|Ph.D Student||Amitai Lee|
|Subject||The Structure of Biogenic Vaterite: The Least Stable|
Crystalline Polymorph of Calcium Carbonate
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Emil Zolotoyabko|
|Professor Boaz Pokroy|
|Full Thesis text|
The calcium carbonate-based mineral system is among the most abundant on the Earth and includes minerals of both geological and biogenic origin. The anhydrous forms of calcium carbonate comprise amorphous calcium carbonate (ACC) and three crystalline polymorphs (in order of decreasing thermodynamic stability under normal conditions), viz. rhombohedral calcite, orthorhombic aragonite, and vaterite. The latter, due to its lower stability, is scarce in nature; nevertheless, vaterite has been found in Portland cement, during oil field drilling, in gallstones, in skeletons of some marine organisms, and even in meteorites.
On the other hand, namely because of lesser stability, vaterite is rather common in synthetic crystallization pathways, serving as a transient phase for end products, i.e. more stable polymorphs as aragonite and calcite. According to the Ostwald rule, vaterite may play an important role in biomineralization of calcium carbonate, which at its initial stage often proceeds via an amorphous precursor.
Despite the cardinal importance to crystal nucleation and growth, the atomic structure of vaterite still remains controversial after nearly 100 years of research. Even lattice symmetry of vaterite is the subject of a long-term dispute; numerous characterization methods and first principle calculations suggest a wide spectrum of symmetries from hexagonal to orthorhombic and even monoclinic and triclinic ones.
Trying to solve this problem, we carried out detailed investigations on biogenic vaterite crystals extracted from the body spicules of the solitary ascidian Herdmania momus. As compared to synthetic vaterite crystals, single crystals of this biogenic vaterite are much larger in size, more perfect and most importantly, facilitated the preparation of TEM samples cut along the desired crystallographic directions. The ultrastructure of the spicules was studied by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and X-ray nano-tomography at synchrotron beam lines, whereas aberration-corrected high-resolution transmission electron microscopy (HRTEM) and high-resolution synchrotron powder diffraction were applied for atomic structure analysis.
Studying the structure of these spicules, revealed a complicated architecture: an elongated cylinder-like polycrystalline main body with micron-sized, hexagonally faceted, single crystal thorns on its circumference. The thorns are organized in pseudo-spirals order and tilted from the main body at an angle of about 26°. The entire main body of the spicule grows along the  direction of vaterite while the individual thorns grow along the  direction.
When solving electron diffraction patterns collected from different zone axes, we found that no single structural model can account for all the observed diffraction spots. This implies that vaterite may actually be composed of different crystallographic structures that coexist within a pseudo-single crystal. As is shown by HRTEM phase contrast imaging, the predominant structure is a hexagonal one, within which nano-domains of another structure are embedded. This finding can explain the discrepancies between the experimental results of different research groups over the last decades due to potential local variations in relative amounts of the coexisting structures.