Pyrite in quartz 12,15 ct

In this sample the “pyrite” discs result green.

This is not a secondary transformation of the pyrite but a primary deposition of iron oxides in some area of the deposit.

This is due to the precipitation of green rust, a group of iron hydroxides probably due to bacterial biomineralization.

More info on this topic in the new book by Marco Campos Venuti, “Biominerals“.