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Thin-section of calcitic ooids from the Carmel Formation, Middle Jurassic, of southern Utah, USA. The largest is 1.2 mm in diameter.

In geology, ooids are small (< 2 mm), spheroidal, "coated" (layered) 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 (the Bahama Platform, 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 meaning they belong to the limestone rock family. Pisoids are similar to ooids, but larger than 2 mm in diameter, often considerably larger, as in 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 (including an aragonite/calcite amalgamation). Most modern ooids are aragonite (a polymorph of calcium carbonate). Some are composed of high-magnesium calcite, or are bimineralic (layers of calcite and layers of aragonite). Ancient ooids are calcite, either originally precipitated as calcite, or formed by alteration (neomorphic replacement) of aragonite ooids (or 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 (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, thusly affecting salinity levels, which in turn facilitate the substitution.

Ooids with radial crystals (such as the aragonitic ooids in the Great Salt Lake (Utah, USA) grow by ions extending the lattices of the radial crystals. The mode of growth of ooids with tangential (usually minute needle-like) crystals is less clear. They may be accumulated in a "snowball" fashion from tiny crystals in the sediment or water, or they may crystallize in place on the ooid surface. A hypothesis of growth by accretion (like a snowball) from the polymineralic sediment of fine aragonite, high-magnesium calcite (HMC) and low-magnesium calcite (LMC), must explain how only aragonite needles are added to the ooid cortex. Both in tangential and in radial ooids, the cortex is composed of many very fine increments of growth. 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.

There are several factors that affect ooid growth: supersaturation of the water with respect to calcium carbonate, the availability of nuclei, agitation of the ooids, a constant location, water depth and the role of microbial organisms.

Kidney stones are similar to ooids in their layered structure; they are sometimes rich in calcium (calcium oxalate) or phosphate (struvite).

• Ooid Formation
• Sculpture Conservation: Ooids and Oolites

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