I essentially agree with everything you say in that message.
So at the mouth of the Mississippi River why is there only quartz and clay and not garnet. The clays (as a mineral) are the result of chemical weathering, the garnet as well is gone, not because it wasn't hard, but because it isn't as resistant to chemical weathering as quartz.
I don't know to what degree garnet (and there are at least 6 different types of garnet) is more chemically stable than quartz. I suspect that garnet is still quite chemically stable.
I think a main reason quartz sand is so common is that it is a very abundant mineral in igneous and metamorphic rocks. Relative to quartz, garnet is a very minor mineral. So, even if it was both physically and chemically as stable as quartz, it would still be uncommon to find as sand.
Look up Bowen's Reaction Series. It shows you the pressure and temp minerals crystallize at, but it also shows you what minerals are less resistant to chemical weathering. The last mineral crystallizing at the lowest temp and pressure is quartz, and therefore the most resistant to chemical weathering at the surface.
It's referred to as the Goldich dissolution series. Yes, it is the inverse of the Bowen reaction series, but I think attributing chemical resistance to formation temperatures and especially pressure is a bit of an oversimplification. Bowen's series doesn't really have anything to do with pressure.
Professor, geology, Whatsamatta U Evolution - Changes in the environment, caused by the interactions of the components of the environment.
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