Clay Mineralogy Assist Prof. Dr. Abbas R. Ali

- If the octahedral layers are contains Mg, (Brucite Structure ) the octahedral layer are named (Trioctahidral layer).

- All the spaces are filled by Mg

- Site of Mg replaces by Fe and Mn.

- If the octahedral layers are contains Al, (Gibbsite Structure ) the octahedral layer are named (Dioctahidral layer).

- 2/3 of the space are filled by Al.

- Site of Al replaces by Fe.

Water H2O existence

• Depending on the presence of water or its absence in clay minerals, Which belong to the three families mentioned above. These minerals can be divided into two parts:

a) Hydrated clay minerals: These minerals, which have layers of water molecules, can be removed without destroying the internal atomic structure of these metals when heated to a temperature of just over 100 ° C.

b) Non-Hydrated clay minerals: This group of clay minerals does not contain particles of water between their layers.

Clay Mineral Family

1. Diphormic Family (I:I).

2. Triphormic Family (I:2).

3. Tetraphormic Family (I:I:2).

Diphormic Family (I:I). 1:1 Layer structure

• 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 O2 ions of the tetrahedral sheets being shared with (and part of) the octahedral sheet . There are three planes of anions (Figure). One plane consists of the basal O2 ions of the tetrahedral sheet, the second consists of O2 ions common to both the tetrahedral and octahedral sheets (marked ‘a’ in Figure) plus OH belonging to the octahedral sheet (‘b’ in Figure ), and the third consists only of OH belonging to the octahedral sheet.

• 2:1 Layer structure The 2:1 layer structure consists of two tetrahedral sheets, with one bound to each side of an octahedral sheet (Figure ). There are four planes of anions. The outer two consist of the basal oxygens of the two tetrahedral sheets, while the two inner planes consist of oxygen's common to the octahedral sheet and the two tetrahedral sheets, plus the hydroxyls belonging to the octahedral sheet.

Triphormic Family (2:1). 2:1 Layer structure

Tetraphormic Family (2:1:1). 2:1:1 Layer structure

• 2:1:1 Layer structure The 2:1:1 layer structure consists of two tetrahedral sheets, with one bound to each side of an octahedral sheet and with additional octahedral sheet (Figure ). 2:1:1 Type Minerals: This silicate group is represented by chlorites.

Kaolinite: • The structure of kaolinite consists of 1:1 layers stacked one above the

other.

• Kaolinite contains Al3 in the octahedral sites and Si4 in the tetrahedral sites .

• The 1:1 layer is electrically neutral and adjacent layers are held together by hydrogen bonding between the basal oxygen's of the tetrahedral sheet and the hydroxyls of the exterior plane of the adjacent octahedral sheet.

types of Kaolinite 1. Kaolinite =1 kaolinit (Triclinic)

(Monoclinic) 2. Dickite = 2 kaolinit (Monoclinic) 3. Nacrite = 4 kaolinite

serpentine group

• The minerals of the serpentine group are layer structures, phyllosilicates, having the general formula

X6Y4O10(OH)8

• where X stands for the atoms in octahedral coordination chiefly magnesium but -also nickel, cobalt, manganese, ferrous iron, zinc, and generally lesser amounts of aluminum, ferric iron, chromium and titanium and

• Y stands chiefly for silicon and in part for aluminum and ferric iron. Because the number of atoms in octahedral coordination for the ideal composition of the serpentine group is equal to 3 for half the unit cell, these minerals are considered structually as trioctahedral.

• Greenalite Fe6Si4O10 (OH)8 (Less common than other minerals)

Berthierine Group

• If the silicon is replaced by aluminum in the tetrahedral layer, a new group will be formed, the Berthierine group.

• This group is characterized by a large amount of aluminum and iron in the tetrahedral layer.

• They also differ from the serpentine group by containing large-size ions in tetrahedral layer.

The following are the main minerals belonging to this group:

Pyrophyllite and Talc • The simple structure of pyrophyllite and Talc are a good starting point for

discussing 2:1 structures.

• Pyrophyllite and Talc consists of 2:1 layers stacked one above the other.

• The tetrahedral sheets contain only Si4 • The octahedral sheet from pyrophyllite mineral contains only Al3 ,

resulting in the ideal formula Al2(Si4)O10(OH)2.

• While the octahedral sheet from Talc mineral contain Mg3 resulting in the ideal formula Mg3(Si4)O10(OH)2.

• The charge is balanced completely within the 2:1 layer, making the layer electrically neutral, and adjacent 2:1 layers are held together only by weak van der Waals forces.

• Pyrophyllite and Talc occurs only rarely in sedimentary rocks and soils, usually only when it is inherited from low-grade metamorphic rocks.

Mica minerals have the 2:1 layer structure described for pyrophyllite but with two important differences.

- First, instead of having only Si4‏ in the tetrahedral sites, one-quarter of the tetrahedral sites are occupied by Al3‏. Because of this substitution, there is an excess of one negative charge per formula unit in the 2:1 layer.

- Second, this excess negative charge is balanced by monovalent cations, commonly K‏, that occupy interlayer sites between two 2:1 layers (Figure ). This gives an ideal formula of KAl2(AlSi3)O10(OH)2 for a mica mineral with Al in the octahedral sites.

Micas

• The octahedral sheet can contain either Al3‏ (the dioctahedral case) or Mg2‏ (the trioctahedral case), and there are several different mica species, because Fe2‏ and Fe3‏ can substitute for Mg2‏ and Al3‏ in the octahedral sheet and Na‏ and Ca2‏ can substitute for K‏ in the interlayer.

• Mica in soils is usually inherited from the parent rock and is likely to occur in soils derived from various igneous and metamorphic rocks, as well as from sediments derived from them. Muscovite, biotite, and phlogopite are the three most common mica group minerals in rocks, and consequently in soils. All three contain K in the interlayer, but they differ in the composition of the octahedral sheet and whether they are di- or trioctahedral. Mica in the clay fraction of soils and sediments differs some what from the macroscopic muscovite mica it most closely resembles. This clay-size mica is often referred to as illite. Glauconite is another mica mineral that is similar to illite, but it contains more Fe and less Al in its octahedral sheet than illite.

• Micas weather to other minerals, particularly to vermiculites and smectites, and the K‏ released during weathering is an important source of K for plants. As a rule, the dioctahedral micas such as muscovite are more resistant to weathering than trioctahedra micas. Thus, muscovite tends to be the most common mica mineral in soils.

Deficient mica Characterized by:

1. It has a lower proportion of (Al) than is found in full mica.

2. Also, the amount of (K) is lower than in the full mica.

group Smectite

The smectite group consists of minerals with the 2:1 structure already discussed for mica and vermiculite, but with a still lower charge per formula weight, namely 0.6–0.2. As in vermiculite, the interlayer contains exchangeable cations.

An idealized formula for a common soil smectite, the mineral beidellite, is: M0.33+‏Al2 (Si3.67Al0.33)O10(OH)2, where M‏ represents exchangeable cations, typically Ca+2‏ and Mg+2.

The most common smectite minerals range in composition between three end-members: montmorillonite, beidellite, and nontronite. All are dioctahedral, but they differ in the composition of the tetrahedral

and octahedral sheets. Smectites do not fix K‏ as readily as do vermiculites because smectites have alower layer charge, but smectites swell more extensively than vermiculite. This is illustrated in Figure 5 by the larger spacing between the 2:1 layers.

• Smectites are important minerals in temperate region soils.

• Many plant nutrients are held in an available form on the cation exchange sites of soil smectites.

• Soils rich in smectite tend to be very effective at attenuating many organic and inorganic pollutants because of the high surface area and adsorptive properties of the smectites.

• Smectites shrink upon drying and swell upon wetting.

• The shrink–swell properties lead to cracking and shifting problems when houses, roads, and other structures are built on smectitic soils.

The most important species that belong to the group of Smactite

1. Dioctahedral Montmorillonite – beidellite series.

2. The Dioctahedral nontronite – beidellite series.

3. The Trioctahedral Stevensite – saponite series.

Vermiculite group • Vermiculites are also 2:1 type minerals in that an octahedral sheet occurs

between two tetrahedral sheets. • In most vermiculites, the octahedral sheet is aluminum dominated (di-

octahedral), although magnesium dominated (tri-octahedral) vermiculites are also present.

• In the tetrahedral sheet of most vermiculite, aluminum is substituted by silicon in most of the sites. This accounts for most of the very high net negative charge associated with these minerals.

• Water molecules, along with magnesium and other ions, are strongly adsorbed in the interlayer space of vermiculites. They act primarily as bridges holding the units together rather than as wedges driving them apart.

• The degree of swelling is, therefore considerable less for vermiculites than for smectite. For this reason, vermiculites are considered limited expansion clay minerals, expanding more than kaolinite but much less than the smectite. The cation exchange capacity (CEC) of vermiculite is higher than all other silicate clays, including montmorillonite and other smectite because of very high negative charge in the tetrahedral sheet.

• Vermiculite crystals are larger than those of the smectite but much smaller than those of kaolinite.

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Chlorites

• This silicate group is represented by chlorites. Chlorites are basically iron magnesium silicates with some aluminum present. In a typical chlorite clay crystal, 2:1 layers, such as in vermiculites, alternate with a magnesium dominated tri-octahedral sheet, giving a 2:1:1 ratio.

• Magnesium also dominates the tri-octahedral sheet in the 2:1 layer of chlorites. Thus, the crystal unit contains two silica tetrahedral sheets and two magnesium-dominated trioctahedral sheets giving rise to the term 2:1:1 or 2:2 type structure.

• The negative charge of chlorites is about the same as that of fine grained mica and less than smectite or vermiculites.

• Like fine micas, chlorites may be interstratified with vermiculites or smectites in a single crystal. Particle size and surface area for chlorites are also about the same as for fine grained micas.

• There is no water adsorption between the chlorite crystal units, which accounts for the non expanding nature of this mineral.

Mixed and interstratified layers:

• Specific groups of clay minerals do not occur independently of one another. In a given soil, it is common to find several clay minerals in an intimate mixture.

• Furthermore, some mineral colloids have properties and composition intermediate between those of any two of the well defined minerals described. Such minerals are termed mixed layer or interstratified because the individual layers within a given crystal may be of more than one type.

• Terms such as "chlorite-vermiculite" and "mica - smectite" are used to describe mixed layer minerals. In some soils, they are more common than single structured minerals such as montmorillonite.

The mixing of these different layers can occur in two ways

1. Regular mixed – layer.

2. Irregular mixed – layer.

a) Mixing between the Dioctahedral layer of minerals.

b) Mixing between the Trioctahedral layer of minerals.

The most common mixed clay minerals in sedimentary rocks :are

1. Illite – Montmorillonite

2. Chlorite - Montmorillonite