https://en.m.wikipedia.org/wiki/Scaly-foot_gastropod
Gastropod that incorporates greigite, pyrite, and graphite on it's shell and foot.
Due to the large quantities of these compounds in dissolved form surround the hydrothermal vents.
Speculation for purpose: the shell is extremely resilient, the metal does improve this greatly. Though whether evolution deemed this adaptation necessary because of an abundance of strong predators, or as a means of detoxification of the injested compounds, is unclear.
The three populations of these snails have varied compositions, one which even being magnetic, due to the different compounds produced by the vents.
Appologies, here is non wiki http://www.esa.org/esablog/research/iron-plated-snail/