kaolinite and montmorillonite's CEC

one of the questions on the midterm (2009) asks why kaolinite's CEC is alot lower than montmorillonite's CEC. I answered that both phyllosilicate are subjected to isomorphous substitution (which makes the phyllosilicate having a net negative charge), but the substitution on the kaolinite is negligible while the substitution is more significant on the montmorillonite. did i answer this right, and is it detailed enough?

Thanks.

Yes, your answer is correct, but you would also need to add an explanation why there are more isomporphic substitutions in montmorillonite than kaolinite. Something along these lines:
Crystalline structure of the montmorillonite is relatively weak and isomorphic substitutions of Mg for some Al in the octahedral sheet of montmorillonite are more common than in the crystalline structure of a 1:1 type of mineral such as kaolinite. Montmorillonite is 2:1 phyllosilicate mineral, which means that there is 1 Al-octahedral sheet sandwiched between 2 Si-tetrahedral sheets. The layers in montmorillonite crystals are loosely held together by cation-to-oxygen bonds. Hence, water molecules and various ions are attracted between layers (into the interlayer space) causing expansion of the crystal. The effective surface of montmorillonite includes both internal and external surface area. Kaolinite is 1:1 phyllosilicate mineral where in a layer of a kaolinite crystal there is 1 Al-octahedral sheet bonded to 1 Si-tetrahedral sheet by shared (apical) oxygen atoms of the tetrahedral sheet. Two adjacent layers are held together by hydrogen bonding. Consequently, the structure is fixed and no expansion occurs between layers when the clay is wetted. The effective surface of kaolinite is restricted to its external surface area. Because of the strong binding forces between their structural layers, crystalline structure of kaolinite is quite stable and as such there are limited numbers of isomorphic substitutions in the crystalline structure of kaolinite. This in turn leads to a low CEC of this mineral.