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Calcite and ? in vug from Ohio

Posted by James Cheshire  
Calcite and ? in vug from Ohio
March 03, 2012 05:55AM
I found this at a road cut of Mississippian siltstone and shale near Newark, Ohio. I wasn't expecting nice minerals there!

The host rock seems to be an iron oxide-cemented siltstone with calcite crinoid stem parts. There are many small vugs with calcite crystals throughout. The pictures are a single vug split in half; field of view is about 14 mm. What are the needle-like crystals? Some are kind of cylindrical, but others are flat. The white angular material is probably calcite (I put 5% HCl on some scrappier examples and got fizz).

open | download - vug1.JPG (107.3 KB)
open | download - vug2.JPG (150.1 KB)
avatar Re: Calcite and ? in vug from Ohio
March 03, 2012 10:24AM
Hi James,

They look like gypsum crystals to me. Nice find!

Re: Calcite and ? in vug from Ohio
March 03, 2012 10:36AM
I think these could be ARAGONITE crystals. We see much the same thing in Indiana and if you go thru my posts of some of my specimens in the "favorite self-collected" thread, "Gail's Favourites of the day" thread and "Midwest Sedimentary Geode" thread you will find a couple of similar specimens. Aragonite seems to often occur with calcite and decaying ferroan dolomite in fossiliferous siltstone deposits similar to the one you describe. Also see several geodes posted by Stephen Rose as they are similar. Also you might confirm by putting a tiny drop of acid and fizzing should occur (but these small crystals will be promptly lost if the acid is left to long!!) BOB
avatar Re: Calcite and ? in vug from Ohio
March 03, 2012 12:36PM
They are aragonite. Joe Vasichko found that material a couple of years ago in a quarry near there.
Re: Calcite and ? in vug from Ohio
March 03, 2012 05:41PM
Yep, aragonite.
Re: Calcite and ? in vug from Ohio
March 03, 2012 06:26PM
That's it! Many thanks for the help!
Re: Calcite and ? in vug from Ohio
March 04, 2012 05:19PM
Dear James;

I agree they are probably aragonite, &, darned nice ones, also. I also think (for experience's sake, if nothing else) you should perform the acid test Bob suggested, EXCEPT, instead of putting the acid on the xls (crystals), you should take a small needle, to pry off a single xl, near the outter edge of the specimen (I see several candidates for this) & test THAT, by dropping it into a drop of acid on glass. It should give very satisfactory results, without damaging your other xls.

Since calcite & aragonite have the same composition (they are called polymorphs, which means it has several forms/minerals with the same composition; graphite & diamonds are a good example of this situation), that means that many locations that have calcite xls, can be expected to also have aragonite, in one form or another. This is especially true in sedimentary deposites.

Your friend, Steve

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