M.Sc Thesis


M.Sc StudentAvigail Bracha
SubjectThe Structure and Micro-Structure of Biogenic Curved
Single Crystals
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Pokroy Boaz
Full Thesis textFull thesis text - English Version


Abstract

Biomineralization is the study of mineral formation controlled by living organisms. With the aid of proteins, organisms are able to control the structure and shape of the mineral crystals, resulting in mechanically and optically superior materials.

Compound eyes are found today mainly in insects, lending them the familiar

multi-lens alien look. However, similar visual systems have been found to exist up to 250 MA in trilobites, extinct marine arthropods. Some trilobite fossils were found to possess lens structures composed mainly of calcite, where each lens is at least partly composed of a single crystal .

Recently, it has been discovered that an extant species of brittle star (Ophiocoma wendtii, of the phylum Echinodermata) possess a lens structure reminiscent of trilobite eyes .

This research focuses on this very brittle star, whose small lens-shaped crystals are found on their arm plates. These lenses focus light onto nerve bundles, allowing the animal to notice moving forms and differences in light intensity. Even more interesting is the fact that these lenses are superior to man-made lenses; corrected for spherical aberrations, birefringence, and exhibiting superior mechanical properties when compared to geological or synthetic calcite.

It has been widely believed that nearly all skeletal elements of echinoderms

are single crystals . In order to achieve a curved complex structure, devoid of

facets, a special crystallization mechanism is required. In this example, it was

found that the arm plates crystallize from an unordered amorphous phase in a

"mold", taking its shape .

We focused on studying the crystallography of the calcite plates, utilizing

various high resolution electron and x-ray diffraction methods, showing these

plates are indeed single crystals. Although this has been proven previously using x-ray diffraction, never before has it been demonstrated with such detail.

Additionally, we employed state of the art microscopy methods, high resolution diffraction techniques and chemical analysis instruments to understand the micro- and nano-structure of the calcite crystals. The different methods all come together to propose a novel toughening mechanism utilized by this brittle star. What appears at first as a single crystal, the calcite arm plates are actually a composite structure, composed of a calcite matrix and embedded with inclusions. These inclusions are also calcitic, but a large part of their Calcium atoms have been replaced with the smaller Magnesium ions, and by staying crystallographically coherent with the matrix, introduce a significant amount of stress to the entire structure.

This biogenic mechanism is another way in which we can perceive the vast

ingenuity of nature in forming and controlling materials with specific purposes

and exceptional properties.