Mollusc shell microstructures and crystallographic textures

X-ray diffraction is used to characterise textures of the aragonite layers of shells from monoplacophoras, bivalves, cephalopods and gastropods. Textures vary in strength, pattern and through the thickness of the shells. The texture patterns exhibited in the studied taxa, which can be quantitatively described by a limited number of parameters, are compared with the microstructure types observed with scanning electron microscopy. Whereas for simple crystallite arrangements, such as nacres, there is a good correspondence between texture and microstructure, this is often not the case in more complex microstructures such as in crossed lamellar layers. Morphologically similar microstructures may have different crystallographic textures, and the same textures may be found in microstructures with different morphology. These two kinds of measurements are shown to be complementary since they provide non-redundant information for many taxa, which suggests that they may be valuable phylogenetic indicators.