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Author Topic:   How to make sand.
Minnemooseus
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From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
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Message 116 of 121 (597338)
12-20-2010 10:09 PM
Reply to: Message 108 by Dr Adequate
12-11-2010 8:44 PM


Mechanical (physical) vs. chemical weathering
Mechanical weathering, and freezing in particular, plays a relatively small part. (You can tell how large a part mechanical weathering and erosion has played by looking at the composition of the sand: more mechanical weathering and erosion corresponds to a higher proportion of non-quartz material such as feldspar.)
Quartz is both more physically resistant and more chemically resistant that feldspar. It's hard to say which is the greater factor. In some situations it may be one, while in another situation it may be the other. To a great degree they work together. Anyhow, more weathering of any variety would create a higher quartz proportion.
Moose
Edited by Minnemooseus, : Firefox was fighting me.

This message is a reply to:
 Message 108 by Dr Adequate, posted 12-11-2010 8:44 PM Dr Adequate has replied

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 Message 117 by Dr Adequate, posted 12-20-2010 11:32 PM Minnemooseus has replied

  
Minnemooseus
Member
Posts: 3941
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 10.0


Message 118 of 121 (597352)
12-21-2010 12:41 AM
Reply to: Message 117 by Dr Adequate
12-20-2010 11:32 PM


Re: Mechanical (physical) vs. chemical weathering
Some definitions:
Physical weathering - Breaking down a rock or mineral by fracturing or abrasion. Relative to feldspar, quartz is much more resistant to physical weathering.
Chemical weathering - Breaking down a rock or mineral (and essentially it is the breaking down of the rock component minerals) via chemical reactions. Quartz is essentially immune to chemical weathering.
Now, both types of weathering are usually happening at the same time, and one will tend to enhance the other. Physically breaking fragments into smaller particles increases the total surface area which enhances chemical weathering. Chemical weathering softens the material which enhances its fracturing and abrasion susceptibility.
Bottom line - It takes a lot of physical wear and tear to destroy a quartz grain.
Now, in the lab, it would be possible to isolate the material from chemical weathering and look at just physical weathering. Say, put 50% feldspar grains and 50% quartz grains in a totally dry tumbler and let it all tumble for a long time. What will you end up with? Rounded quartz grains and relatively small amounts of quartz dust, and feldspar dust. Of course, as mentioned above, that feldspar dust is now in prime condition for chemical weathering to kick in if water based solvents/reactants are introduced.
There are real world examples of (essentially) purely chemical weathered rocks. I'm not going to here get into such much, but see bauxite and laterite. In my quick readings of those cites, I didn't notice much specific mention of quartz, but I'm thinking that quartz is not much effected by the process (but I may be wrong).
Moose

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 Message 119 by petrophysics1, posted 12-21-2010 7:46 PM Minnemooseus has replied

  
Minnemooseus
Member
Posts: 3941
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 10.0


Message 121 of 121 (597483)
12-21-2010 8:30 PM
Reply to: Message 119 by petrophysics1
12-21-2010 7:46 PM


Re: Mechanical (physical) vs. chemical weathering
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.
Moose

Professor, geology, Whatsamatta U
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"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

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