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Amanda and I recently collected fossils from the Grant Lake Formation in the Cincinnati Arch and numerous examples of brachiopod geodes were collected. Some of the structures were spectacular. Has any work been done to identify the minerals forming the crystalline structures. I am including several pictures.
The first one looks kind of like calcite to me…
…and the others look like Quartz, maybe with some Limonite (Iron Oxide)
@lcone @vperez @william-howat Lee had asked me about this clear crystal earlier but I hedged on trying to ID from a photo.
I can tell you for certain that the orange crystals are Dolomite. Since the Cincinnatian is pretty much all carbonates, the in-filling minerals are usually Calcite or Dolomite. I have also seen Barite. I know of only one site with silicification around here and it is at the top of our Ordovician exposures way above the Grant Lake. I suppose it isn’t impossible that it is quartz but that is less likely than these other minerals.
You made me get my mineral books out! Right now I’d say the larger clear crystal is one of the many forms of Calcite. Calcite can be clear and has over 100 different crystal forms including combinations of those forms so the outward resemblance to quartz isn’t unexpected. If you Google Calcite crystals and look at images you’ll see what I mean. Sometimes calcite will fluoresce so you can try that if you have a SW UV light. I have a similare brachiopod geode here (photo attached) and the internal crystals in mine do not fluoresce. A scratch test with glass would tell a lot too but I don’t know how you’d do that inside the brachiopod geode.
Lee, if you come to Geofair here on May 6 and 7, you can ask our mineral identification experts what it is!
Jack
Jack @jkallmeyer – Thanks so much for the detailed explanation and information. I am writing a short article for the Friends newsletter on the brachiopod geodes, and will add this information with credit. I would also like to include the photo that you posted, if that is ok. That is a spectacular example! Thanks to Victor and Bill for the input, as well. I, too, thought that the outer crystals were probably quartz, so I really learned a lot from the forum thread. Thanks myFOSSIL community!
@lcone Sure you can use the photo. The mineral guys here ID’d the long white crystal in my geode as probably Celestite by the way. The orange pink stuff is Dolomite. The rest of the crystals along the walls of the brach area all Calcite. Make sure you tell people not to break brachs for the pretty crystals – mine are all pre-broken by nature or weathering as I believe you said yours were. 🙂
Jack
@lcone @jkallmeyer It’s really hard to ID Calcite or Dolomite by a photo. You can use acid test – place a drop of dilute hydrochloric acid on a mineral and watch for releasing bubbles of carbon dioxide. If reaction is weak – it is Dolomite.
@matteo-di-angelo @lcone Matt, your chemical tests are spot on for Dolomite. In the cases here (Lee’s samples and mine) the crystal shape is the dead giveaway. These crystals exhibit a form not seen in Calcites. They are very thin curved rectangular shapes – kind of like a potato chip that is rectangular rather than oval. The crystals lining the brach have been shown to be Calcite by tests like yours. I attached a cropped photo of the one I sent earlier to show (hopefully) the curved crystal shape of the Dolomite.
@jkallmeyer I just said that the identification of minerals on the photos is quite a difficult task. 🙂 Mineralogy was my hobby at school, and I had a small collection.
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