Atomic structure and ultrastructure of the Murex troscheli shell.
The structure of the gastropod Murex troscheli shell was investigated by using high-resolution scanning and transmission electron microscopies and high-resolution synchrotron X-ray powder diffraction. Rietveld refinement of X-ray diffraction patterns revealed anisotropic distortions of the orthorhombic aragonite unit cell, reaching a maximum of about 0.2% (along the c-direction) with respect to geological aragonite. Lattice distortions are effectively relieved under mild annealing at temperatures between 150 °C and 250 °C. This relaxation correlated well with the loss of organic substance during annealing, measured by thermal gravimetric analysis. The shell ultrastructure consists of three orders of hierarchy, which is typical for crossed-lamellar layers. The third-order fiber-like lamellae, nearly 50 × 250 nm² in cross-section, are heavily twinned, producing specific contrast features in transmission electron microscopy micrographs. So, higher orders of hierarchy are, in fact, twins, which boundaries are free of organic matter.