Unfortunately, we don’t have enough mineral posts, but here’s one from ESCONI VP Dave Carlson about crystals in the Maquoketa Formation. If any has an article or information they’d like to contribute, please send us an email at esconi.info@gmail.com.
I wanted to share pictures of some crystals found over the years at the Vulcan DeKalb quarry.
The first picture shows a small, 1 cm vug in a piece of dark shale. The shale contains fossils of wide, flat bryozoans some of which have dissolved leaving space for crystals to grow.
The vug contains clear, white, yellow and orange crystals of calcite and bits of oxidized pyrite. Also present are many disc or blade-like brownish structures.
The “blades” have ridges on the surface which appear to be the edges of rows of “sub-crystals”. These are smaller crystals deposited as the structure grows.
Whether these are calcite, siderite or something else I don’t know. I did find pictures on the internet of similar, but much larger structures called bladed siderite. Maybe this is how it starts.
Some of the blades (center of picture above) are angular with a rhombohedral outline. Those in the bottom left (above) are viewed on edge, and appear to have a thin margin with increasing thickness toward the back. Perhaps the shape depends on what kind of structure these crystals are growing on.
Please contact me if you have any questions if you have any comments or suggestions on the identity of these structures.
Dave Carlson
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