083120d

 

Compound ooid. An ooid is a tiny sphere of calcium carbonate that grows in agitated warm water, often nucleating on some small pre-existing particle. Ooids are abundant in the tropics, in places like the Bahamas. In this case, an ooid grew on a small chunk of microcrystalline calcium carbonate. After the ooid grew for a while a blob of microcrystalline carbonate formed on one side of it, perhaps under the influence of bacteria (suggested by the two white ovals and dark patches within it). Then, the ooid was reactivated and added a couple more layers. It was finally buried by a mixture of mud, sand, and various kinds of shells. Mississippian, North Alabama, image 1 mm wide.