>> Most shells are aragonitic, with alternating rows of interlocking,
>> elongate prisms of aragonite.  Nacre (mother-of-pearl) consists of
>> flat plates of calcite.  Both are calcium carbonate, but with
>> different crystal forms, and different reaction rates chemically.
>> There are also less common microstructures, like in muscle scars.

Actually, true nacre (mother of pearl) is always aragonite, as are
muscle attachment areas.  Crossed lamellar structure, typical of most
mollusks, is generally aragonite.  Oysters, scallops, wentletraps,
nerites, and some fossil squid-like animals (belemnites) are among the
mollusks with large components of calcite in their shells.

The flat plate-like structure is what makes nacre irridescent.  Like a
bit of oil on water, the colors result from thin layers reflecting
light.

----------------------------------------
Dr. David Campbell
425 Scientific Collections
University of Alabama, Box 870345
Tuscaloosa AL 35487
"James gave the huffle of a snail in
danger But no one heard him at all"  A.
A. Milne

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