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Mica Group

If we start with a simple 2:1 phyllosilicate such as talc or pyrophyllite and replace some of the Si with Al, the charge on the 2:1 layer will decrease by -1 for every Si replaced. The negatively charged layers would now attract positive cations (e.g., Na, K, Ca) which can then hold the layers together. As far as rock-forming minerals are concerned, the most important example are the micas. Here is the structure of muscovite:

(100) Plane) (001) Plane

Mica Compositions

The general formula of micas is A0-1Y2-3(Z4O10)(OH,F)2 where A are the interlayer cations (K+, Ca2+ and Na+), Y are the octahedral-layer cations (Fe, Mg and Al), and Z are the tetrahedral cations (Si and Al).
Mineral A Y Z
Muscovite K Al2 AlSi3
Paragonite Na Al2 AlSi3
Phlogopite K Mg3 AlSi3
Annite K Fe2+3 AlSi3
Lepidolite K Li2Al Si4
Brittle Micas
Margarite Ca Al2 Al2Si2
Xanthophyllite Ca Mg3 Al2Si2

Optical Properties of Micas

Micas are fairy easy to identify under the microscope. Biotite, for example, always shows a strong characteristic pleochroism:


Intermediate between the smectites and micas is vermiculite. This phase is defined by its intermediate layer charge. An important feature of both smectites and vermiculites is their ability to exchange cations between the interlayer and a coexisting aqueous solution.

Mineral Layer Charge per Formula Unit Cation Exchange Capacity
Micas 0.9-1.0 0.20-0.40
Vermiculite 0.6-0.9 1.50
Smectites 0.2-0.6 0.80-1.00


Illites may be thought of as potassium-deficient muscovites. Depending on how the interlayer charge deficiency is made up we can get illite, hydromuscovite or phengite:

Mineral X Y Z (OH)- O2-
Muscovite K2 Al4 Si6Al2 4 20
Hydromuscovite K2-x Al4 Si6Al2 4+x 20-x
Illite K2-x Al4 Si6+xAl2-x 4 20
Phengite K2-x Al4-x(Mg,Fe)x Si6+xAl2-x 4 20

Illites are the dominant clay minerals in shales and mudstones. The presence of interlayer K prevents H2O and organic molecules from entering the interlayer sites. Consequently, illites do not expand and have a low cation exchange capacity. Smectites transform to illites and also form complex illite-smectite mixed layer clays during diagenesis.

Chlorite Group

Chlorite consists of 2:1 layers that are held together by Mg(OH)2 sheets:

The Clay Minerals

The building blocks: octahedral and tetrahedral layers:

The phyllosilicates are based on sheets of (Si,Al)O4 tetrahedra. Note below that there are two types of oxygens: bridging (shared by two tetrahedra) and apical (bonded to one tetrahedron). If the tetrahedral sites are occupied by Si(4+), then each bridging oxygen will receive a total Pauling bond strength sum of 2 x (4/4) = 2. The apical oxygens receive a net bond sum of 4/4 = 1. Hence, the apical oxygens need to be bonded to something else.

Tetrahedral layer with stoichiometry (Si4O10)4- composed of corner-sharing SiO4 tetrahedra making an infinite 2-dimensional sheet.

That something else is the octahedral layer made up of (Fe,Mg)O6 or AlO6 octahedra.

We can derive a first-order picture of the phyllosilicates in terms of the tetrahedral and octahedral layers:

1:1 Phyllosilicate Structures

The simplest phyllosilicates result from bonding one silicate layer to one octahedral layer. Because of this arrangement, we call these the 1:1 phyllosilicates.


The most important 1:1 phyllosilicate is the mineral kaolinite. The dioctahedral analogue is the serpentine mineral antigorite.

2:1 Phyllosilicate Structures

The next group of phyllosilicates have one octahedral layer sandwiched between two tetrahedral layers. We call these the 2:1 structures.

Talc and Pyrophyllite

The simplest 2:1 structure is that of talc; the dioctahedral analogue is pyrophyllite.

Structure of talc: Si2O10 tetrahedral sheets (blue) and Mg(O,OH)6 octahedral sheets (yellow) form 2:1 layers. The 2:1 layers are held together by very weak Van der Walls forces. This is why talc is so slippery.

Montmorillonite Group (Smectites) Clay Minerals

The montmorillonite structure resulting from replacing by Al (yellow octahedra) by Mg (blue octahedra) in the octahedral layer of pyrophyllite. The charged double layers are held together by interlayer cations Ca and Na (purple spheres) which are surrounded by water molecules (not shown). Because variable amounts of water can be held between the layers, the layer spacing can expand and contract depending on the hydration. This causes a great deal of structural damage to buildings sited on soils with a high smectite clay content.
Other smectites are nontronite, biedellite and saponite. These result from variable modes of substitution in the octahedral and tetrahedral layers of pyrophyllite and talc:
Di-Octahedral Clays
Mineral Z Y A (exchange cation)
Pyrophyllite Si8 Al4 --
Montmorillonite Si8 Al3.34Mg0.66 (0.5Ca,Na)0.66
Beidellite Si7.34Al0.66 Al4 (0.5Ca,Na)0.66
Nontronite Si7.34Al0.66 Fe3+4 (0.5Ca,Na)0.66
Tri-Octahedral Clays
Mineral Z Y A (exchange cation)
Talc Si8 Mg6 --
Saponite Si7.34Al0.66 Mg6 (0.5Ca,Na)0.66
Hectorite Si8 Mg5.34Li0.66 (0.5Ca,Na)0.66
Sauconite Si6.7Al1.3 Zn4-5(Mg, Al, Fe)2-1 (0.5Ca,Na)0.66

Montmorillonite and beidellite result from the alteration of volcanic ash to give bentonite clay deposits. In conditions of good drainage, Mg will be leached and kaolinite will form instead of montmorillonite. Smectites also result from the weathering and alteration of basic rocks. Nontronite results from the alteration of basaltic glass.

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