Orientation patterns of aragonitic crossed-lamellar, fibrous prismatic and myostracal microstructures of modern Glycymeris shells.

The shells of the bivalves Glycymeris glycymeris and Glycymeris nummaria are widely used for environmental studies. They consist of aragonite and comprise four different microstructures and textures from outer to inner shell surfaces: crossed-lamellar, myostracal, complex crossed-lamellar and fibrous prismatic. We characterize with SEM, EBSD, laser-confocal microscopy and AFM imaging mineral unit size, morphology and orientation of crystallites in the different microstructural arrangements and at the transition from one microstructure to the other. We also characterize the microstructure and texture of adductor and pedal retractor myostraca and address structural characteristics at the transition from crossed-lamellar to myostracal assemblies. We find that the crossed-lamellar layer has a three-dimensional crystallographic orientational order. Each set of first-order lamellae consists of twinned aragonite; the two sets of first-order lamellae are misoriented to each other by about 30 to 40° while retaining an approximately parallel a-axis; they do not show any particular twin relationship. Myostracal aragonite grows homoepitactically onto the crossed-lamellar aragonite, but is clearly a separate microstructure, with its own crystallite size and morphology. Within adductor and pedal myostraca, prisms increase in size towards inner surfaces. In contrast to the other shell layers, the myostraca form through competitive growth. The complex crossed-lamellar aragonite initially inherits the three-dimensional texture of the crossed-lamellar microstructure, but with growth develops an axial texture, which is transmitted to the underlying fibrous prismatic microstructure. With this work we provide a modern, unaltered, reference for fossil Glycymeris shells to be used for detection of diagenetic overprint in fossil Glycymeris analogs.