Sandberg, Philip A. Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois.
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A deposit containing spheroidal grains with a mineral cortex, most commonly calcite or aragonite, accreted around a nucleus formed primarily of shell fragments or quartz grains. The term ooid is applied to grains less than 0.08 in. (2 mm) in diameter, and the term pisoid to those greater than 0.08 in. (2 mm). Accretionary layering (growth banding) is usually developed clearly. A flattened or elongate shape may occur if the nucleus shows that form. Ooids formed on nuclei of shells and shell fragments and composed of fine, radial calcite are shown in illus. a. These ooids are cemented by coarse, clear calcite. Growth banding is visible in most ooids. The pisoids shown in illus. b are composed of many thin layers of very small, tangential (lighter layers) and radial (darker layers) aragonite crystals. These pisoids are cemented with fibrous aragonite.
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