Soil Colloids: Classification, Clay Minerals, and Formation

Soil Colloids: The Active Fraction of Soil

Colloids represent the active fraction of soil, playing a crucial role in soil properties and fertility.

Classification of Soil Colloids

Soil colloids can be broadly classified into two main categories:

• Organic: Humified fraction of organic matter.
• Inorganic: Silicate clays (phyllosilicates), Al and SiO₂ gels (allophane and imogolite), and oxides and hydroxides of Fe and Al.

Nature of Clay Minerals

It’s important to differentiate between clay minerals and the clay fraction. The clay fraction, based on particle size, is arbitrarily defined as particles smaller than 2 μm.

Crystal Structures of Clays

Individual atoms in clays are arranged in layers with a repeating pattern. Two basic structural units are common to all crystalline silicate clays:

• Tetrahedron: A silicon atom at the center, surrounded by 4 oxygen atoms in a tetrahedral arrangement.
• Octahedron: An aluminum atom at the center, surrounded by 6 oxygen atoms in an octahedral arrangement.

Isomorphic Substitution

Structural changes involving ion substitutions can occur during clay formation in both tetrahedral and octahedral sheets. These substitutions, such as replacing Si atoms in tetrahedra or Al atoms in octahedra, are limited by atomic radius and coordination number (4 for tetrahedra and 6 for octahedra).

Clay Mineral Groups

Different groups of clay minerals exhibit distinct properties:

• Kaolinite Group: Low cation exchange capacity (CEC), relying on external surfaces and broken edges. Large relative size (0.1-5 mm).
• Montmorillonite Group: 2:1 type clays with high CEC, both internal and external surface area. Small size (0.01-1 mm).
• Hydrated Mica (Illite) Group: 2:1 type clay with intermediate properties. Non-elastic, with a lower amount of isomorphic substitutions.
• Chlorite Group: 2:2 type clay. 2:1 structure with an intermediate layer of aluminum. Limited presence in Chilean soils.
• Vermiculite Group: 2:1 type clay with high internal surface area. K⁺ ions are exchangeable, and the layer can fix cations.

Origin of Clays

Clays can originate through various processes:

• Inheritance: Clays are part of the parent materials, typical in sedimentary soils. The deposited materials depend on the location and time.
• Transformations: Limited changes in primary minerals, usually involving degradation.
• Neoformation: Significant changes involving the breakdown and reconstruction of primary minerals. This involves separating the crystalline structure and ions from tetrahedra and octahedra.

Conditions for the Formation of Different Clay Minerals

• Montmorillonite: High concentration of silica and Mg, persistent weathering at a high level in neutral to alkaline conditions.
• Vermiculite: Moderate concentration of H⁺, high concentration of Si and low Al. Mica present in the starting material.
• Illite: Mica present in the starting material, H⁺ at low to moderate levels, displacement of K⁺ in interlayers. Medium to relatively high concentration of Si and Al for stability.
• Kaolinite: High concentration of H⁺, absence of bases.
• Halloysite: Same concentration of Al and Si and high concentration of H⁺.
• Allophane: Medium to high concentration of H⁺, in systems with high moisture content or saturated conditions.
• Gibbsite: High concentration of H⁺ and low concentration of Si, no bases.