Mechanisms of Mineral Decomposition

The decomposition of several common soil minerals has been examined in the laboratory and probably represents the mechanism of mineral weathering in soils as well. The decomposition rate of montmorillonite is proportional to the H+ concentration of the attacking solution  

the rate of kaolinite decomposition depends on both the acidity and the kaoli-nite concentration. As kaolinite decomposes, its rate of decomposition decreases, and complete disappearance should theoretically require infinite time. Indeed, kaolinite is quite resistant to weathering. Kaolinite is nonexpanding so its exposed surface is small. Inasmuch as few soil minerals expand, Eq. 7.8 probably characterizes soil mineral weathering better than does Eq. 7.7. Smaller mineral particles tend to decompose first, leaving behind the larger particles. The weathering rate thus dimin- 

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