The structure of sea urchin spines, large biogenic single crystals of calcite

Sea urchin spines are porous, single crystal Mg-rich calcite [(Mg, Ca)(CO3)] with a three dimensional meshwork architecture. The crystallographic orientation of large (∼10 cm in length) spines from Heterocentrotus trigonarius was determined using the X-ray back-reflection Laue technique; the long axis of all spines is parallel to the 〈00·1〉 direction of calcite. The internal structure was studied using SEM and TEM. The macropore structure dominated the SEM images, whereas many small (∼80 nm) protein occlusions can be observed within the single crystal spines using TEM. The spines appeared crystallographically perfect in the TEM. The bend strengths of a group of spines were also determined and varied between ∼13 and 41 MPa.