Pyritized Brachiopod fossil, near Toledo, Ohio

Description
Good example of the delicacy of replacement of original textures by secondary mineralization. After the limestone was buried, but probably before lithification, ground water containing iron sulfide (in solution) progressively replaced the original shell material with fool’s gold (pyrite).
Whereabouts

Back in 1967 I collected this fossil in a vast limestone quarry in Sylvania, Ohio, just outside of Toledo.  I was a graduate student at the time at The University of Michigan, which is just up the road.  That same afternoon I collected a beautiful trilobite (Phacops).  I had been ‘working’ a productive layer with a knife blade, and suddenly spotted the trilobite, and its dark brown-black calcium phosphate shell in stark contrast to the light grey host limestone.  When it came out in one piece I gave a shout that could be heard in Ann Arbor.  Years later our front door slammed hard, and knocked a bunch of minerals and fossils off nearby glass shelves; the trilobite was trashed during that event.

Age & Formation
Silurian fossil collected in limestone, ~430 Ma