Microstructural Investigation of Biogenic Carbonates in Mollusc Shells
with Raman Spectroscopy
Annette Dowd1, Elizabeth A. Carter2, Michael B. Cortie1, and Peter A. Lay2
1
Institute for Nanoscale Technology, University of Technology Sydney, Broadway, NSW 2007,
Australia; 2Vibrational Spectroscopy Core Facility, University of Sydney, Sydney, NSW 2006,
Australia
E-mail: annette.dowd@uts.edu.au
Complex composite carbonate biomaterials are ubiquitous in Nature but there are still
many aspects of their formation and function that are poorly understood. While Raman
spectroscopy has been used for some years for the identification of biogenic carbonates [1, 2],
advances in Raman microscopy now make high spatial resolution mapping feasible. The
specimen preparation is more straightforward than that used for SEM, the usual technique
used for biogenic mineral mapping, as the sample needs only to be polished to optical
flatness.
We have used Raman microscopy to analyse the shell microstructure of the Australian
gastropod Ninella torquata. Two polymorphs of crystalline calcium carbonate, aragonite and
calcite, are mapped at high spatial resolution to reveal subtle changes in the mineral gradient
in the shell cross-section. No evidence was found for vaterite. The ordering of the mineral is
also investigated. Within the calcitic outer layer, the calcite at the boundary between calcite
and aragonite is more disordered than calcite near the periostracum, possibly because of
increased Mg2+ substitution. Crystal orientation can also be investigated by comparing the
relative scattering intensities of the lattice modes and we show there is little to no change in
aragonite orientation throughout the nacreous material. An organic matrix is deposited
alternately with crystalline material during shell growth. We map this organic phase which
clearly reveals growth layers 5 – 20µm apart and a lamellar mineral structure that extends
through most of the thickness of the shell except near the nacreous inner surface. The results
can be combined with the output of neutron diffraction studies of the same shell to provide a
detailed picture of the microstructure, crystallographic texture and high fracture toughness of
these mollusc shells.
Fig. 1. (a) Map of fluorescence induced by organic molecules showing lamellar structure, (b)
map of calcite (red) and aragonite (green) showing clear interface, (c) white light image. Scale
bar is 500 µm.
References
[1] J. Urmos, S. K. Sharma, and F. T. Mackenzie, Am. Mineral. 76, 641 (1991).
[2] L. Bergamonti, D. Bersani, S. Mantovan and P. P. Lottici, Eur. J. Mineral. 25, 845
(2013)
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