Clay Minerals in Soil

Clay minerals form from the breakdown of rocks.

Clay minerals form from the chemical weathering of rocks and are mostly present on the Earth's surface. Various silicate minerals, such as feldspars and micas in igneous rocks, will breakdown into clay minerals in the presence of water. Clay minerals are important components in soil because they hold water and nutrients necessary for plant growth. There are several types of minerals in the clay family.

Kaolinite Clay Mineral

Kaolinite is a very common clay mineral that forms through the weathering or hydrothermal alteration of feldspar minerals. Feldspar minerals are common in igneous rocks, such as granite. Kaolinite is a white clay that looks like a powder. It is common in the Kaolin regions of China, which is the source of the name for the clay. This particular clay mineral does not absorb water readily and is mined for its use in pottery and china.

Illite and Chlorite Clay Minerals

Illite clay is the result of the weathering of minerals such as feldspar and muscovite. This clay mineral is dominant in soil of the Midwest; therefore, its name came from the state of Illinois. Illite is the most dominant clay mineral in old shales and mudrocks. Chlorite clays form from the weathering or hydrothermal alteration of mafic minerals such as biotite, pyroxene, amphibole and cordierite. Chlorite clay often occurs with illite clay in Midwest soil.

Vermiculite Clay Minerals

Vermiculite clay forms as the result of the weathering of mica minerals. This clay is able to absorb and hold water. Because of its ability to hold water, vermiculite is commonly added to soil where it does not occur in significant amounts. Vermiculite will help soil to retain moisture and will aid in the slow release of fertilizer nutrients. This clay is often a component of potting soil and is a common packing material for flower bulb shipping and storage.

Smectite Clay Mineral

Smectite clay forms from the weathering of feldspar minerals and volcanic rocks. This clay is also known as Montmorillonite clay or Bentonite clay. Smectite is a swelling clay, which means that as it comes in contact with water, it absorbs the water and swells to form a water-tight barrier. Bentonite is a common clay that drillers use when they are setting a water well. The bentonite swells to seal the well screen from potential surface contaminants. Sodium-rich smectite will swell more than a calcium-rich smectite.

References

• "Manual of Mineralogy, 19th edition"; Cornelius S. Hurlbut, Jr. and Cornelis Klein; 1971
• University of Minnesota; Clay Minerals; Terence H. Cooper; 2009
• Tulane University; Weathering & Clay Minerals; Stephen A. Nelson; 2010
• USGS; Environmental Characteristics of Clays and Clay Mineral Deposits; Nora K. Foley; 1999
• Oakton Community College; Introduction to Clay Minerals & Soils; William K. Tong; 2000