Clays are Clays, or are they?

'Cation exchange capacity' (CEC), is a term that gets thrown around when discussing the electrical properties of clays, but what does it actually mean?

The literal definition is:
 "...the number of exchangeable cations per-dry-weight that a soil is capable of holding, and available for exchange with the soil water solution."
What does this actually mean?

Going right back to basics, charges can be divided in anions (an-eye-ən) and cations (kat-eye-ən), representing negative and positively charged ions, or 'non-equally charged atom or molecule', respectively.

A high cation-exchange-capacity therefore means there is a large number of positively charged, mobile ions available to 'exchange' with other ions.

What about the physical values?

Soil colloid—CEC meq/100 g
Sesquioxides—0–3
Kaolinite—3–15
Illite—25–40
Montmorillonite—60–100
Vermiculite—80–150
Humus—100–300
Table from Wikipedia (https://en.wikipedia.org/wiki/Cation-exchange_capacity)

'meq/100g' represents the 'milliequivalent of hydrogen per 100 grams of dry-soil'...

...and humus is very, very high.

Not to be confused with Hommus. 

Humus is organic soil matter, for your interest. (https://en.wikipedia.org/wiki/Humus)

Back on topic, however, is why CEC is relevant to geophysics.

Clays are made up from plate-shaped 'grains' (for lack of a better word), which gives them a vast specific surface area when compared to sand, and spherical sand grains. The surface of these clay platelets has a negative charge as a result of the structure of the clay (a topic for another time), which promotes concentrations of cations from pore-water.

Generally speaking, this is why clays are more conductive than sandstones.


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