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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?

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  • $\begingroup$ Wouldn't it increase porosity? link.springer.com/article/10.1007%2Fs12665-011-1015-y Unless I wonder if the granodiorites you are studing are weathered and kaolinite and other neoformed minerals may plug the holes if they are at surface and weathered. $\endgroup$ – user12525 Jun 6 '18 at 9:31
  • $\begingroup$ They are slightly weathered. So it seems the production of kaolinite increases porosity? $\endgroup$ – Yaboy33 Jun 6 '18 at 9:37
  • $\begingroup$ The weathering and chemical proccess is asociated. They should be porous because of the proccess, but clays plug the holes, so porosity is still big, but effective porosity has decreased because of clays. $\endgroup$ – user12525 Jun 6 '18 at 9:39
  • $\begingroup$ @Universal_learner What about the transition of granodiorite to a saprolite form. These are apparently more-porous and clay-based (kaolinite). Am I missing something? $\endgroup$ – Yaboy33 Jun 6 '18 at 9:44
  • $\begingroup$ The transition -at my climate on spain- I read is K-feldspar->kaolinite. I understand you are at the transition between mass rock and saprolite. Clays are very waterproof, water reservoirs are placed on clays frequently. The weathering proccess is what has started porosity. Saprolite will be very porous. it will become a regolite. Of course the gradient is porosity increase if you look at the hole proccess of soil formation. $\endgroup$ – user12525 Jun 6 '18 at 10:02
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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.

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  • $\begingroup$ Another part of my research plans on linking ion presence in an adjacent lake to the weathering of this granodiorite since the area I am studying is fairly humid year round. I believe groundwater would eventually trickle into the river hence carrying these ions with them. I assume it's also possible for ions to get "stuck" in the rigid confines of the mineral and react with excess water to form more oxides augmenting decomposition that way? $\endgroup$ – Yaboy33 Jun 7 '18 at 11:21
  • $\begingroup$ @Yaboy33 Usually in wet environments it's Hydroxides that form rather than Oxides, drying is usually a key step in forming Oxides rather than Hydroxides. $\endgroup$ – Ash Jun 10 '18 at 15:27

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