Mystery white mineral fossil preservation

[ScottBlooded]

I opened this rock up the other day to expose this really cool plate of orthocone nautiloids (among a bunch of other stuff). Naturally, the first thing that jumped out was the very white, almost feathery looking preservation inside the one nautiloid. I’ve worked in this formation a lot (Needmore, in WV) and I only ever see this very white, very fragile material inside of fossils. So I suppose my question is, is there any way to know what that material is? Why it forms? The other specimens I’ve collected with it are likewise nautiloids or in a few cases goniatites. Always on the inside of the shell, never a replacement for the shell itself.

When I found this one today though it did put me in mind of this other find I had last year that I couldn’t identify (pictures also included). Likewise had a portion of a 3 dimensional interior replaced by this fragile white mineral. Submitted this one to the forum already and the only responses I got were that it’s a concretion. Being an amateur, I’m always very ready to accept any professional opinion on here. This was the only case where I’ve been hesitant. Having worked in this formation a lot, I really don’t find concretions like that. Anything I’ve ever found that has been made up of these materials has been a fossil of some sort.

So anyway! Main question is in regards to the white mineral and what it might be, at least in regards to the nautiloid. And also, if someone wants to come along and magically vindicate me on that second specimen, it wouldn’t hurt my feelings.

[Lone Hunter]

Amateur guess, Aragonite?

[Rockwood]

47 minutes ago, Lone Hunter said:

Amateur guess, Aragonite?

Another. Would most likely recrystallize as calcite in that time.

The feathery material is cameral deposit that was deposited in life by the animal. It may be recrystallized. The other mineral must be primarily replacement. It likely will weather to a rust color from the iron content present.

[Ludwigia]

Calcite and aragonite are possibilities, but other minerals like gypsum or baryt could also come into question. For a visual diagnosis, one would need to be able to distinguish form and habit of the mineral crystals, but this does not seem to be possible here. Hardness and streak tests are probably also practically impossible. It would help if you could study the mineral paragenesis involved in this Formation at this site to at least find out which minerals have been involved. It is probably a pseudomorphosis which occurred at a later time, perhaps even more recently.