Oolite

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.

Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, chert, dolomite or iron minerals, including hematite. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone.

Oolitic hematite occurs at Red Mountain near Birmingham, Alabama, along with oolitic limestone. They are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments, though some are formed in inland lakes.

The mechanism of formation starts with a small fragment of sediment acting as a 'seed', e.g. a piece of a shell. Strong intertidal currents wash the 'seeds' around on the seabed, where they accumulate layers of chemically precipitated calcite from the supersaturated water. The oolites are commonly found in large current bedding structures that resemble sanddunes. The size of the oolite reflects the time they have had exposed to the water before they were covered with later sediment.

Ollite sample. From sandatlas.org

Osmington Oolite Formation, Corallian Group, Osmington Mills, southern England. From Ian West.

Bibliography

• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• M. J. Hibbard (1994): Petrography to Petrogenesis. Prentice Hall editore

Photo
Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)
Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)
Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)
Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)
Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)
Ooids in a Oolitic Grainstone, with sparry calcite matrix. XPL image, 2x (Field of view = 7mm)	Ooids in a Oolitic Grainstone, with sparry calcite matrix. PPL image, 2x (Field of view = 7mm)	Bioclastic fragmant within an Ooid. PPL image, 10x (Field of view = 2mm)
Bioclastic fragmant within an Ooid. XPL image, 10x (Field of view = 2mm)	Bioclastic fragmant within an Ooid. PPL image, 10x (Field of view = 2mm)	Bioclastic fragmant within an Ooid. PPL image, 10x (Field of view = 2mm)
Bioclastic fragmant within an Ooid. PPL image, 10x (Field of view = 2mm)	Bioclastic fragmant within an Ooid. XPL image, 10x (Field of view = 2mm)	Bioclastic fragmant within an Ooid. PPL image, 10x (Field of view = 2mm)
Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)	Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)	Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)
Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)	Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)	Ooid with radial-fibrous structure. PPL image, 10x (Field of view = 2mm)
Pressure solution surface. PPL image, 10x (Field of view = 2mm)	Pressure solution surface. PPL image, 10x (Field of view = 2mm)	Pressure solution surface. PPL image, 10x (Field of view = 2mm)