Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Biologische Systeme und Medizin Poster: Mi., 14:00–16:30 M-P187
News from the crystallographic structure of sea urchin spines of
Heterocentrotus mammillatus
S. Castorph 1 , R. Hock 2 , M. Baier 2
1 Institute for X-Ray Physics, University of Göttingen, Friedrich-Hund-Platz 1, 37077
Göttingen – 2 Chair for Crystallography and Structural Physics, University of Erlangen-
Nürnberg, Staudtstr. 3, 91058 Erlangen
Sea urchin spines consisting of high magnesian calcite are considered single crystals
with the c-axis of calcite oriented parallel to the long spicule axis. As a biogenic carbonate
mineral with a content of about 0.1 weight% proteins, an abnormal conchoidal
fracture as compared to calcite and their sponge like interior structure, they have been
attracting the interest of crystallographers [1,2,3]. We investigated spines of sea urchin
Heterocentrotus mammillatus by powder diffraction and single crystal methods like
the ordinary Laue technique on a Mo x-ray source and the high energy focussing Laue
technique at energies of up to 65 KeV. For the detection of the single crystal diffraction
patterns an image plate system was used. Chemically, spines were analysed by ICP
Emission Spectroscopy. For powder diffraction measurements material from the outer
dense shell of the spines and the inner sponge like calcitic stucture were prepared. The
Mg content of the two radially separated regions of the spines is 9.4(4) and 7.6(4).
Powder data from the inner part high magnesium calcite shows a superstructure with
a modulation in [104] direction. Diffractograms from the shell material do not show
superstructure reflections. Even so the diffractograms of the inner part material look
at a first glance like those from ordinary high magnesian calcite, Rietveld fits within
the R-3c model and a statistical Ca/Mg distribution are not satisfactory. To explain
the superstructure, possible models for the symmetry of the biogenic calcitic material
will be discussed and tested by Rietveld refinements. Namely a modified “inverse”
µ-model according to Wenk et al. [4] with simultaneous ordering of cations and distortions
in the carbonate groups and a tentative monoclinic variant of high magnesian
calcite crystallised in C2/c with cation ordering in [104] sheets (in original calcite cell
R-3c) together with distortions of the carbonate groups. Laue diffractograms with Mo
radiation confirm the single crystalline nature previously reported, even so the term
single crystal must be questioned in the light of the powder diffraction data. Diffuse
scattering is observed in transmission and the variation of the Laue patterns after soaking
samples in water and drying them at temperatures from 60 ◦ C up to 300 ◦ C were
studied. Laue diffractograms measured with high energy in transmission nondestructively
reveal the mosaicity of the spines as a function of position along the spicule axis.
[1] C.D. West J. Paleontology 11 (1937) 458-459
[2] U. Magdans H. Gies, Eur. J. Mineral. 16 (2004) 261-268
[3] X. Su et al. J. Mat. Science 35 (2000) 5545-5551
[4] H.R. Wenk et al. Phys. Chem. Minerals 17 (1991) 527-539