7 - Lecture notes for Clay Mineralogy

Required reading:
Classification of hydrous layered silicates

1:1 Structures

Serpentine group minerals: Most common forms are Chrysotile, Antigorite, Lizardite



1:1 layers common to structure of serpentines

Clay mineral: Chrysotile
T-O combination:  1:1
Octahedral type: Trioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: none described


Chrysotile occurs with a fibrous or asbestosform habit.  In the parlance of clay mineralogy the term asbestos connotes silicates minerals that have an abestoform or fibrous habit. Other common asbestoform minerals are the actinolite, amosite, anthophyllite, crocidilite, riebeckite, and tremolite forms of amphibole. Asbestoform minerals are used in commercial applications because they have beneficial properites of high tensile strength, resistivity to chemical attack, electrical insulation, and high temperature stability.

Chrysotile

Transmission Elecrtron Micrograph of chysotile. Misfit between the smaller Si-rich tetrahedral sheet and larger Mg-rich octahedral sheet results in curl. (Yada, K., 1971, Acta Cryst., A27 659-664)
<>Clay mineral: Antigorite
T-O combination:  1:1
Octahedral type: Trioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: Fe
Mixed-layering: none described

Antigorite is found associated with talc, amphiboles, and metamorphposed dolomite.




Periodic inversion of the tetrahedra creates a macroscopically platey minerals
(Kunze, G., 1961 Fortschr Miner. 39, 206-324)




Calculated X-ray diffraction pattern of antigorite. (Cu-radiation)

Clay mineral: Lizardite
T-O combination:  1:1
Octahedral type: Trioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 2H1, 1T
Chemical varieties: limited
Mixed-layering: none described

Lizardite is most common of the serpentine group minerals. It accomodates the misfit by substitution of Al for Si. This results in a platey habit.



2H1 Polytype             1T polytpe



Calculated X-ray diffraction patterns of Lizardite polytypes (Cu radiation). Note polytypes are distinguished by higher order reflections. Also be aware that mixtures of polytypes are common in geologic samples.                                      


Paragenesis -

Kaolin group minerals (sometimes referred to as the khandites  - kaolinite - halloysite - nacrite - dickite).  Do not confuse the term "kaolin" with kaolin group minerals. The term "kaolin" is a rock term that describes a rock dominated by one of kaolin group minerals (most oftern kaolinite. 

Clay mineral: Kaolinite
T-O combination:  1:1
Octahedral type: Dioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited (Fe substitutes for Al up to a level of about 1:30)
Mixed-layering: kaolinite/smectite

The combination of vacant and filled octahedra site in the octahedral sheet distort the hexagonal pattern. Vacant polyhedral sites are larger than the filled sites (i.e., Thre is no central cation to "pull" the neighboring anions in).  The resultant fit between Octahedral sheet and Tetrahedral sheet is good.  This results in a stable structure (i.e., ~7Å 001  repeat).





Note in the figure below that the top of octahedral sheet is composed of all hydroxyls. Bonding to basal oxygens of tetrahedral sheet is by hydrogen bonding.

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(Bish, D., 1993 )
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Orienation of hydroxyl group is influenced by octahedral sheet charge distribution.



TEM image of kaolinite crystals. Overlay of layer-model not to scale, but shown to demonstrate orientation of crytallographic axes.

Clay mineral: Dickite
T-O combination:  1:1
Octahedral type: Dioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: na

<>Compositionally similar to 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)



Clay mineral: Nacrite
T-O combination:  1:1
Octahedral type: Dioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: na

Compositionally similar to kaolinite and dickite but it has a six-layer structure.


Clay mineral: Halloysite
T-O combination:  1:1
Octahedral type: Dioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: hydrated/dehydrated (10Å/7Å) forms


Think of this mineral as a kaolinite layer with a layer of water (2.9Å) in the interlayer space. The layer thickness is therefore, 10Å. There is also lots of disorder within and between layers.  Fe substitutes for Al in the octahedaral sheet. Often occurs as cylinders or spheroidal shapes (due to hydrogen bonding with water molecules).


Paragenesis - Likely a precursor phase in the formation of kaolinite and primary residual of weathering and hydrothermal alteration of felspars in igneous rocks



Scanning electron image of halloysite from NW Turkey.  Hydrothermal alteration of andesite.


Halloysite tubes. TEM Photo scale is 0.5 µm wide.


Halloysite spheres. TEM Photo scale is 3 µm wide.


Order/disorder in 1:1 structures



Hinckley Index. - The Hinckley index is an empirical measure of crystal defect density in kaolin group minerals. The higher the index, the lower the defect density.  There is a general positive relationship between Fe content and defect density as suggested by the plot below. One must be cautious with this generalization, because it can also be seen that within the same group of samples, this trend is actually non-existent or negative.




The H.I. for the top sample is about 0.64.  The H.I. for the bottom sample is about 1.28.





Here's another data set, showing other reflections for a well-ordered and a poorly-ordered kaolinte.




There is a generally good relationship between Fe content and H.I.  A close inspection of the data shows that scatter that may be attributed to factors other than crystallographic disorder.

Ordering of isomorphous subsititution may influence mineral properties.






2:1 Structures



Talc - Pyrophyllite Group

(R2+3, R3+2 )T4 O10 (OH)2

These are phyllosilicates with no net layer charge They are useful to study because they serve as end-member models for discussing structures found in other clay minerals.

Talc

Clay mineral: Talc
T-O combination:  2:1
Octahedral type: Trioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: na

Sheets are held together by van der Waals bonding. Therefore giving rise to its soft and slippery nature.

Paragenesis - Talc ---> Low-grade metamorphism of silicous dolomite


Pyrophyllite


Clay mineral: Pyrophyllite
T-O combination:  2:1
Octahedral type: Dioctahedral
Layer charge: 0
Interlayer cation:  none
Polytype: 1M
Chemical varieties: limited
Mixed-layering: na


Sheets are held together by van der Waals bonding. Therefore giving rise to its soft and slippery nature.

Paragenesis - Pyrophyllite ----> Low-grade metamorphism of Al-rich volcanic and sed. rocks.



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<>Structure above from Wyckoff (1969) Crystal structures, Vol. 4, page 365


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Relatively pure pyrophyllite is greasy white to gray in color. At Graves Mountain pyrophyllite is often found stained with red-brown hematite. Pyrophyllite's crystal habit is displayed as unique radiating stellated (star-like) aggregates. Individual stars range from 5 to 25 mm in diameter.