The examples from the Chattanooga Shale shown on this page, however, suggest that they might be of primary origin. I would be most interested to hear about it. Photos are low resolution for faster loading. Click on the image to get a larger, high resolution picture. SEM images from broken pyrite ooid.
Bottom photograph shows concentric nature of pyrite cortexes.
This observation suggests that pyrite ooids in core formed essentially at the seafloor, and were pyritic prior to incorporation into ooid. Scale bar 0.1 mm long . Py-ooid etched with HNO 3 . Scale bar 0.5 mm long. Scale bar 0.5 mm long.