# Allochem

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{{Short description|Identifiable grains in carbonate rocks}}
[[File:OoidSurface01.jpg|thumb|right|250px|[Ooids](/source/Ooids) on the surface of a [limestone](/source/limestone); Carmel Formation (Middle [Jurassic](/source/Jurassic)) of southern Utah, USA. Largest is 1.0 mm in diameter.]]

The term '''Allochem''' was introduced  by [sedimentary petrologist](/source/Petrology) [Robert L. Folk](/source/Robert_L._Folk) as part of his [petrographic classification of limestones](/source/Folk_classification)<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](/source/carbonate_rocks). Examples would include [ooids](/source/ooids), [peloids](/source/peloids), [oncolite](/source/oncolite)s, [pellets](/source/Pellets_(petrology)), [fossil](/source/fossil) or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations – for example, if an [aragonite](/source/aragonite) shell were to dissolve and be later replaced by [calcite](/source/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](/source/Matrix_(geology)) of [micrite](/source/micrite) (< 5 μm in [lime mud](/source/Limestone)) or sparry calcite (larger calcite crystals in the size range 20–100 μm).

==References==
{{reflist}}

Category:Sedimentology
Category:Sedimentary rocks
Category:Limestone

{{Sedimentology-stub}}

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