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 (1959) 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, 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.
The allochems are typically embedded in a matrix of micrite (lime mud) or sparry calcite.
References
Folk, R.L., 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.
