How does Montmorillonite clay swell?


I’m a field hand actually, yet sometime it’s interesting to know how thing works in microscopic scale. So fun! I’ll explain 3 common clay minerals that mostly found in my workplace right now: Montmorillonite (Smectite), Illite and Kaolinite. In recent years, Smectite is more widely recognized, while montmorillonite is typically associated with aluminius base exchange.

Montmorillonite/Smectite

It has been known as highly swelling clay. This clay mineral has three layered plate-like clay with each unit layer is roughly 10 Angstroms thick. That means, there is 1 million layer clay every 1 mm of thickness 😯 . It also very thin, highly flexible and has a huge surface area.

Montmorillonite

or, in detail structure, it shows as below:

Don’t get intimidated by this structure, it’s just a detail version of a layer clay 🙂

How could the clay be negatively charged in the surface and positively charge in the edge? Here is the mechanism :

  • Mg2+ substitutes Al+++. This Mg++ could be coming from water that is rich in Mg++
  • Now, the surface become surplus of electron (negative ions) since Al has 3, while Mg has only 2.
  • Because the surface now is negatively charged, it is enable to absorb cations such as Na+, K++, Ca++, Mg++. Those cations is exchangable. The CEC measure the amount of cations (in meq) per gram minerals.
  • if significant amount of these ion is Sodium (Na+), then it’s called Sodium montmorillonite, if Calcium (Ca++), it’s called Calcium montmorillonite.

or in a nutshell, it might explained below:

A brief mechanism of cation exchange on montmorillonite

A typical property of montmorillonite is that interlayer swelling (hydrating) with water. The layer absorb water and swell to the point where the forces that holding them together become weakened and the layer and the individual layer can be separated from the packs.

Comparison Ca-Montmorillonite and Na-Montmorillonite. Na+ swell so badly compare to Ca++. Why? attractive force Ca and layer is stronger, since Ca++ is stronger cations than Na+

This is the comparison among clay minerals :

….to be continued….

Reference: MI Mud Manuals

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