>Relatively recent fossils, in which the original calcium carbonate ground
>substance is still present....  Older fossils, in which the
>original ground substance has been replaced by harder, non-carbonate
>minerals, (especially non-calcific non-carbonate minerals), would probably
>not be susceptible to this kind of chemical decomposition.

Age of a fossil has nothing to do with replacement by other minerals,
actually; it's the burial history ie depth of burial, with the
resultant temperature & pressure regime, which allow groundwater to
replace elements in a fossil. The simplest case is mere
recrystallization... the calcium carbonate is dissolved and then
replaced with calcite; when these break the fracture is sugary in
texture. Silica can replace carbonates; this is acidproof if the
replacement is complete; too bad if the matrix is also siliceous or
zeolite-cemented (zeolites are hydrated aluminosilicates, nasty stuff
in fossils). Here in NZ our Triassic-Jurassic fossils are generally
utterly dissolvced, with no replacement, so the fossils are empty
molds with steinkerns (if fossils were hollow); these are good for
taking silicones to convert to polyester replicas. Many are also
replaced by chlorite (YUK) or zeolites (double yuk). Zeolitized ones
are virtually unpreparable if the matrix is siliceous, as acid will
remove neither matrix nor infilling, and mechanical prep doesn't work
at all well on these, as the zeolites adhere to the matrix to grim
death.

>If you use a very strong acid (like 100% muriatic
>acid), and a substantial piece of shell, the bubble production may be so
>violent as to cause the acid solution to splash out of the container, so
>that is not recommended.

"Muriatic" is a neophyte term for hydrochloric, HCl. Beware of ANY
concentrated acids; acetic is NOT merely vinegar, and is as dangerous
as any other normal acid. Concentrated acetic freezes at 18ºC, which
is a pain if you need to mix a solution up to work on fossils...
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut