Microstructure and crystallography of abalone shells

Guo, Du-Jiao (2011) Microstructure and crystallography of abalone shells. MSc(R) thesis, University of Glasgow.

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Biominerals are biogenic mineralized products comprising both mineral and organic components. Calcium-bearing minerals comprise about 50% of biominerals with the most common being polymorphs of calcium carbonate, e.g. calcite and aragonite.
Abalone (Haliotis asinina Linnaeus, 1758; Haliotis rufescens Swainson, 1822 and Haliotis gigantea Gmelin, 1791) are marine snails, belonging to the Phylum Mollusca, class Gastropoda, family Haliotidae, genus Haliotis. They can be easily recognized by the row of apertures, which are closed, half-closed and open ones, along the spiral ridges and towards the shell longitudinal growth direction of the dorsal margin.
This study considers the microstructure and crystallography of three species of abalone shells, Haliotis asinina from Australia, Haliotis rufescens from America and Haliotis gigantea from Japan; as well as the aperture infill of H. asinina. The microstructure and crystallography are analysed through Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) respectively.
Abalone shells all have an outer prismatic and an inner nacreous layer. It is confirmed that the shell of H. asinina is comprised of aragonite in both prismatic and nacreous layers, with c-axis orientation of crystals throughout the prismatic and nacreous layers of shell. Towards the prismatic-nacreous interface, crystallographic alignment of prismatic layer becomes much more tightly constrained. In nacre, crystallographic continuity persists across several laminae. Along the longitudinal shell growth direction (from posterior part to anterior), there is a trend of gradually thickening tablets towards the shell interior. In addition to this trend from exterior to interior, the tablets formed at the anterior are thicker than those formed earlier at the posterior. It is also confirmed that the nacreous layer in H. rufescens and H. gigantea is aragonite. The prismatic layer of H. rufescens shell is composed of calcite, the c-axis of which is normal to the shell surface. The prismatic layer of H. gigantea shell consists of calcite and aragonite, the c-axis of aragonite is normal to the shell surface and that of calcite is parallel to the shell surface. In these three species, there is higher crystallographic constraint in the nacreous layer than in the prisms. Furthermore, the crystallographic continuity of the nacreous layer persists across as many as 40~50 laminae.
H. asinina is about 800 µm thick, H. rufescens is the thinnest at around 600~700 µm and H. gigantea shell is the thickest at about 1 mm. Comparing the three species, the prismatic layer to the shell thickness of three species, the calcite prismatic layer (H. rufescens) takes up about 50% of the shell thickness compared with aragonite prismatic layer (H. asinina) of about 20~30%, and the prismatic layer with calcite and aragonite (H. gigantea) is between the two at over 30% of total shell thickness.
Aperture infill of H. asinina is also composed of aragonite with both prismatic and nacreous layers. The crystallographic orientation of infill prismatic layer is parallel to the shell surface unlike that of the shell where the crystallographic orientation of the prismatic layer is perpendicular to the shell surface. There is a prism-like layer between aperture infill and the shell. The contact with the shell always occurs within the nacreous shell layer. Aperture infill grows on this prism-like layer and the growth rate of aperture infill is in- keeping with that of the shell growth rate. When individuals reach six months and older, shell growth and aperture infill occur at approximately the same speed.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Keywords: abalone shell, microstructure, crystallography, aperture infill, SEM, EBSD
Subjects: Q Science > QD Chemistry
G Geography. Anthropology. Recreation > G Geography (General)
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Cusack, Professor Maggie and Kamenos, Dr. Nick
Date of Award: 2011
Depositing User: Ms DUJIAO GUO
Unique ID: glathesis:2011-2401
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 01 Mar 2011
Last Modified: 10 Dec 2012 13:54
URI: https://theses.gla.ac.uk/id/eprint/2401

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