Oolites

Oolites are sedimentary grains and the rocks they form, characterized by spherical to ellipsoidal ooids, which are calcium carbonate coatings around a central nucleus. The grains themselves are typically about 0.25 to 2 millimeters in diameter, and when numerous ooids accumulate and lithify, the resulting rock is called an oolitic limestone (or oolitic dolostone if dolomite is predominant).

An ooid forms by concentric accretion of CaCO3 around a nucleus—often a sand grain or shell fragment—within warm, shallow, agitated marine waters where calcium carbonate is supersaturated. The turbulence keeps grains in motion and facilitates the growth of a rind, yielding laminated, concentric layers that distinguish the grain.

Ooids are usually composed of calcite or aragonite, and diagenetic processes can alter them to dolomite. The typical ooid size and the presence of concentric laminae are key petrographic features of oolitic textures.

Oolitic rocks are widespread in the geological record and can form extensive beds in ancient carbonate platforms as well as in modern environments. Modern ooid-rich sediments occur in warm, shallow seas such as the Bahamas and parts of the Persian Gulf, where carbonate shoals and beaches accumulate extensive oolitic limestones and dolostones.

Economically, oolitic rocks have been used as building materials and can act as hydrocarbon reservoirs in some sedimentary basins, reflecting their porosity and diagenetic history.