Clay Mineralogy

 

Clay minerals are hydrous aluminium silicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations.

 

Clays form flat hexagonal sheets similar to the micas. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products.

Clay-rich shales has been usually called “shales” while non-clay shales have been called “silts”. Petrophysical analysis deals with minerals, not particle size, so it is confusing when a zone is geologically described to be shale when the logs show little clay is present.

Clay minerals include the following groups :

1.   Kaolin group which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al₂Si₂O₅(OH)₄). Some sources include the kaolinite-serpentine group due to structural similarities. Usually occurs in the pore system as discrete particles, which do not attach securely to sand grains, so when become dislodged they clog the pore throats.  

 

 

 2.   Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite.

If in contact with water it swells by retaining a good amount of water between layers, plugging the pore throats.

 

 3. Illite group which includes the clay-micas. Illite is the only common mineral.

4. Chlorite group includes a wide variety of similar minerals with considerable chemical variation. Usually occurs as a pore lining around individual sand grains or in clusters.