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 structural 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 structural formulas 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 (Mg,Fe2+)Al Si
4 O10 (OH)2

Distinguishing Di- from Trioctahedral structures can be accomplished in several ways. Distortion of the octahedral sheet related to vacancies in the dioctahedral structure cause a shortening of the b lattice dimension. In the case of triocthedral sheets, where all sites are occupied, the b lattice dimension increases (and more so as the octahedral cations become larger). This is manifested in the (060) reflection as seen in the diffraction patterns below.

060 reflections from micas

The increase in symmetry afforded by triocthedral structures also has an effect of phase shifting the X-rays in a way that decreases the intensity of the (002) reflections.  The diffraction pattern below demonstrates decrease in the (002) reflection of trioctahedral structures. Ratios of odd:even (00l) reflections make excellent indices for di- versus trioctahedral content, in addition to the positions of the (060).

001 to 002 micas

Other analytical methods can also be used to distinguish octahedral and tetrahedral content and structure.

Here are some papers (and references therein) that discuss other techniques:

Infrared Spectroscopy:

Schroeder, P.A. 1990 Far infrared, X-ray powder diffraction and chemical investigation of potassium micas, American Mineralogist. v. 75, 983-991.
Schroeder, P.A. 1992 Far infrared study of the interlayer torsional-vibrational mode of mixed-layer illite/smectite. Clays and Clay Minerals. v. 40(1), 81-91.
Schroeder, P.A. (2002) Infrared Spectroscopy in Clay Science. in Teaching Clay Science, (ed.) Steve Guggenheim and Audrey Rule, Clay Minerals Society Workshop Series. The Clay Minerals Society, Aurora. CO. v. 13. 181-202. 
Diaz, M., Robert, J-L. Schroeder, P. A. and Prost, R. 2010 Far infrared study of the influence of the octahedral sheet composition on the K+-layer interactions in synthetic phlogopites. Clays and Clay Minerals, v. 58(2) 263-271.

Nuclear Magnetic Resonance (NMR) Spectroscopy

Schroeder, P.A. 1993 A chemical, XRD and 27Al NMR investigation of Miocene Gulf Coast shales with application to understanding illite/smectite crystal chemistry: Clays and Clay Minerals, v. 41(6). 668-679.
Schroeder, P.A
. and Pruett, R. 1996 Iron ordering in kaolinites: Insights from 29Si and 27Al NMR spectroscopy. American Mineralogist, v. 81, 26-38.
Schroeder, P.A., Pruett, R.J., and Hurst, V.J., 1998 Effects of secondary iron phases on kaolinite 27Al MAS NMR spectra: Clays and Clay Minerals, v. 46(4), 429-435.

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 hydroxide 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


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

Below is an XRD pattern of the clay fraction of a Triassic sandstone (Dora J. Truluck #1  South Carolina Upper Triassic). It contains quartz, illite, albite, and a chlorite group mineral. It is hypothesized that the chlorite mineral is sudoite. Here is a bibliography of sudoite research.

XRD pattern