Reply To: What minerals form the brachiopod geode crystals?

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Jack Kallmeyer

@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!


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