> I know of a few incidents of this from the conservation literature, but
> I would like to get a picture of just how widespread this kind of
> deterioration really is. Any reports or stories will be highly
> appreciated.
The best summary I know of is :

Shelton, S. (1996) : 'The shell game : mollusks (sic) shell deterioration
in collections and its prevention'. Festivus vol. 28 (7) : 74-80.

This gives a good overview of the research to that date, and lists the
literature cited.