Mollusks: (gastropods, pelecypods,
cephalopods)
Composed initially of aragonite, allochems of the major mollusk
groups occur as open molds (secondary pores) or molds filled with calcite
spar (replacements) in most rocks older than the Pleistocene. Rare
preservation of aragonite is known, even from Paleozoic rocks, but the majority
of these allochems are vastly modified by diagenesis. Allochems from these
groups are especially difficult to identify when broken into small bits.
Mollusks: (rudists, oysters)
The extinct rudists
and extant oysters are an exception to the general rule that mollusk shells are
made of aragonite. These groups secrete calcite fibers that are generally
arranged parallel to the shell wall, as in brachiopods. Additional complexity in
the rudists however comes in the form of porous layers and also interlaminations
of aragonite that, in its usual fashion, can be replaced by sparry calcite.
Discrimination from brachiopods is not generally a problem because calcitic
mollusks are a feature of Mesozoic, Cenozoic and Quaternary rocks in which
brachiopods play a diminished role. These mollusks are also characterized by a
greater size and shell thicknesses that generally exceed those of
brachiopods.