Oolite

Oolite, rock, usually limestone, made up of small egg-shaped particles of calcium carbonate, a chalky mineral. The rock is sometimes referred to as oolith and the individual particles can be called ooliths, ooids, or oolites. Individual grains vary in size from 0.25 to 2.0 mm (0.01 to 0.08 in) in diameter, but are usually 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter.

An ooid is made up of concentric layers of calcium carbonate or radiating needle-shaped crystals. Scientists have found ooids in rocks dating from the Cretaceous Period that are about 100 million years old. Today ooids are formed in warm, shallow, very salty waters like the Bahamas, the Persian Gulf, and the Great Salt Lake in Utah. Geologists think ooids form when a nucleus of quartz or carbonate is rolled by the wave action of the water, allowing layers of carbonate or crystals to be deposited around the nucleus.

Even though the ooids in rocks are usually made of a type of calcium carbonate called calcite, the ooids forming today are mostly another kind of calcium carbonate called aragonite. Scientists think this is because conditions have changed through geological time. In the Cretaceous period, when the ooids in rocks were being formed, sea level was about 300 m (984 ft) higher than it is today, the average temperature was about 10°C (18°F) higher, and there was more carbon dioxide in the air and water than there is today. Those conditions caused more calcite to be deposited, while conditions today favor aragonite precipitation.