Ooid

Ooids (/ˈoʊˌɔɪd/,[1] from Ancient Greek ᾠόν (ōión) 'egg stone')[2] are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals.

After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite.

Moldic ooids (or molds later filled in by calcite cement) occur in both young and ancient rocks, indicating the removal of a soluble polymorph (usually aragonite).

This has been shown in some examples to be due to temperature fluctuations in marine environments, which affects salinity levels, which in turn facilitate the substitution.

Some modern (and ancient) ooids partially or totally lack clear layering and have a micritic (very fine grained) texture.

Examination of such micritic ooids by scanning electron microscopy often shows evidence of microbial borings later filled by fine cement.

Ooimmuration: Enhanced fossil preservation by ooids, with examples from the Middle Jurassic of southwestern Utah, USA.

Modern ooids from a beach on Joulter Cays , The Bahamas.
Ooids on the surface of limestone; Carmel Formation (Middle Jurassic ) of southern Utah, USA.
A thin slice of calcitic ooids from the Carmel Formation, Middle Jurassic , of southern Utah, USA.
Fossils ooimmured in ooids from the Carmel Formation (Middle Jurassic of southern Utah).