I learned about this mixture in my soils class last week, called plinthite. I am not sure whether or not it is considered a rock, or soil. It is a mixture of clay, quartz and other minerals that hardens when it gets wet, supposedly irreversibly hard. I find this rather interesting, because I would think that if it hardens irreversibly it would be a rock, but it it doesn’t have cement holding it together, and is not compacted, as a sedimentary rock would be. that leaves this as a type of soil, but one that is incredibly hard. Any thoughts?

http://www.isric.org/isric/webdocs/docs/major_soils_of_the_world/set6/pt/plinthos.pdf

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