Geology Reference
In-Depth Information
(a) (b) (c) (d)
Figure 4 Structures of (a) ortho (isolated); (b) single chain; (c) double chain; and (d)
sheet silicates
In Table 3, susceptibility to weathering increases down the list as fewer
silicon-oxygen bonds need to be broken to release silicate. Conse-
quently, quartz and feldspars especially, but also mica in temperate
soils, are common inherited minerals in the coarse particle size fractions
of soil (the silt and sand fractions, 0.002-2 mm). The amphiboles,
pyroxenes, and olivine are much more easily weathered. Thus, soils
derived from parent material with rock containing a predominance of
framework silicates (e.g. granite, sandstone) tend to be more sandy,
while those derived from rocks containing the more easily weathered
minerals tend to be more clayey.
The chemical composition of the silicate minerals affects the inherent
nutrient properties of soil. Coming down the list of silicates in Table 3,
there is a change from the dominance of Si, Al, and Na in quartz and the
feldspars to greater contents of Ca, Mg, and Fe in the amphiboles,
pyroxenes, and olivine. These latter make up the so-called 'ferromagne-
sian' group of minerals, and soils in which they are an important
component of the parent material have better inherent nutrient proper-
ties. In particular, such soils are better buffered against the natural
acidification process that occurs due to loss of exchangeable cations,
especially Ca
21
and Mg
21
, by leaching (see Sections 5.4 and 5.5).
5.2.3 Secondary Minerals
Secondary minerals (which dominate the clay-size fraction of soil,
o
0.002
mm) are formed in soil by the action of various weathering processes on
