1. Introduction
   
2. Lattice structure of silicate minerals
   a) Formation of Si-O-Si bridges
   
3. Systematics of silicate minerals
   a) Isomorphous substitution
   
4. 2-layered and 3-layered clay minerals
   a) Kaolinite
   b) Smectite
   c) Illite
   

• Primary minerals are chemically not modified since deposition and crystallization from lava. Through weathering and soil forming process, the chemical composition of the primary minerals is altered and secondary minerals are formed. The major processes during weathering leading to the formation of secondary minerals are:
  

1. Modification (alteration) of the structure
   
2. Complete dissolution (decomposition) in ions and colloidal decomposition products
   
3. Recombination of clay minerals from dissolved weathering products (very slow process)
   

Fig.1 Weathering and clay minerals

( Source: Brady and Weil, 2002.)

• Of particular interest in Soil Science are: silicate minerals, oxides, carbonates, phosphates and sulfides. In this section we will focus on silicate minerals. Pictures on primary minerals please click  here
  

Fig.2 The two most common minerals in the Earth's crust. Crystals of feldspar (green) and quartz (gray) from Pikes Peak, Colorado. This specimen is 20 cm across.

Fig.3 Si-tetrahedron

( Source: Adopted from Press and Siever, 1996.)

• Fig.3: Tetrahedron-shaped silicate anion (a) and [SiO₄]^4- complex (b).
  

Si-tetrahedron:

Further informations at  www.gly.uga.edu/schroeder/geol6550/CM06.html

• The basic module of all silicate minerals is a tetrahedron. The smaller Si-Ion in the center is associated with O-atoms and forms the SiO₄^4- complex. The [SiO₄]^4- tetrahedron is an anion with 4 negative charges and needs to be counter-balanced by 4 positive charges either through binding with cations and formation of Si-O-Si bridges where the O-ions are shared with other Si-tetrahedrons.
  

Fig.4 Silica tetrahedra

( Source: http://www.origins.rpi.edu/claycatalyzed.html)

Octahedron

• Larger cations like Al³⁺, Fe²⁺ and Mg²⁺ are associated with 6 O- or OH-ions and form an eight-sided octahedron.
  

  Fig.5 Octahedron

  ( Source: Brady and Weil 2002, p. 322.)

• When 3 Mg²⁺-atoms are coordinated with the 6 O/OH ions the sheet is called a tri-octahedral sheet.
  
• When the 6 O/OH ions are coordinated with 2 Al³⁺-atoms the sheet is called di-octahedral.
  
• The negative surplus charge of an octahedron is ¾ of the total charge, for example [AlO₆]^9-.
  
• Similar to the Si-tetrahedron, the negative surplus charges are counter-balance through cations or higher-level structures.
  

• The silicate minerals are composed of sheets which are held together by joint O-atoms. This leaf-like or plenar structure is termed phyllosilicates.
  
• Adjacent tetrahedrons share 3 O-atoms [Si₂O₅]^2- and adjoining octahedrons share 3 OH-ions [Al(OH)₃].
  
• Based on the layer sequence one distinguishes between 1:1 two-layer minerals or 2:1 three-layer minerals. In the interlayers are adsorbed cations and water.
  

  Fig.6 Si-O-Si bridges

  ( Source: Brady and Weil 2002, p. 322.)

Fig.7 Polymerization of common silicate minerals

( Source: Press and Siever 1995, p. 37.)

• The diversity of silicate minerals is based on 3 properties:
  

1. Polymerization: Si-O-Si (see fig.7)
   
2. Formation of multiple salts
   
3. Isomorphous substitution (see below)
   

• Based on the basic structure silicates are divided into 6 groups (see fig.8).
  

Fig.8 Basic systematic structure of silicates

Fig.9 Isomorphic substitution

( Source: Brady and Weil 2002, p. 323.)

• The isomorphous substitution in the crystal lattices of clay minerals: one atom or element fills a position usually filled by another of similar size without alteration or modification of the lattice structure (see fig.9 above).
  
• It takes place at the formation of minerals from lava flux (not afterwards during weathering) due to deficiency or surplus of individual elements in the lava flux. The molecules fit by magnitude (size) but, posses a different charge.
  
• However, as the number of O- or OH-ions in the tetrahedron or octahedron is fixed, negative surplus charges are generated. Hence neutralizing (counter-balancing) cations are inserted. They determine the cation-exchange capacity ( CEC) and the nutrient availability of soils.
  
• Note: the charge deficit is permanent in the sense that it is a structural feature of the clay particle and persists irrespective of changing conditions such as soil pH and soil solution composition.
  

Fig.10 Tetrahedral and octahedral sheets

( Source: Brady and Weil 2002, p. 325.)

• Kaolinite is a 2-layer clay mineral (1:1). Alternating tetrahedral and octahedral sheets are stacked on top of one another (see fig.10).
  
• The OH of the O-sheet in one layer are adjacent to the O of the T-sheet of the next layer. Adjacent layers of kaolinite are bound together by  hydrogen bonding (O.....OH).
  
• Because of the interlayer  hydrogen bonding, the structure of kaolinite is fixed and no expansion takes place when clay is wetted. Therefore cations and water do not enter between the structural layers. This results in:
  

1. the effective surface is restricted to the external surface area
   
2. lack of significant isomorphous substitution and hence low net negative charge and small capacity to adsorb exchangeable cations.
   

• Kaolinite is the single most occurring clay mineral and is the end product of weathering. Kaolinite is prevalent in many tropical soils.
  
• Example of low or non isomorphous substitution of kaolinite and the calculation of the net charge is given below:
  

Example:

Fig.11 Charge characteristics of kaolinite ( Source: Juma 2002, p. 186.)

• The 2:1 type clay minerals are composed of one octahedral sheet sandwiched between two tetrahedral sheets. Four general groups of silicate clays have this basic crystal structure:
  

1. smectite (montmorillonite) expanding-type
   
2. vermiculite
   
3. fine micas (illite) none-expanding
   
4. chlorite
   

Fig.12 Smectite

( Source: Brady and Weil 2002, p. 327.)

• The top and bottom planes of each layer consist of O-atoms and the adjacent layers are only loosely bound by very weak O-O or O-cation-bridges (see fig.12).
  
• Therefore, exchangeable cations and water easily can enter the interlayer spaces which may result in interlayer expansion. This leads to:
  

1. High plasticity
   
2. Problems in land use: sticky and wet during rainy season and hard and soil cracks during dry season
   
3. Change in volume: swelling-shrinking

   Fig. 13 The different swelling tendencies of kaolinitic (left) and montmorillonitic clay (right) is illustrated. All 4 cylinders initially contained dry, sieved clay soil. An equal amount of water was added to the 2 center cylinders, termed “wet”. The kaolinitic soil settled a bit and was not able to absorb all the water. The montmorillonitic soil swelled about 25 % in volume and absorbed nearly all the added water.

   ( Source: R. Weil, University of Maryland, USA.)

   
   
4. But also: very high total specific surface area
   

• Smectites have a high negative net charge through isomorphous substitution of mainly Mg²⁺ for A³⁺ in the octahedral sheet.
  
• The exchange capacity is 20-40 times higher than of kaolinites.
  
• The predominant formation of smectites occurs through weathering of basic igneous rocks and volcanic ashes, if Mg is present/available. Weathering products in semi-arid regions and lower landscape positions (accumulation of Mg).
  
• An example of the isomorphous substitution of montmorillonite and the calculation of the net charge is depicted in fig.15:
  

Example:

Fig.15 Charge characteristics of montmorillonite ( Source: Juma 2002, p. 187.)

Fig.16 Illite

( Source: Brady and Weil 2002, p. 329.)

• Illites have isomorphous substitution in the tetrahedron but little or none in the octahedron (see fig.16 above).
  
• K-ions are present in the interlayer. The ionic radius of K fits snugly into the hexagonal holes between the tetrahedral oxygen groups. Thus adjacent tetrahedral sheets are strongly bound together, preventing the type of expansion that characterizes smectite clays.
  
• The behave more like kaolinite than smectites with regard to adsorbing water and cations. This takes place mainly at the external surfaces.
  
• An example of the isomorphous substitution of illites and the calculation of the net charge is presented in fig.17:
  

Example:

Fig.17 Formula illite ( Source: Juma 2002, p. 189.)