Allochem

The term Allochem was introduced by sedimentary petrologist Robert L. Folk as part of his petrographic classification of limestones^[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]

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).