Allochem

Allochem is a term introduced by Folk^[1] to describe the recognisable "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.^[2]

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

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