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.