulting from the action of ice; alluvium, deposited
from water; and aeolian or wind-blown deposits. w
5.2.2 Primary Minerals
The primary soil minerals, sometimes called 'inherited' minerals,
are those that are derived from the rocks of the parent material. They
are distinguished from the secondary, or 'pedogenic' minerals, which are
formed during the process of soil formation (see next section). The main
primary minerals are silicates (Table 3), all based on the SiO 4 4
tetrahedron, in which three oxygen atoms are in a triangular plane
below the silicon atom and one oxygen is above the silicon (Figure 3a).
There may be isomorphous substitution of silicon by aluminium. The
charge is balanced by cations such as K 1 ,Na 1 ,Ca 21 ,Mg 21 , and Fe 21 .
The silica tetrahedra can be linked in a number of ways to form
silicates of differing structures, and hence susceptibility to weathering
w A description of the origins and morphology of specific soil types is outwith the scope of this
chapter. An excellent description of the major soils of the world can be found at http://
soils.usda.gov/education/.