The shells of the mollusks are built, in general, according to one scheme.
Initially the shell consists of three layers:
the top layer consists of protein
the two lower layers consist of calcium carbonate (CaCO3).
Calcium carbonate in the shell of mollusks can be in the form of three modifications:
Aragonite- characteristic of the most ancient mollusks; The pearlescent layer always consists only of aragonite.
Calcite- apparently, this modification is a later acquisition of mollusks.
Veterite- used for repair.
It is hypothesized that a huge amount of carbon is bound and stored on the ground by such a method. And, as necessary, it is released into the atmosphere as a result of heating from volcanic activity.
up! 
up! 