Ooids are near-spherical grains, usually less than 1 mm in diameter, that form by precipitation of a cortex of calcium carbonate (and in some instances Fe-oxides, clay minerals, and even halite) around a nucleus grain. Formation of carbonate ooids typically occurs in environments of strong and persistent currents where agitation drives CO2 loss that promotes carbonate precipitation. Storms can disperse ooids into many possible environmental settings.

Ooid nuclei may be any allochem type, including broken fragments of older ooids, and also siliciclastic sand grains. Ooids typically display concentric lamination within the cortex when viewed in cross-section. Most modern ooids have cortices made of aragonite. At the microscale, tiny aragonite needles are arranged tangentially to the concentric laminae, yielding a striking uniaxial extinction pattern when viewed in cross-polarized light. A minority of modern ooids, and the majority of some Paleozoic ooid assemblages, have calcitic cortices with a radial structure that may or may not occur in combination with concentric laminations. Ooids of mixed calcite and aragonite mineralogy are also observed, typically with alternating layers of contrasting mineral composition. As with other aragonitic allochems, aragonite ooids and aragonite layers within ooids of mixed mineralogy, are prone to being replaced by sparry calcite during diagenesis. In contrast, calcite ooids have cortical structures that are more resistent to alteration.

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