3.3: Clay Minerals

Clay Minerals Defined

In Earth Science the word “clay” has two meanings. Clay is broadly defined as any unconsolidated material with a grain diameter less than 0.004 mm. That is about 1/100th as big as the period at the end of this sentence, so is not big enough to see with the naked eye. This could include finely ground up quartz, feldspar, or any other mineral. “Clay” also refers to the clay minerals, which are sheet silicates as previously discussed. Remember these form as silica tetrahedra are strongly bonded into sheets. The sheets are then stacked and held together by weaker bonds. Clay minerals typically only exist as very tiny crystals, so most true clays also conform to the fine-grained meaning of the word “clay”.

Most of the clay present in rocks at surface has formed as a result of weathering of other silicate minerals, primarily feldspars, micas, pyroxene and amphibole. The reactions involved are hydrolysis reactions as previously explained.

In one example, feldspar reacts with water and carbon dioxide to form kaolin (clay mineral) plus bicarbonate ions. The potassium and some of the silicon that were originally present in the feldspar, are removed in solution. The carbon dioxide comes from the atmosphere, and over geological time, this is type of reaction plays an important role in controlling the atmosphere’s composition and hence the greenhouse effect.

Clay minerals can also be formed when hot waters (known as hydrothermal solutions) circulate through a body of rock. As is the case for weathering, the hot solutions lead to alteration of pre-existing minerals. Hydrothermal solutions are often also associated with the formation of metal deposits (such as porphyry copper deposits) and the surrounding clay-mineral halos can be an important guide in the exploration for such deposits.

Unlike the primary silicate minerals that they form from, clay minerals are soft and easily eroded into tiny fragments and then transported. They accumulate mostly as sediments in low-energy deposition environments (e.g., deep ocean or in lakes), sediments that are eventually turned into shale.

Properties of Clay Minerals

It is worthwhile to understand some of the properties of clay minerals, as they have important implications for many aspects of environmental geology. They play a role in a great variety of processes, from soil chemistry to the causes and effects of earthquakes, to the permeability of rocks. Some of their important properties are as follows:

They are soft and weak, primarily because of the weak bonds between sheets and the resulting tendency for the sheets to slide past each other under stress. Talc is number 1 on the Mohs scale, and most other clay minerals are similarly soft. The weakness of clay minerals has implications for slope failure (as noted above) because clay bearing rocks also tend to be weak, and for earthquakes, because a plate boundary with clay-rich rocks is likely to slide smoothly, and so less likely to stick and cause large earthquakes.

Most clays are malleable when wet—also because of weak inter-layer bonds—and so can easily be formed into useful shapes for artistic, domestic, industrial and scientific purposes.

Clay minerals are crystals like other minerals, but they typically only form as very small crystals, so clay deposits are almost universally fine grained. Although a body of clay has significant porosity, the pores are extremely small and most of the water within them is close enough to a grain boundary to be held tightly by surface tension, making a clay deposit significantly impermeable. This has implications for groundwater flow and for waste disposal which will be discussed later in this text.

The tetrahedra that make up clay minerals have negatively charged ions (anions) on their outsides making the surfaces of the individual layers negatively charged, and therefore attractive to positively charged ions (cations) in solution. Most metals exist as cations and many organic pollutants have positive charges, and so clay minerals are efficient scavengers of environmental pollutants and can be used as barriers to prevent dispersal of contaminants and also in environmental rehabilitation projects. Different clays have different capacities to absorb cations (known as “cation exchange capacity”), and some of these are listed in the table below.^[1] Smectite has a much higher cation exchange capacity than other clays because cations can get onto the sites in between the molecular layers within a crystal, as opposed to just the outside surfaces of the crystals.

Table \(\PageIndex{1}\) Surface Area and Cation Exchange Capacity of Some Clay Minerals
*Meq/g is milli-equivalents per gram, or milli-moles. E.g., 10 meq/g of Cu = 0.29 g of Cu per g of clay

–	Effective Surface Area in m2/g	–	Cation Exchange Capacity
Mineral	Interlayer	External	Meq/g*
Kaolin	0	15	1 to 10
Chlorite	0	15	<10
Illite	5	15	10 to 40
Smectite	750	50	80 to 150

Clays, especially smectites, have a unique ability to absorb water molecules in interlayer sites, and thereby will expand or swell when wet. Some swelling clay is shown on Figure \(\PageIndex{1}\). The clay is present within a depression and was wet. On drying it shrank in the typical mudcrack pattern. Swelling clays have numerous important industrial and domestic uses, but they also have some serious geological implications. A swollen wet smectite is even weaker than a dry one and can weaken slopes significantly. Bodies of swollen clay can distort the materials around them and contribute to slope failure or problems for building or road foundations.

Figure \(\PageIndex{1}\): Smectite-Bearing Clay in the volcanic Krafla Area, Iceland. On drying the clay shrank and the decrease in volume was accommodated by cracking.

Media Attributions

• Figure \(\PageIndex{1}\): Photo by Steven Earle, CC BY 4.0

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1. The data in Table \(\PageIndex{1}\) are based on information in Wilson, M. (2004). Weathering of the primary rock-forming minerals, processes, products and rates. Clay Mineralogy, 39(3), 233-66. doi:10.1180/0009855043930133; and in Deer, W., Howie, R., and Zussman, J. (1991). An introduction to the rock-forming minerals (2nd ed). Longman. ↵