Ooids

The term ooid is applied as a general name to coated carbonate grains that contain a nucleus of some kind - a shell fragment, pellet, or quartz grain - surrounded by one or more thin layers or coatings (the cortex) consisting of fine calcite or aragonite crystals. ^[1] In some ooids, the nucleus may be too small to be easily seen. ^[1]

These coated grains are sometimes also referred to as ooliths; however, the term ooid is preferred. ^[1] Carbonate rocks formed mainly of ooids are called oolites. ^[1]

Normal (Mature) Ooids

Spherical to subspherical ooids that exhibit several internal concentric layers with a total thickness greater than that of the nucleus are called normal or mature ooids. ^[1]

Formation Conditions

Ooids form where strong bottom currents and agitated-water conditions exist and where saturation levels of calcium bicarbonate are high. ^[1] Currents and waves keep the grains moving and intermittently suspended, allowing more or less even precipitation of calcium carbonate on all sides of the grains. ^[1] Both supersaturation of the water and intermittent burial and resuspension of the ooids owing to agitation appear to be necessary for most ooids to form, although some ooids are known to form in quiet water. ^[1]

Mineralogy

The coatings on modern ooids are composed mainly of aragonite, whereas ancient ooids are composed principally of calcite. ^[1] Many of these ancient ooids were composed originally of aragonite that later transformed to calcite; however, petrographic evidence suggests that other ancient ooids originated as calcite. ^[1] Precipitation of ancient calcitic ooids appears to have been particularly important during middle Paleozoic and middle Mesozoic time. ^[1]

Sea Level Control

Variations in ooid mineralogy appear to be related to sea levels. ^[1] High stands of the sea apparently favor formation of calcite ooids because CO₂ levels tend to be higher and Mg/Ca ratios lower during such times; low stands favor aragonite ooids because of lowered CO₂ levels and elevated Mg/Ca ratios. ^[1] High Mg/Ca ratios favor precipitation of aragonite rather than calcite because Mg ions inhibit crystallization of calcite. ^[1]

Radial Ooids

Although most ooids display an internal structure consisting of concentric layers, some ooids show a radial internal structure. ^[1] Radial ooids that also display concentric layers probably formed by recrystallization of normal ooids; however, radial ooids may form also by primary sedimentation processes. ^[1]

Superficial Ooids (Pseudo-ooids)

The coating on some ooids consists only of one or two very thin layers, which have a total thickness less than that of the nucleus. ^[1] Such ooids have been called superficial ooids or pseudo-ooids. ^[1]

Pisoids

Coated grains that have an internal structure similar to that of ooids but that are much larger - that is, greater than 2 mm - are called pisoids (a rock composed of pisoids is a pisolite). ^[1] Pisoids are generally less spherical than ooids and are commonly crenulated. ^[1] Some pisoids may be of algal origin, formed by the trapping and binding activities of blue-green algae (cyanobacteria) in the same way that stromatolites are formed. ^[1]

Oncoids

Spheroidal stromatolites that reach a size exceeding 1 to 2 cm are called oncoids. ^[1]

Role of Microorganisms

Cyanobacteria or other microorganisms may influence the formation of ooids - possibly by trapping carbonate grains on organic films or by mediating carbonate precipitation through removal of CO₂. ^[1] The quantitative importance of organic influences on the formation of ooids is not well understood. ^[1]

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