Nora Bryan wrote,
"Off the wall thought - Could the shell be effectively thin-sectioned and
looked at under a polarizing microscope?  If iron was substituted for
calcium in calcium carbonate wouldn't you get siderite (Fe2CO3 ???) or
something between siderite and aragonite. I would think this would have
different optical properties from pure aragonite.  Maybe it's not possible
for an organism to do that though, I don't know."
 
Sure, thin sections of shells are quite useful. Basic sources of
information include:
 
Horowitz, A. S., and Potter, P. E., 1971, Introductory petrography of
fossils: New York, Heidelberg, and Berlin, Springer-Verlag, xiv + 302 p.,
100 pl.
 
Majewske, O. P., 1969, Recognition of invertebrate fossil fragments in
rocks and thin sections: Leiden, E. J. Brill, 101 p., 106 pl.
 
Scholle, P. A., 1978, A color illustrated guide to carbonate rock
constituents, textures, cements, and porosities: Tulsa, Oklahoma, American
Association of Petroleum Geologists, Memoir 27, xiii + 241 p.
 
All are profusely illustrated. Only a small part of each work is on
mollusks. Horowitz and Potter wrote a little poem to start off their book:
 
"Shells to bits,
Bits to dust,
Aragonite to calcite,
The microscope's a must."
 
Why the books' emphasis on fossils? It's because fossils make up limestone,
and a lot of petroleum is found in limestone... but that's another story.
 
Molluscan shells are composed mainly of two different minerals of calcium
carbonate, aragonite and calcite. The atoms are arranged in different, but
regular lattices in each mineral. For the mineral buffs on Conch-L, calcite
is trigonal (i.e., having 3-sided symmetry) and aragonite is orthorhombic
(having the symmetry of a brick). The crystals in molluscan shells are tiny
and do not have neat crystal faces, but they are crystalline nonetheless.
 
Gastropods, chitons, and scaphopods are almost completely made of
aragonite, as are most bivalves. A few groups of bivalves are made almost
exclusively of calcite (oysters, scallops), and some others have layers of
both minerals (e.g., Spondylus, Plicatula). Calcite is stable under
near-surface conditions, but aragonite tends to dissolve or recrystallize
as calcite, which is why there are so many oysters lying around on
Cretaceous outcrops and so few snails.
 
Now, according to Deer, Howie, and Zussman (or, as we called them in one
petrography class, "Here, Zowie, and Dustman"), iron (Fe++) will substitute
for calcium (Ca++) in calcite, but it's not very common, and the amount of
iron is limited to about 10 mol. per cent. Likewise, in natural iron
carbonate (siderite), calcium can substitute for iron, but only about 10 to
15 percent. The Fe++ and Ca++ ions are not of the same size, so there is
evidently some difficulty in having them substitute freely for one another.
 
D H & Z say that "Most aragonites are relatively pure and conform to the
ideal formula". Iron does not fit well into the aragonite crystal lattice.
 
As to different optical properties, siderite and calcite are virtually
identical in thin sections if they are fresh, and must be stained to be
told apart accurately. Weathered siderite reveals itself as other iron
minerals form in the fractures and on the edges, generally turning the
crystals brown.
 
Well, that's it for tonight. You Conchlers are really making me work!
 
Andrew K. Rindsberg
Geological Survey of Alabama
 
Additional reference
 
Deer, W. A., Howie, R. A., and Zussman, J., 1966, An introduction to the
rock-forming minerals: London, Longman, xi + 528 p.