Pyrite and marcasite are very common sulfide minerals that are found in chalks, limestones, and shales.  Although pyrite and marcasite have the same chemical composition (FeS2) , they have different crystal structures.  Marcasite tends to weather more readily in the presence of air and water than pyrite, and fossils preserved in marcasite commonly disintegrate as they sit in your display case.

For more information go to the Mineralogy Database Website → Pyrite Page

Basal Austin Group, Atco Formation:  collected by Keith Minor from Williamson County, TX.

Fredericksburg Group, Kiamichi Formation:  collected by Keith Minor from Bosque County, TX.

Basal Austin Group, Atco Formation:  collected by Keith Minor from Dallas County, TX.

Upper Eagle Ford Group, South Bosque Formation:  collected by Keith Minor from Travis County, TX.