The Post Up a Rock on Tuesday thread.

Oolite is a sedimentary rock made up of ooids (ooliths) that are cemented together. Most oolites are limestones — ooids are made of calcium carbonate (minerals aragonite or calcite). Ooids are spheroidal grains with a nucleus and mineral cortex accreted around it which increases in sphericity with distance from the nucleus. Nucleus is usually either mineral grain or biogenic fragment. The term “ooid” is applied to grains less than 2 mm in diameter. Larger grains with similar genesis are pisoids (pisoliths). Rocks made up of pisoids is pisolite.

The terms “oolite” and “ooid” are derived from the Greek word for fish roe (oon) which ooids resemble.

Ooids usually possess a clearly developed growth banding. Ooids may be spherical but some are elongated, depending on the shape of nucleus. Most ooids are marine, forming in shallow (less than 10 m, preferably even less than 2 meters), warm, and wave-agitated water such as the Persian Gulf and the Bahama Platform. Ooids in these places form a distinct type of sand — ooid sand. Ooids are kept moving by waves which enables accretion to occur on all sides. This is also the reason why ooids are so well-polished. Warm water is needed to lower the carbon dioxide content in water (higher temperature reduces the ability of water to keep gases dissolved) and thereby enhance the precipitation of calcium carbonate. It is believed that ooid formation is generally abiogenic process. However, the exact formation mechanisms are still unresolved.

Most modern ooids are composed of mineral aragonite. Some ooids form in non-marine environments, the Great Salt Lake is probably the best known example of ooid formation in saline lake. Some ooids form in fresh-water lakes, caves, caliche soils, hot springs, and rivers. Even ooids made of evaporite minerals gypsum and halite have been reported1.

Some ooids are made of silica (chert), dolomite or fine-grained phosphatic material (collophane). Such ooids are formed by replacement of original calcium carbonate, but they may be also primary. Especially phosphatic and iron-bearing ooids, composed of hematite and goethite seem to have been formed as such.

commonly thin tabular, square, with flat or rounded vicinal faces, may be elongated, or pyramidal, with the pyramid truncating or replacing; more rarely pseudo-octahedral; and very rarely either cubic or short prismatic pyramidal. Commonly exhibits additional forms, some exhibiting pyramidal hemihedrism.