Preferred orientation of hydroxylapatite in the scales and fins of seawater Lateolabrax japonicus determined using XRD pole figures and ODF

Pole figures measured using X-ray diffractometry were used to analyze the orientation of crystalline hydroxylapatite (HAP) in the scales and fins of seawater Lateolabrax japonicus. An orientation distribution function (ODF) was calculated based on the pole figures of the lattice planes (002),(130), and (211) of HAP in the scales and those of (031),(120), and (132) in the fins. The pole figures indicate that the c-axis of HAP prefers three primary orientations in the scales and five primary orientations in the fins. In the scales, the preferred orientations of the HAP c-axis are parallel to the normal line of the scale face and intersect the normal line at about 39° and 63°. In the fins, the HAP c-axis intersects the normal line of the cross-section at 3°, 9°, 17°, 24°, and 36°. However, the calculated ODFs show that the preferred orientations of the HAP c-axes are nearly parallel to the face of the scales and perpendicular to the cross-section of the fins. As the pole figure only reflects a two-dimensional projection of the crystalline orientations, the preferred orientations determined by the pole figures are not very accurate. Conversely, three-dimensional ODF is more suitable for analyzing the preferred orientation of biominerals than the two-dimensional projection. The tendency of the c-axis to be parallel to the collagen fibers is controlled by organic matter and provides a good mechanical performance in hard tissue.