{{Short description|Identifiable grains in carbonate rocks}} [[File:OoidSurface01.jpg|thumb|right|250px|Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Largest is 1.0 mm in diameter.]]
The term '''Allochem''' was introduced by sedimentary petrologist Robert L. Folk as part of his petrographic classification of limestones<ref name="Folk1959a">Folk, R. L. (1959) ''Practical petrographic classification of limestones.'' American Association of Petroleum Geologists Bulletin. 43, pp. 1–38.</ref> to describe the recognizable "grains" in carbonate rocks. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations – for example, if an aragonite shell were to dissolve and be later replaced by calcite, the replacement would still be deemed an allochem.<ref name=" ScholleOther2003a">Scholle, P. A., and D. S. Ulmer-Scholle (2003) ''A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis''. American Association of Petroleum Geologists Memoir no. 77. Tulsa, Oklahoma, American Association of Petroleum Geologists. 474 pp. {{ISBN|0-89181-358-6}}</ref>
The allochems are typically embedded in a matrix of micrite (< 5 μm in lime mud) or sparry calcite (larger calcite crystals in the size range 20–100 μm).
==References== {{reflist}}
Category:Sedimentology Category:Sedimentary rocks Category:Limestone
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