>Both calcite and aragonite are easily soluble in acid.

Fossil shells are not normally under acid conditions. Calcium
carbonate in nature is an extremely long-lived compound when
sheltered, as a fossil within a rock. There are plently of Ordovician
and Cambrian fossils to attest to that! Of course bone (calcium
phosphate) is somewhat more resistant yet...

>Aragonite is also unstable (on a geological time frame) and
>gradually changes into calcite,

Not necessarily. The shells are dissolved slowly by fluids in the
rocks. If the shell is dissolved (and not replaced by some mineral)
after the rock is indurated (hardened) a natural mold results. If
however the shell is dissolved BEFORE induration the shell may be
lost entirely, leaving a "ghost" as the sediment squeezes into the
space formerly occupied by the shell wall, said ghosts ranging from
being be so faint that it is difficult to see to being well-preserved
and identifiable down to species level; it may also have both
external and internal characters impressed on the same surfaces (eg
radial ribs and hingeteeth superimposed in decalcified Latiarca). I
have also seen dissolved shells with the organics still preserved as
a carbon film... black ammonoids, bivalves & gastropods with internal
& external details superimposed.

Sometimes aragonitic shells may be dissolved and the calcium
carbonate recrystallizes as calcite, but such shells have a "sugary"
texture when broken, quite unlike the layers of ordered original
crystals. This can also happen to originally-calcitic shells.

>  which usually involves crumbling away, hence the loss of original
>aragonitic shells from many fossil deposits.

A buried shell doesn't crumble. It may be plastically squashed or
nonplastically fractured as the sediments dewater and compact, but it
won't fall to bits.

Nacreous shells can (rarely) disintegrate eventually when lying on
the sea floor for long enough under the right conditions, as my
undescribed Calliostoma from the NZ fjords attests...


--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
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