Lecture 10 Synopsis:
Secondary Minerals in Soils
Though not always true, it is useful to consider
that the gravel, sand, and silt fractions of soils are commonly dominated
by primary silicates and that the weathering products of these larger particles
are concentrated in the clay fraction. Later we will analyze the
cation exchange capacity of the clay fraction to make a rough assessment
of its mineralogical composition. We also examined
the weathering pathways (and products) that primary minerals undergo in increasingly
vigorous weathering environments. Weathering "intensity" can be viewed as
consisting of two processes: (1) rate of weathering (driven by temperature,
precipitation, vegetation) and (2) effectiveness of removal of soluble weathering
products (leaching rate). An important concept is that as "intensity" increases,
the desilication of the soil incrases. We review a
weathering flowchart
to learn how common primary minerals undergo chemical weathering (see attached
figure). Secondary phyllosilicates are classified by the number of Si tetrahedral and Al octahedral sheets. The 2:1 silicates (especially) have considerable cation exchange capacity. In addition to storing cations, these minerals are physically reactive depending on the (1) proportion of Na in the soil water (sodium adsorption ratio) and (2) the total concentration of salts (positive ions) in the soil water. High Na ratios, combined with dilute solutions, cause clays to disperse and create unusual structure (columnar) as well as management problems. |