Ooid

Ooid is a small, nearly spherical or ellipsoidal carbonate grain, typically about 0.25 to 2 millimeters in diameter, that forms as concentric lamellae around a central nucleus such as a sand grain or shell fragment. The coating layers grow by mineral precipitation, producing a grain with visible concentric rings when examined under a microscope.

Ooids form in supersaturated carbonate waters that are in motion, where a grain is kept in suspension by waves or currents. This agitation repeatedly exposes fresh surface area for precipitation, leading to the characteristic layered structure. The mineral composition is usually calcium carbonate, in the form of calcite or aragonite; in some settings silica-rich ooids can occur, though these are less common. The nucleus and the environmental conditions influence the final appearance and composition of the ooid.

Many modern ooids occur in warm, shallow, high-energy carbonate environments, such as tropical and subtropical seas, carbonate platforms, and shoals. When ooids accumulate and lithify, they form oolitic limestones, an important and widespread carbonate rock type in the geological record. Ooid-rich deposits are especially prominent in certain Mississippi Valley-type and Cretaceous carbonate sequences.

In the rock record, ooids can be distinguished from similar grains like peloids or pisoids by their well-developed, concentric lamellae and their typically uniform, rounded shape. The formation of ooids is discussed as potentially abiotic precipitation in agitated water, though some hypotheses involve biological mediation by microbes or algae in shaping microenvironments.

Ooids are valued for reconstructing past hydrodynamic conditions, water chemistry, and the diagenetic history of carbonate rocks.