Clay Minerals

246 CLAY MINERALS CLAY MINERALS D G Schulze, Purdue University, West Lafayette, IN, crust, therefore, can be characterized as large O atoms USA in an approximately close-packed arrangement held ß 2005, Elsevier Ltd. All Rights Reserved. together by attraction to smaller cations located in the interstitial space. Most of the elements in the crust and in soils occur in Introduction minerals, and the elements listed above are major con- The clay-size fraction of soils consists of mineral stituents of the most abundant minerals, including clay particles that are less than 2 m in equivalent diam- minerals. Building on the concept of packing O atoms eter. This is the realm of exceedingly small, crystalline in space, we will consider atoms as rigid spheres, real- particles dominated by planar arrays of SiO4 struc- izing that this is an oversimplified but convenient tural units and many structural hydroxyls and water. model for developing the key structural concepts. These ‘clay minerals’ crystallize in the aqueous envir- onment at the Earth’s surface from the constituent Basic Structural Concepts ions released by dissolving (weathering) ‘primary minerals’ such as olivines, pyroxenes, feldspars, Tetrahedra and Octahedra micas, quartz, and others that were formed under Two distinct structural features occur within the crys- extreme heat and pressure deep within the Earth. tal structures of soil clay minerals as a consequence Clay minerals are responsible for many of the soil’s of packing the large O2À ions together in space. The most important and characteristic physical and chem- first consists of four O2À ions packed closely together, ical properties. Fundamental soil properties such as and can be described as three O2À ions arranged in cation exchange and shrink–swell properties, as well a triangle with the fourth O2À occupying the dimple as practical considerations such as how well a par- formed by the other three (Figure 1). The centers of ticular soil will attenuate a specific pollutant, or how the four O2À ions form the apices of a regular tetra- much fertilizer phosphorus will be fixed and unavail- hedron, and the small space in the center is called a able to crops, are all influenced by molecular-scale ‘tetrahedral site.’ Cations located in tetrahedral sites differences in soil clay minerals. are in fourfold or tetrahedral coordination, because Clay minerals are distinguished on the basis of their they are surrounded by and bonded to four O2À ions. different crystal structures, and there is a close rela- The second structural feature consists of six closely tionship between the crystal structure and the corres- packed O2À ions. Three of them are arranged in a ponding bulk physical and chemical properties of a triangle in one plane, and the other three, also in a particular type of clay. We begin by considering some triangle but rotated 60 relative to the first three, are of the properties of the major chemical elements that in a second plane so that the two triangular groups make up the clay minerals. intermesh (Figure 1). The centers of the six O2À ions form the apices of a regular octahedron, and the small Major Element Composition of space in the center is called an ‘octahedral site.’ Clay Minerals Cations located in the octahedral site are said to be in sixfold or octahedral coordination because they are Most of the mass and volume of the Earth’s crust surrounded by and bonded to six O2À ions. is made up by only a few chemical elements. O and Tetrahedral and octahedral sites differ in another Si alone account for almost 75% of the mass, with important way. The space that can be occupied by a most of the remainder, in order of decreasing abun- cation in a tetrahedral site is smaller than the space dance, consisting of Al, Fe, Ca, Na, K, Mg, Ti, H, P, that can be occupied in an octahedral site. Since ca- and Mn. On a volume basis, oxygen alone accounts tions vary in size, smaller cations tend to occur in for more than 90% of the total volume. O, as O2À,is tetrahedral sites, somewhat larger cations tend to oc- the only abundant anion, while the other abundant cur in octahedral sites, and the largest cations must fit elements are all cations. Most of these cations have into spaces that are even larger than octahedral sites. only one stable oxidation state at the Earth’s surface Cations with sizes intermediate between the optimum (Al3þ,Ca2þ,Naþ,Kþ,Mg2þ,Ti4þ,Hþ,P5þ); Fe for two sites can occur in either site. The Al3þ ion, for (Fe2þ,Fe3þ) and Mn (Mn2þ,Mn3þ,Mn4þ) are the example, can occur in either octahedral or tetrahedral exceptions. The O2À anions are much larger than sites. Table 1 summarizes the structural sites in which most of the positively charged cations. The Earth’s cations tend to occur in clay minerals. CLAY MINERALS 247 Table 1 Type of structural sites in which common cations tend to occur in phyllosilicate mineral structures Type of site Cation Tetrahedral only Si4þ Tetrahedral or octahedral Al3þ,Fe3þ Octahedral only Mg2þ,Ti4þ,Fe2þ,Mn2þ Interlayer sites Naþ,Ca2þ,Kþ in the same plane and are referred to as basal oxygens. Note that adjacent tetrahedra share only one O2À between them (the tetrahedra share apices or corners). The fourth O2À ion of each tetrahedron Figure 1 Spheres closely packed to form a tetrahedron and an is not shared with another SiO4 tetrahedron and is octahedron. Note the appearance for the three different ways of free to bond to other polyhedral elements. These drawing the model: as a sphere-packing model (top row), ball- 2À and-stick model (middle row), and a polyhedral model (bottom unshared O ions are referred to as apical oxygens. row). (Adapted from Schulze DG (2002) An introduction to soil Since each basal oxygen contributes a charge of À1to 4 mineralogy. In: Dixon JB and Schulze DG (eds) Soil Mineralogy with each Si þ ion, the addition of Hþ ions to the apical Environmental Applications, pp. 1–35. Madison, WI: Soil Science oxygens to form hydroxyls should result in an elec- Society of America, with permission.) trically neutral tetrahedral sheet. Such individual tetrahedral sheets do not form stable mineral structures by themselves and only occur in combination Representing Crystal Structures with octahedral sheets, as described below. Octahedra and tetrahedra are commonly represented Figure 2 shows all of the apical oxygens pointing using different types of models, each of which por- in the same direction, namely, out of the plane trays the same concept, but highlights different of the paper toward the reader. This is the most structural features. Sphere-packing models give an common arrangement, but structures also occur in impression of the space occupied by the atoms, ball- which the apical oxygens point alternately in opposite and-stick models highlight the bonds, while polyhe- directions. dral models emphasize the tetrahedral and octahedral Octahedral Sheet units (Figure 1). There is some ambiguity associated with each representation, and it is important to un- Analogous to the tetrahedral sheet, we can consider derstand the correspondence between, and limita- the octahedral sheet illustrated in Figure 3 as an as- tions of, each type of model. Polyhedral models are semblage of octahedra in which adjacent octahedra used here, along with some sphere-packing models. share two oxygens with one another. In other words, adjacent octahedra share edges. For the arrangement Tetrahedral and Octahedral Sheets of octahedra shown in Figure 3, the octahedral sites are occupied by trivalent cations, typically Al3þ,and The most common and abundant clay minerals for charge balance, a proton (Hþ) must be associated belong to a group of minerals called phyllosili- with each O2À. (The Hþ takes up very little space, and cates (from Greek ‘phyllon,’ meaning ‘leaf’) or sheet the OHÀ ion can be considered a sphere of roughly silicates. A common structural theme in all phyllosi- the same size as an O2À ion.) Each OHÀ contributes licates is the presence of SiO4 tetrahedra arranged one-half a negative charge to each cation because into sheets. Octahedra arranged into sheets are also each OHÀ is shared between two octahedra. Each present in the structures of phyllosilicates and in some Al3þ cation is therefore effectively surrounded by hydroxide minerals as well. Different combinations 6 Â 0.5 ¼ 3 negative charges and the sheet is electric- of the two sheets give rise to the different clay mineral ally neutral. Note the pattern of empty and filled structures. octahedral sites. Two of every three possible octahedral sites are filled when trivalent cations are present Tetrahedral Sheet in the octahedral sites. This arrangement is called The tetrahedral sheet consists of SiO4 tetrahedra ar- dioctahedral and is the most common in soil clay ranged such that three of the four O2À ions of each minerals. If the octahedral sites are filled with diva- tetrahedron are shared with three nearest-neighbor lent cations such as Mg2þ then every possible octahe- tetrahedra (Figure 2). These shared O2À ions are all dral site must be occupied to produce an electrically 248 CLAY MINERALS Phyllosilicate Minerals Common In Soil Clays Phyllosilicates are divided into two groups, 1:1- and 2:1-type minerals, based on the number of tetrahedral and octahedral sheets in the layer structure. 1:1-Type Minerals 1:1 Layer structure The 1:1 layer structure consists of a unit made up of one octahedral and one tetrahedral sheet, with the apical O2À ions of the tetrahedral sheets being shared with (and part of) the octahedral sheet (Figure 4). There are three planes of anions (Figure 4b).

Load more