Ooids

Ooids are small, roughly spherical to ellipsoidal carbonate grains, typically 0.25 to 2 millimeters in diameter. Each ooid has a nucleus—often a sand grain or shell fragment—around which calcium carbonate minerals precipitate in concentric laminae, forming a laminated cortex. When ooids accumulate and become cemented, the resulting rock is called oolitic limestone or oolite.

Formation occurs in supersaturated carbonate waters that are agitated by waves or currents, such as in shallow tropical seas. The constant rolling keeps the grain in motion, exposing fresh surface area for mineral precipitation and enabling the growth of concentric layers. The mineralogy is usually calcite, though aragonite can occur in some cases. Growth can be influenced by biological processes or abiotic chemistry, and the exact mechanisms are a topic of ongoing research.

Ooids are common in modern and ancient carbonate environments. Modern deposits are found in warm, shallow shelf settings such as the Bahama Banks and parts of the Persian Gulf. In the rock record, abundant ooids define oolitic limestones, which can form extensive sedimentary sequences and serve as important stratigraphic markers.

Significance lies in their environmental implications: ooids indicate warm, agitated, shallow marine conditions and provide insights into diagenetic histories of carbonate rocks. They also have practical importance as components of reservoir rocks and, in some settings, as aquifers.