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Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah, USA. Largest is 1.0 mm in diameter.

Allochem is a term introduced by Folk[1] to describe the recognisable 'grains' in carbonate rocks. Any fragment from around ½mm upwards in size may be considered an allochem. 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.[2]

The allochems are typically embedded in a matrix of micrite (lime mud) or sparry calcite.


  1. ^ Folk, R.L. (1959) Practical petrographic classification of limestones. American Association of Petroleum Geologists Bulletin. 43, pp. 1-38.
  2. ^ 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