Fibrous radiating crystal form of iron pyrites in shale laminae

I have often seen similar objects embedded in the rock platform on the beach at Kimmeridge Bay in Dorset. I had thought they were all ammonite fossils that had been transformed into iron pyrites or Fool’s Gold. Now I am not so sure because the shapes of some of them are very distorted to be fossils, and I have just learned about the existence of Angel Wing Pyrite.

Iron pyrites is iron sulphide, FeS², and under normal conditions it would form cube shapes, or pyritohedrons. However, when the crystals develop in the restricted confines between the compressed layers of bituminous shales, they grow out laterally in a radiating fibrous form. Sometimes these can be perfectly circular growths commonly known in the United States as a Pyrite Sun or Pyrite Dollar. Sometimes overlapping discs along adjacent bedding planes can look like Pyrite Flowers.

The examples shown here lack that regularity of shape and can assume various shapes which are locally called Angel Wings. They occur in the Maple Ledge Shales of the autissiodorensis Zone of the Kimmeridge Clay deposits.

Fibrous radiating crystal form of iron pyrites in shale laminae

Fibrous radiating crystal form of iron pyrites in shale laminae

Fibrous radiating crystal form of iron pyrites in shale laminae

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved

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