Unit layers, clay

The unit layers stack together face-to-face and are held in place by weak attractive forces. The distance between corresponding planes in adjacent unit layers is called the c-spacing. A clay crystal structure with a unit layer consisting of three sheets typically has a c-spacing of about 9.5 X 10 mm. 

Two types of swelling may occur. Surface hydration is one type of swelling in which water molecules are adsorbed on crystal surfaces. Hydrogen bonding holds a layer of water molecules to the oxygen atoms exposed on the crystal surfaces. Subsequent layers of water molecules align to form a quasi-crystalline structure between unit layers which results in an increased c-spacing. All types of clays swell in this manner. 

Osmotic swelling is a second type of swelling. Where the concentration of cations between unit layers in a clay mineral is higher than the cation concentration in the surrounding water, water is osmotically drawn between the unit layers and the c-spacing is increased. Osmotic swelling results in larger overall volume increases than surface hydration. However, only certain clays, like sodium montmorillonite, swell in this manner. 

In terms of composition, the simplest of the marine clay minerals is kaolinite in which tetrahedral and octahedral layers alternate (Figure 14.5a) creating a two-layer repeating imit. In three-layer clays, the repeating unit is composed of an octahedral... 

The CEC of clay minerals is partly the result of adsorption in the interlayer space between repeating layer units. This effect is greatest in the three-layer clays. In the case of montmorillonite, the interlayer space can expand to accommodate a variety of cations and water. This causes montmorillonite to have a very high CEC and to swell when wetted. This process is reversible the removal of the water molecules causes these clays to contract. In illite, some exchangeable potassium is present in the interlayer space. Because the interlayer potassium ions are rather tightly held, the CEC of this illite is similar to that of kaolinite, which has no interlayer space. Chlorite s CEC is similar to that of kaolinite and illite because the brucite layer restricts adsorption between the three-layer sandwiches. 

Chlorite and Vermiculite. Chlorite is a 1,4-nm (14 A) clay mineral that cannot be expanded or collapsed by traditional laboratory procedures. Structurally, the unit layer of chloride is composed of a 2 I layer combined with a ().4-nin Mg or Al interlayer or hydroxide sheet. 

There is a large number of clays which are not pure mineral types but consist of interstratified units of different chemical composition. (In detail, this may include nearly all the 2 1 layer minerals.) These are called mixed-layer clays. The two or possibly three different units can be regularly interstratified ABABAB or more commonly randomly interstratified AABABBABA. The most common regularly interstratified clay mineral, corrensite (Lippman, 1954), consists of alternate layers of chlorite and vermiculite or chlorite and montmorillonite. 

Mg-rich chlorites do not seem to form readily in low-temperature marine environments. Mixed-layer chlorite-vermiculites form fairly easily in magnesium-rich environments but complete development of the brucite sheets must be considerably more difficult. Mixed-layer chlorite-vermiculite is the predominant clay in the Lower Ordovidian limestones and dolomites of southern United States, usually over a thousand feet thick and extending over 500,000 square miles, yet very little chlorite is present (Weaver, 1961a). These mixed-layer clays and others like them appear to have formed on extensive tidal flats where the clays were exposed to alternating evaporitic... 

Mixed-layer or interstratified clay minerals are those in which individual crystals are composed of unit cells or basic unit layers of two or more types. It is quite probable that the great majority of clay minerals are composed of interstratified layers of differing composition. In most instances these differences are not detected by routine methods of analysis. From a practical standpoint, clay minerals are classified as interstratified when the layers are sufficiently different in character and sufficiently abundant that the presence of the two or more layer types can be established by X-ray analysis. 

Normally, because of the small size of soil particles and the presence of small capillaries and pores in the soil, the water phase is not totally independent of soil solid matter. Water present in larger spaces in soil is relatively more available to plants and readily drains away. Water held in smaller pores or between the unit layers of clay particles is held much more firmly. Water in soil interacts strongly with organic matter and clay minerals. 

The compression of the interlayer water between the unit layers of the vermiculite clay is probably a consequence of the large attractive force between the negatively charged unit layers and a densely populated layer of sodium ions midway between the unit layers. This large attractive force also keeps the unit layers from swelling beyond a two-layer complex. 

Experimental Details. A fair comparison between the apparent densities of clays immersed in water and of clays with a certain number of preadsorbed monolayers immersed in n-decane requires that each preadsorbed monolayer of water between the unit layers is completed, so that no vacant space within a mono-layer exists. The clay should be in the same state of hydration in the entire system. The selectively accessible void space should be completely filled, as well as capillaries in the clay aggregates. The homogeneous distribution of the adsorption water was achieved by slowly equilibrating thin flakes of clay with almost saturated water vapor. After about one month of equilibration, the uniform state of hydration of the clay was shown by the sharpness and order of the x-ray diffraction pattern. The completion of the monolayers was judged from the amount of water taken up by the clay, with the knowledge that about 100 mg. of water is needed per gram of clay for the formation of a monolayer. 

Unit layers are held together in various ways to produce a clay crystal. These are regular, rigid systems and the distance between equivalent points in adjacent unit layers, the c spacing or basal spacing, can be measured by X-ray diffraction and is used to identify the clay minerals (Figure 5). 

Clay crystal Stacks of unit layers held together by various bonding mechanisms, depending on the type of clay... 

The similar structure of illite and smectite allows mixing or interstratification of 2 1 units to form mixed-layer clays. Most illites and smectites are interstrati-fied to a small degree, but they are not classified as such until detectable by X-ray diffraction. As one might expect, illite-smectite mixed-layer clays have intermediate cation exchange capacity between the end-member compositions. 

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