Phyllosilicate Colloids and their Surface Chemistry
Phyllosilicate Colloids and their Surface Chemistry
Rivka Fidel
Upon completing this chapter, you should be able to:
- Define phyllosilicates
- Explain:
- the difference between colloids, clays and phyllosilicates
- the difference between a 2:1 and 1:1 phyllosilicate
- what happens when phyllosilicates swell and why swelling is important
- where phyllosilicates get their surface charges from
- Match types of charges (permanent and pH-dependent) with their definitions and where they come from
Review of Colloid Types
Recall from the previous chapter that colloids are all particles less than 1 micrometer in diameter, and that there are 4 types:
- Crystalline silicate clays (phyllosilicates): layered minerals containing SiO4
- Poorly crystalline silicates: sphere and tube-shaped minerals containing SiO4
- Oxides and hydroxides: highly weathered minerals containing O2-, OH–, and OOH3-, bonded to transition and post-transition metals (mainly Fe and Al)
- Organic colloids: particles of biological origin <1 μm in size that contain C-H and/or C-C bonds, also called “colloidal humus”
Here we will focus on the first type, crystalline silicate clays. The remaining types will be discussed in the next chapter.
Crystalline Silicate Clays (Phyllosilicates)
Crystalline silicate clays are the most abundant colloids in most soils, and have a disproportionate effect on soil properties. We’ve learned that the term “clay” can refer to any mineral particle less than 2 µm in diameter. The majority of these clay particles are crystalline silicate clays, also known as phyllosilicates. From Greek, “phyllo-“ refers to layer, and “silicate” refers to SiO42-. As reflected in their two names, these important minerals have 3 main properties:
- Crystalline: all phyllosilicates have atoms arranged in a regular, near-infinitely repeating structure
- Layers: the crystal structure of phyllosilicates is arranged in flat, paper-like layers
- Silicate: phyllosilicates all contain silicate tetrahedra. Each tetrahedron has the formula SiO42-, but since oxygens are shared, the stoichiometric formula of silicate is frequently written as SiO2 (this is the formula of quartz for example, the dominant mineral in most sand particles).
Phyllosilicate sheets and layering patterns
Phyllosilicates, also known as “layer clays,” can be classified according to their layering pattern and chemical composition. Layering patterns are defined by how tetrahedral and octahedral sheets are arranged.
Tetrahedral sheets are composed primarily of silicate tetrahedra, SiO42-. In each tetrahedron, the Si is at the center, and is bonded to 4 oxygens arranged in a pyramid-like shape with a triangular base. These can be drawn in various ways.
Octahedral sheets are composed primarily of aluminum and magnesium octahedra. In each octahedron, the aluminum or magnesium is located at the center, and is bonded to 8 oxygens. The overall shape resembles two pyramids with square bases, fused at the base.
The two main patterns