Mineralogy
C. Lee Burras
- Discuss types of minerals present in the soil.
- Understand the importance of mineralogy for soil science.
- Relate mineralogy with Cation Exchange Capacity
Introduction
Soils are minerals. Even the O horizon of a Histosol is more than 70% minerals on a dry mass basis. In any other soil, minerals account for 95 to even 99.99% of the dry mass. The importance of a given mineral in soil is dependent on its prevalence and, especially, its degree of reactivity. It is important to note that soil minerals can be inherited, lost (e.g., dissolved), gained and/or moved in the profile and the landscape depending on natural and human-controlled processes.
What are minerals?
Minerals are crystalline solids. Most are naturally occurring and inorganically formed. Individual minerals (“species”) are identified and distinguished from one another based on their chemical composition and atomic framework. Restating that, a mineral cannot be uniquely identified from just its chemical composition even though on a day-to-day basis that is often how they are identified. In reality the atomic lattice is a key feature and must be known. For example, “diamond” and “graphite” are independent isomorphic mineral forms of pure “C”, yet – as everyone realizes – they have tremendously different physical and chemical properties. Their isomorphic differences are what control their hugely different value in human societies. Those difference are the function of their bonding strengths across their respective crystalline lattices – aka “atomic frameworks.”
Types of minerals
“Calcite” and “aragonite” are more mundane mineral polymorph in soils. Both are composed entirely of “CaCO3” but only calcite is the one commonly found in soil parent materials and calcic horizons. Calcite is also the one mined for ag liming and such. Aragonite is interesting in its own right, though, given it is biologically created mineral. Snails, mollusks, and other gastropods manufacture it into their shells.
Most soils have tens to hundreds of mineral species comprising their solid fabric. Those minerals commonly fall into three common groups: (a) silicates, (b) carbonates and (c) oxides/hydroxides. It is important to note that soil regions in the world also contain significant sulfate, halide and/or phosphate minerals; however, this chapter focuses on the silicates, carbonates and oxides/hydroxides. Two important subgroups within the silicates are the tectosilicates (3D lattice framework of Si-tetrahedrons) and phyllosilicates (2D framework of aluminosilicates comprised of Si-tetrahedral layers bonded to Al- or Mg-octahedral layers – e.g., see Nelson. 2015., Gaston. 2015).
Learn more about soil minerals, their composition, and main functional importance: Soil minerals[1]
Importance of soil minerals
The importance of a mineral in soil is dependent on its prevalence and/or its degree of reactivity. Prevalence matters because minerals they form the soil’s physical framework, accounti