Hi Moose and Dr.A.,
First, sand is a grain size. It has nothing to do with the mineral content.
Read that again. When I describe a sand I first say if is VF, or F, or M, or C or VC, or a mix in which case it is not well sorted. Then I descibe how rounded or angular it is, then how spherical.
At that point I would describe its color and then mineral content, fossils, cement, clay content etc.
When I say the word sand.......I AM ONLY TALKING ABOUT A GRAIN SIZE.
Mechanical weathering depends on the physical resistance of minerals, their hardness. Feldspars are not as hard as quartz, but quartz is not as hard as garnet.
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.
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.
Quartz does chemically break down, otherwise where would silica cement come from? Polycrystalline quartz sand grains weather faster than monocrystalline quartz sand grains. ( A sand grain looked at in a petrographic scope with crossed nicols will show different angles of extinction if it is polycrystalline, but if monocrystaline the whole grain will extinguish at once.)
One can see the mineral content of sands change, along with an increase of polycrystalline quartz as one gets closer to the source rocks/mountains.
There is no way a giant flood could have done this. We see it in sediments today, and we see it in the old rocks if we actually look at them and understand how they were laid down.
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