>I've heard nacreous conchs were a pasting of Aragonit with a proteinic glue.

Pinnids are largely calcitic, as are pinnids, mytilids, ostreids,
anomiids, limids, pectinids, propeamussiids and epitoniids (as well
as brachiopods, echinoderms and crustaceans). How do I know? The
regional limestone which was deposited over most of New Zealand in
the latest Oligocene/earliest Miocene has generally undergone
diagenetic alterations, during which all of the aragonitc specimens
were dissolved; those infilled by phosphates may remain as rare vague
"phantoms" or rarely proper steinkerns. Those shells remaining intact
are all in the families listed above. In only a few localities has
the diagenesis not affected the shells, and there we find rich
assemblages of aragonitic taxa.

I believe calcitic molluscs have a thin aragonitic inductura.

It appears that nacre can be either aragonitic or calcitic. Calcite
is far more stable over time under tough conditions, though in our
Triassic and Jurassic almost everything everywhere was dissolved,
both aragonitic and calcitic, leaving just cavities.
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut