Question

I was taking the soil components section quiz and this question came up: " As soils age and weather through the eons they may change from montmorillonite to kaolinite. Also, the amount of oxides increases. As soils follow this trend of weathering, the charge on the soil particles will: a) Increase b) Fluctuate c) Decrease d) Remain the same

I said it would decrease (and it turns out to be right) but I don't know if my explanation is appropriate. So I approached this question by first thinking about the structure. If it is kalionite (1:1), CEC decreases and there is less isomorphic substitution (hence the surface will be "more negative"). Also oxidation and weathering "washes out" the cations loosely bound at the cation exchange surface, making it "more negative" as well. Is my thinking right?

Thanks, Maria

If the change in composition of minerals leads to reduction of montmorillonite (which has a high CEC) to minerals such as kaolinite and Al/Fe oxides (that both have relatively small CEC) then that change will result in the decrease in number of charges on the soil particles.

As for why does montmorillonite have higher CEC than kaolinite - the former has a relatively unstable crystalline structure in which there are more isomorphic substitutions and consequently more negative charges.

Thanks!!

re: CEC on Montmorillonite, also consider the increased specific surface area. As Montmorillonite is a shrink-swell clay, it has both external and internal surfaces which can attract and hold cations. In contrast, with Kaolinite the micelles (1:1 layers) are held by H-bonds, so there is only external surface area (i.e. no internal surfaces are exposed)