Calcium Carbonate Concretions: Ooids, Oolite, and Pearls
Image: surface of an ooidic limestone from Gunlock, Utah. From wikimedia commons, in the public domain.
Ooids are spheroidal grains less than 2mm in diameter displaying growth banding around a central nucleus or “seed”. They usually form in warm, shallow marine water, where the agitation of waves allows accretion around the nucleus on all sides. Warm water helps enhance the precipitation of calcium carbonate, which is the primary component of most ooids, usually in the form of aragonite. Larger grains formed similarly are known as pisoids. Ooids may also be composed of phosphate, chert, dolomite, or iron minerals such as hematite or goethite.
Oolite is a sedimentary rock formed from ooids, spherical grains, cemented together. Oolitic limestone is composed of calcite or aragonite ooids with calcite acting as the cement.
Aragonite ooids may sometimes resemble pearls, which are also calcium carbonate (either aragonite or a mixture of aragonite and calcite) deposited concentrically around a central “seed”. Ooids are not iridescent, but then neither are most pearls: almost all shelled molluscs are capable of forming pearls, but only those produced by specific oysters and mussels are the perfectly round, iridescent, gem-quality concretions that is most commonly associated with the word “pearl”.