2-2 Clay Minerals

For section 2-2 you should read section 10.5 of Chapter 10 of the textbook and complete the exercises embedded in that chapter, and then answer questions 11 to 13 at the end of Chapter 10. This document includes interactive versions of some of the exercises in the textbook.

Clay minerals are special because they are phyllosilicates that have relatively weak inter-layer bonding (Figure 10.5.2 in the textbook), making the rocks and sediments they are within quite weak. This is because they have negatively charged surfaces and so are attractive to cations, and also because they tend to exist as very small crystals so that a mass of clay grains has a very high surface area (Figure 10.5.4). There is a significant emphasis on clay minerals in this course because their existence has many implications for Environmental Geology and Earth Systems. Some of these implications are discussed below, but they are also covered in several other parts of this course.

Clay minerals are common products of chemical weathering of silicate minerals (by hydrolysis) and, as summarized on Table 10.5.1 in the textbook, the most common weathering products are kaolin and smectite. An example of that is the chalky look of the kaolin on the surface of feldspars in the weathered granite shown on Figure 10.2.1. Clay minerals also form at temperatures higher than those of weathering at surface, and this takes place at relatively shallow levels in volcanic regions and at greater depth elsewhere. Clay mineral formation is particularly common in situations where there is water flowing through the rock.

Two examples of that are illustrated on Figure 2-1 below. On the left, sea water is circulating through oceanic crustal rock. On the right, fresh water is circulating through older crustal rock and more recent volcanic rock. In both cases the convective systems have been created by the heat of the volcanism. The oceanic crustal rock is basalt and gabbro that are dominated by pyroxene, olivine and plagioclase feldspar. The likely alteration product in that situation is the clay mineral chlorite, although serpentine might also be formed from any ultramafic parts of the oceanic crust or upper mantle. The continental crustal rocks surrounding a subduction-related volcano can be highly variable in composition, so several different clay minerals could form.


The image is describing two different activities. The left side is under the oceanic crust. The title is Groundwater circulation and hydrothermal clay-mineral alteration a spreading-related volcanism. There is a large yellow arrow coming up from the bottom of the image pushing the oceanic crust up. There are white arrows radiating from the crust, going down then coming up right beside the peak of the yellow arrow which appears to be pushing up the oceanic crust. there are white arrows pointing in different directions in the upper mantle. Between the image on the left is a green upper mantle yer, with the oceanic crust on top veering down towards the right. The veering down is called the subduction. On the right hand side the title is Groundwater circulation and hydrothermal clay- mineral alteration at subduction- related volcanism. There are two layers with a volcano on top. The volcano has a red line running through it and going through the layers from below the upper mantle layer. The hole at the top of the volcano has two white arrows curving in from the top layer and our towards the hole at the top of the volcano. The lowest layer is called the upper mantle and it is a light green colour. The layer closest to the volcano is called the continental crust.
Figure 2-1 Sites of Hydrothermal Alteration in Volcanic Settings at Divergent and Convergent Boundaries

Figure 10.5.5 in the textbook shows the types of clay mineral transformations that can take place within sedimentary rocks during burial. Smectite and kaolin form near to surface (as is the case in weathering environments), while chlorite, illite, and the non-clay mineral muscovite are more common at greater depths.

Exercise 2-2 Properties of Clay Minerals

Clay minerals are important to several Earth Systems processes, and some of the key roles are listed in the textbook. Make sure you understand why clays are important in those roles. Some of the important properties of clay minerals are listed in the table below. In the right-hand column provide an explanation for the property listed.

Table 2-1 Important Properties of Clay Minerals
Property Explanation
Soft and weak
Attractive to cations
Ability to swell


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