Hardouinia mortonis "Geode" with internal crystallization

Hardouinia mortonis
Peedee Formation
Maastrichtian Age, Late Cretaceous (~70 Ma)
Holden Beach, Brunswick County, North Carolina, USA

Self collected in September, 2023
 

A bit of an unusual find, this Hardouinia mortonis specimen had a significant amount of crystallization occur within the internal cavity, where the sediment settled and left a pocket. It's not too terribly uncommon to find these specimens with a layer fine crystals in these pockets, and I have found a few broken ones with such layers from the area. However, this one stood out because it was mostly intact, and the crystals began to form in what was likely a small crack, which ended up splitting it open over time as they grew. I have been calling it my "geode" echinoid.

I am not sure what kind of crystals they are. They are usually very small and grey, forming a "film" over the walls of their cavity. I think it's either a type of quartz, or a type of calcium carbonate. The entire test of this one seems to have a slight degree of mineral replacement to it, mostly noticeable on the oral surface (there is some dense crystal filling the peristome and the features surrounding it).