Ooid

Ooids are small (≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas (around the Bahamas, for example, or in the Persian Gulf). After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad (Karlovy Vary) in the Czech Republic.

An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most modern ooids are aragonite, a polymorph of calcium carbonate; some are composed of high-magnesium calcite, and some are bimineralic (layers of calcite and aragonite). Ancient ooids can be calcitic, either originally precipitated as calcite (as in calcite seas), or formed by alteration (neomorphic replacement) of aragonitic ooids (or the aragonite layers in originally bimineralic ooids). 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).

Whether ooids become calcitic or aragonitic can be linked to strontium/calcium substitution within the crystalline structure. 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. Marine calcitic ooids were typically formed during calcite sea intervals, especially during the Ordovician and the Jurassic Periods. The geochemistry of these seas was a function of seafloor spreading and fluctuating Mg/Ca ratios. Low Mg/Ca ratios favor the precipitation of low-magnesium calcite.

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