Microstructure and thermal change of texture of calcite crystals in ostrich eggshell Struthio camelus

Eggshell from ostrich Struthio camelus, pristine and thermally treated in the range from room temperature to 550 °C, was investigated with low vacuum scanning electron microscopy (LVSEM), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and X-ray powder diffractometry (XRD). Different zones of the eggshell were analysed, including the protein-related, non-crystalline, inner organic membrane. After the high-temperature treatment (>500 °C), only crystallised calcite phase was found showing two main textures depending on the shell zone and the treatment temperature. In the crystal layer of the untreated samples, nanosized calcite crystals are organized with their c crystallographic axes highly aligned normal to eggshell surface (a very sharp gaussian angular distribution, σ=0.14, was obtained by using the Rietveld method to model the preferred orientation function in the X-ray powder diffraction pattern). Elemental analysis revealed more Mg2+ in the crystal layer than in cone layer of the eggshell. A high nitrogen content in the organic membrane is associated to a proteinaceous phase. The cone and palisade layers are composed of needle-shaped calcite crystals, which are more crystallized than in the crystal layer and in average with their c crystallographic axes oriented in all directions except for the one perpendicular to the eggshell surface. Due to the complex structure and the amorphous/crystal phase interactions, the heating at about 500 °C texturizes the crystals orienting them mainly along the c-axes normal to the inner eggshell surface.