Poorly crystalline silicate, oxide, and organic colloids
Poorly crystalline silicate, oxide, and organic colloids
Rivka Fidel
After completing this chapter, you should be able to:
- Compare and contrast types of colloids in terms of:
- General composition
- Type or origin of surface charge
- Where you would expect to find them in abundance
Review of Colloid Types
Recall from the previous chapters that there are 4 types of colloids in soil:
- 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”
Previously, we discussed the crystalline silicate clays, also known as phyllosilicates or “layer clays”. Here we will discuss the remaining 3 types of colloids.
Poorly crystalline silicates
Poorly crystalline silicates are SiO4-based minerals containing what is called “short-range order”, meaning that the patterns in which their elements are arranged only repeat a few times instead of thousands of times or more. They tend to have sphere- or tube-shaped structures, and form during weathering of volcanic ash. Volcanic ash is unique in that it contains very small pieces of volcanic glass, so non-crystalline SiO4, instead of more common minerals like quartz, feldspar, and mica. Glass, or “glassy silica” lacks crystallinity because it forms from very rapid to instantaneous cooling of lava. It is from this non-crystalline, glassy precursor that the poorly crystalline silicates get their lack of crystallinity. Indeed, some soil scientists and minerologists consider the crystallinity of these minerals to be so poor that they call them “non-crystalline silicates” instead, grouping them with glass.
Two of the most common poorly crystalline silicates are allophane and imogolite. Their sphere and tube shapes help them retain more plant-available water than phyllosilicates and oxides or hydroxides (the latter is discussed later in this chapter). This water retention greatly improves the fertility of Andosols (volcanic ash soils). Additionally, allophane and imogolite have substantial pH-dependent charge, helping them hold onto cations and/or anions, depending on the pH.
Oxides and Hydroxides
Oxide and hydroxide minerals are the most weathered of the mineral colloids, and are therefore most concentrated in highly weathered soils like Ultisols and Oxisols. They can be crystalline, poorly crystalline, or non-crystalline. Oxide and hydroxide minerals are named after their anions, oxide, O2-, and hydroxide, OH–. When oxide and hydroxide are together, they form oxyhydroxide, OOH–. For simplicity, we refer to oxyhydroxide minerals as a type of oxide mineral, since they contain oxide. In oxide and hydroxide minerals found in soil, the negative charges of