I started studying the effects of weathering on granodiorite (see previous posts) and plan on referencing the link between the chemical weathering of feldspar plagioclase leading to kaolinization, a process I am relatively unfamiliar with. In brief terms, how would this reaction reduce the porosity of the rock? Also, how would the release of $Ca$ and $K$ influence further degradation of the rocks porosity?
Kaolinite "grains", for want of any better term, are swollen with chemically bonded water compared to the crystals they weather out of so porosity is decreased by particle expansion while at the same time overall density drops. Kaolin also has very strong particle-particle adhesion so any mechanical deformation of the material tends to compress it leading to further reductions in bulk porosity. The influence of Calcium and Potassium on the ongoing weathering of the rock and it's overall porosity depends greatly on the weathering regime; for example if weathering is continuously under wet conditions then Calcium and Potassium will be mobilised in ground water and removed. On the other hand in a wetting-drying cycle Calcium and Potassium liberated from the parent rock become oxides when the material dries out, those subsequently react strongly with water in the next wetting cycle acidifying the environment and accelerating rock decomposition.