CEC and surface area of common soil minerals

CEC and surface area of common soil minerals

Mineral	Type	CEC (surface charge cmolc/kg-1)	Surface area (external m2/g-1)
Smectite	High activity clay	-80 to -150	80 to 150
Vermiculite	High Activity clay	-100 to -200	70 to 120
Fine Mica	High activity clay	-10 to -40	70 to 175
Chlorite	High activity clay	-10 to -40	70 to 100
Kaolinite	Low activity clay	-1 to -15	5 to 30
Gibbsite	Al-oxide	+10* to -5	80 to 200
Goethite	Fe-oxide	+20 to -5	100 to 300
Allophane	Amorphous	+10 to -150	100 to 1000
Humus	Organic	-100 to -500	Variable

* Positive sign indicates that the minerals no longer exhibit a cation exchange capacity, but rather an anion exchange capacity.
Adapted from Table 8.1, Brady and Weil (2002).