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Tiny crystals in sandstone
  
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Ken




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PostPosted: Mar 17, 2020 21:59    Post subject: Tiny crystals in sandstone  

This sandstone was found in the California desert. I was breaking it apart digging out some very old gastropod and bivalves. These stones were extremely hard, one I kept coming across these small cavities that were filled with tiny crystals. How would you crystal formations inside of sandstone, and is this something common?


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Kevin Schofield




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PostPosted: Mar 18, 2020 05:54    Post subject: Re: Tiny crystals in sandstone  

Ken wrote:
This sandstone was found in the California desert. I was breaking it apart digging out some very old gastropod and bivalves. These stones were extremely hard, one I kept coming across these small cavities that were filled with tiny crystals. How would you crystal formations inside of sandstone, and is this something common?


Hi Ken,

to answer your second question first, yes, this is quite a common phenomenon.

The sandstone you have is very rich in shell fragments and small "coaly" or carbonaceous fragments (the black whisps that you see in all three of the bigger fragments). I presume that the abundance of shell fragments is why you were out looking for shell fossils...
When the various bivalves and molluscs deposited their shells, they were likely made up of a mix of the two polymorphs of calcium carbonate, calcite and aragonite, or possibly (depending on the bug) wholly calcite or wholly aragonite.
Aragonite is much more stable at the earths surface and during very shallow burial (a few metres) than it is as burial continues. Calcite on the other hand is a lot more stable at all depths. As a result, as the shells are buried, aragonite may either dissolve, or alter through a process of recrystallization called "neomorphism".
If they go the dissolution route, the rock is left with a whole bunch of cavities where the shells used to be (extra-large pores called vugs). The dissolved calcium carbonate goes its merry way, but if the solutions become hyper-saturated (which if a lot of shells dissolve quickly is very likely), then the carbonate has to reprecipitate.
It either does that between the grains of the sandstone to form a "cement", or redeposits in the large pores that were formed when the shells dissolved. If that cement phase does not completely fill the pores, then you get cavities such as yours lined with crystals. This process also explains, of course, why your sandstones are so hard...
Thus I expect that most of the crystals in your sandstones are calcite. Intriguingly, the more needle-like crystals in your Photo 6 look as if they are probably aragonite, which implies that at some point during burial and uplift, the physico-chemical conditions again became suitable for aragonite to grow as the stable phase.
If you ant to test the hypothesis, pour a little dilute hydrochloric acid (or spirit vinegar) on the rock. It should fizz as the calcite dissolves.

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Bob Harman




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PostPosted: Mar 18, 2020 07:10    Post subject: Re: Tiny crystals in sandstone  

With all due respect to Kevin S, I question the basic rock being sandstone in the first place and in some of the photos, to me, the crystals lining the small cavities look more like iron stained druzy quartz. The very first photo, above, does not, to me, show a sandstone and quartz lines the small cavity. The other photos also, to me, show quartz lining the cavities.
I do agree that one area does look like aragonite.

As an addendum, look at Ken's other recent posting. A California desert non-sandstone rock with tiny druzy quartz crystals lining small cavities. Some similarities there......

Basically, this shows the difficulty of identifying small rocks/minerals from just a few photos. Bob
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Ken




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PostPosted: Mar 18, 2020 23:01    Post subject: Re: Tiny crystals in sandstone  

Wow, that's very interesting. That's a lot to take in but I do believe you are on to something with your explanation of these crystals. The reason I say this is because I kept coming across specimen of snails and clams that were made of crystals. Some were sediment and crystals but there were hundreds that were all crystals. Some were yellow, transparent white, brown and even a reddish color. I think you may have just explained why this is so. Thanks Kevin S.
The first photo is of the type of sandstone the specimen were found in. The others are of a snail that is part sediment and part crystal.



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