We've been talking about the same things, but with
conflicting terminology.  Hopefully this clarifies it a bit.
These issues are mainly problems when dealing with
fossils, though all shells are vulnerable to acid.

>>Both calcite and aragonite are easily soluble in acid.<
>Fossil shells are not normally under acid conditions. <

Even very slightly acidic solutions, such as most rainwater
and some groundwater, can react with carbonates such as
calcite and aragonite.  Another situation that is associated
with acidity is anoxia, where hydrogen sulfide and organic
acids produced by decay can dissolve carbonates.  The
cases you mention where the organic part of the shell is
preserved but the shell is gone would fit this situation.

>>Aragonite is also unstable (on a geological time frame)
and gradually changes into calcite,<<
>Not necessarily.<

Aragonite and calcite are different crystal forms of calcium
carbonate.  Under conditions similar to those at the earth's
surface, aragonite is thermodynamically unstable and,
given the opportunity, will change into calcite.  This
destroys the crystal structure of a shell, contributing to its
breaking down.  Fossil shells starting on this process will
be somewhat chalky.  Over a longer term, the remains of
the shell are lost, although a mold, cast, or ghost may be
produced.  However, if the sediment is suitably hardened,
the transformation can take place through recrystalization,
where the crystal structure has changed but the shape of
the shell is retained.  This may even preserve traces of
shell microstructure.  Aragonite is more stable than calcite
at very high pressures (like diamond versus graphite) , so
deep burial may help preserve aragonite longer.  No
original aragonite is known from shells older than
Pennsylvanian, and even mid-Mesozoic aragonite is rare.

>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.<

True; it requires dissolution to actually remove the shells,
but a shell that has been degraded through partial
recrystalization may crumble as soon as it is exposed.


    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
    [log in to unmask]

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