ooides

Ooids are small, spherical or ellipsoidal accretionary sedimentary grains, typically less than 2 millimeters in diameter. They are formed by the concentric or radial layering of calcium carbonate around a nucleus, such as a shell fragment or a pellet. The formation process typically occurs in high-energy, shallow marine environments where there is sufficient agitation to keep the nucleus suspended and rolling, allowing for continuous deposition of mineral layers. These environments are often characterized by warm, clear waters with supersaturation of calcium carbonate.

The internal structure of ooids can be radial or concentric, depending on the mineralogy and depositional conditions. Aragonite is the most common mineral component, though calcite can also be present. Ooids are commonly found in carbonate platforms and banks, and their presence can indicate specific paleoenvironmental conditions. They are distinct from oncolites, which are larger, irregularly shaped algal-laminated structures, and pisoids, which are similar but larger than 2 millimeters. The study of ooids is important for understanding past marine environments and the processes of carbonate precipitation.