8 - Lecture notes for Clay Mineralogy

Required reading:

Moore and Reynolds, 146-148
Brindley and Brown, pages 46-80
Newman, pages 22-128

Classification of hydrous layered silicates

2:1 Structures.
True Mica Group X~1

Idealized end-member strucutral formulas are more the exception than the rule when it comes to representing the composition of natural samples. They do however, provide a reference frame for discussing variations in the cystal-chemical seen in natural phyllosilicates. Below are presented the strucutral fromulas for the end-member minerals. <>
Trioctahedral Group (K,Na)1-2z(Mg,Fe2+, [ ] z)3Al3+ Si3O10(OH,F)2

Phlogopite - KMg3AlSi3O10(OH)2

Biotite - KMg2FeAlSi3O10(OH)2


Annite - KFe2MgAlSi3O10(OH)2

not shown


Dioctahedral Group (K,Na) (Al,Fe3+)2-y(Mg,Fe) yAl 1-ySi3+y O 10 (OH)2

Muscovite - K Al2AlSi3O10 (OH)2

Properties: Flexible and elastic

Phengite - K Al 1.5Mg0.5Al0.5Si3.5 O10 (OH)2
Celadonite - K MgAl Si4 O10 (OH)2

* Octahedral-Tetrahedral Misfit - Type of sheet distorsion to accomodate lateral misfit.
 Tertrahedral rotation through ideal angles α ±30°, i.e., Hexagonal (0°) ->Ditrigonal (>0° to <30°) --->Triangular (30°). Ditrigonal symmetry can be related to the b lattice dimension through the trigonometric relationship cos α = b(obs)/b(ideal) where: ideal refrers to the unconstrained tetrahedral sheet. Lateral reductions typcially range from about 1% to 13%.




Apical oxyten and corregation of basal oxygen.

2:1 Brittle micas X~2

Clintonite - Ca Mg 2 Al SiAl3O10(OH) 2 Trioctahedral

Margarite - Ca Al 2Si2 Al 2O10(OH) 2 Dioctahedral





2:1 Chlorites X~1

Consist of a negatively charged 2:1 layer that alternate regularly with a positively charged interlayer sheet of brucite-like or gibbsite-like layers.



Four possible combinations of layer types:
Tioctahedral chlorites: Both 2:1 and interlayer hydoxide sheet are trioctahedral

Clinochlore - Mg3 Si3Al O10(OH )2 Mg2Al (OH)6


Chamosite - Fe3Si3Al O10(OH )2 Fe2Al (OH)6



Chamosite (Fe-chlorite) rosettes from subsurface Norphlet formation, Alabama. The scanning electron micrograph is about 5 µm accross.

Tri-Dioctahedral chlorites: ?
Di-Dioctahedral chlorites:
Donbassite - Al2 Si3Al O10(OH )2 Al2.3(OH)6

Conatins excess Al in the gibbsite-like sheet
Di-Trioctahedral chlorites:

Cookeite - Al2Si3Al O10(OH )2 Li Al2(OH)6

Li-rich

Sudoite - Al2 Si3Al O10(OH )2 Mg2Al (OH)6