1:1 Clay Mineral Structures: Kaolinite and its relatives


Recall that each layer of the 1:1 minerals is made from one tetrahedral sheet linked to one octahedral sheet. The octahedral sheet can be either dioctahedral or trioctahedral, which splits the 1:1 clay minerals into two subgroups. The kaolin subgroup minerals are dioctahedral, while the serpentine subgroup is trioctahedral.

Differences between the dioctahedral and trioctahedral variants:

The inner hydroxyl (the hydroxyl whose O atom is in the apical O plane) is oriented differently. The O-H bond points more or less perpendicular to (001) in trioctahedral clay minerals but is more or less in the (001) plane in dioctahedral clays (the O-H bond seems to be inclined at an angle of 0 to 30 relative to (001) in the latter case).


Tetrahedral sheet distortion due to "misfit" with octahedral sheet. The ideal tetrahedral sheet has a=5.28 and b=9.15 , while the values for gibbsite are a=5.08 and b=8.68 and those for brucite are a=5.44 and b=9.43 . The octahedral sheet is generally more rigid than the tetrahedral sheet, so the tetrahedral sheet does most of the adjusting to make this fit (remember that Si-O-Si is quite flexible). Thus, for a dioctahedral clay mineral, the tetrahedral sheet must shrink a bit to fit the octahedral sheet. It does so by rotating and tilting individual tetrahedra relative to their neighbors. For a trioctahedral clay, the tetrahedral sheet must expand a bit to fit the octahedral sheet, which it does by a more uniform tilting that can lead to a curling of the layers. Here are the basal planes of the dioctahedral kaolinite (left) and trioctahedral antigorite (right):


Minerals of the kaolin subgroup: Kaolinite, dickite, nacrite, halloysite

Each layer in the kaolin subgroup minerals has approximately the composition Si2Al2O5(OH)4, so we know that the average layer charge is near zero and that the layers are dioctahedral. The minerals and their properties include:

Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Kaolinite

 1:1

 dioctahedral

 ~0

1Tc, 1M

5.1535

8.9419 7.3906 91.926 105.046 89.797

Kaolinite is the most common member of the kaolin subgroup. Economically important.

Classic hexagonally platy morphology to more disordered phases. Also "books." Wide variety of particle sizes.

Structure: DL Bish, 1993, Clays Clay Minerals 41:738-744.







 

Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Dickite

 1:1

 dioctahedral

 ~0

2M

5.1474

8.9386 14.390 90 96.483 90

 

Compositionally the same as kaolinite but it has a two layer structure. Can be considered as the regular alternation of right- and left-handed kaolinite layers.

Much more well ordered than most kaolinites (i.e., less stacking faults from layer to layer).

Structure: DL Bish and CT Johnston, 1993, Clays Clay Minerals 41:297-304.

 


Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Nacrite

 1:1

 dioctahedral

 ~0

2M, 6R

8.909

5.146 15.697 90 113.7 90
Compositionally the same as kaolinite and dickite but it has a two layer structure (some say six).

Structure: AM Blount, IM Threadgold, SW Bailey, 1969, Clays Clay Minerals 17:185-195.


Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Halloysite

 1:1

 dioctahedral

 ~0

1M

5.20

8.92 10.25 90 100 90

Seems very similar to kaolinite but with a single layer of water (2.9) in the interlayer space. 

Often occurs as cylinders or spheroidal shapes (due to hydrogen bonding with water molecules).




Serpentine minerals: Lizardite, chrysotile, antigorite

Each layer in the serpentine subgroup minerals has approximately the composition Si2Mg3O5(OH)4, so we know that the average layer charge is near zero and that the layers are trioctahedral. Individual minerals include:

Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Lizardite

 1:1

 trioctahedral

~0?

1H, 1T, 2H1

5.31

5.31 7.31 90 90 120
Lizardite is probably the most common of the serpentine minerals. It accommodates the tetrahedral-octahedral misfit by substitution of Al for Si. Is it charged then? No, it seems to balance the (-) charge thus created by also substituting Al for Mg in the octahedral sheet, thereby creating excess (+) charge. 

Platy in morphology.

Structure has also been solved as orthorhombic 5.31x9.20x7.31, I. Kristanovic, 1968, Z. Krist. 126:163-169.

Another variant that is similar is amesite: Two-layer hexagonal with a=b=5.31 , c=14.04 (H. Steifink and G. Bruntun, 1956, Acta Cryst. 9:487-492.

 


 

Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Chrysotile

 1:1

 trioctahedral

 ~0

2M?

5.34

9.20 14.65 90 93.25 90

Chrysotile solves the problem of misfit between the smaller Si-rich tetrahedral sheet and larger Mg-rich octahedral sheet by curling into cylindrical rolls.

Needle-like morphology is called asbestoform. Common tube sizes reported to be 13 nm outside diameter and 5 nm inside diameter. 

Structure: EJW Whittaker, 1956, Acta Cryst. 9:855-862.



(Yada, K., 1971, Acta Cryst., A27 659-664)

Clay mineral

Layer type

 Octahedral sheet

 Layer Charge

 Main polytypes

Unit cell dimensions (, degrees)

a b c a b g
Antigorite

 1:1

 trioctahedral

 ~0

2M?

5.32

9.50 14.90 90 101.9 90
Antigorite - periodic inversion of the tetrahedra by 180. Seems to happen along the a-axis, about every 43 angstroms.

A macroscopically platy mineral with lath morphology despite the inversions.



(Kunze, G., 1961 Fortschr Miner. 39, 206-324)