042421b

 

Beautiful ooid surrounded by calcite crystals. Ooids are spheres, formed when seafloor sand-size sediment is rolled regularly, typically by tides. Concentric layers form in warm water with the right chemistry.Ooid cortices usually form around nuclei, in this case a fine quartz grain. Viewed with cross-polarized light, long dimension of field of view = 1 millimeter, Mississippian, North Alabama.

Part of a study of a very small fossil reef made by a consortium of corals and diverse microbes. The ooid was washed in from a nearby shoal during a storm, and overgrown by the reef.