ABSTRACT SUBMISSION FORM
CRYSTAL 24 - MARYSVILLE 2005
24th Biennial Conference of the Society of Crystallographers In Australia and New Zealand
ABN: 13 007 652 018
Submission deadline for abstracts is 14th Jan 2005. Abstracts should be submitted,
preferably via email, to Maureen Rendall (Maureen.Rendall@csiro.au ).
Postal address: CMIT, Private Bag 33, Clayton, VIC 3169, Australia.
Abstracts should be formatted according to the guidelines given in the example abstract
appended to this form.
Presentation type (preferred): Oral
Title: On calcite twinning and superstructures in sea urchin tests ............................
Name of presenting Author:Ann-Kristin Larsson ..................................................
Affiliation:Applied Maths, RSPhysSE, ANU ...........................................................
Address: 0200 ACT ..................................................................................................
...................................................................................................................................
Email/Phone/Fax:ankie.larsson@anu.edu.au,
6125 4692 ................................
Please indicate which of the following areas best describe the general subject
matter of your abstract (tick more than one box if appropriate):
‫ ڤ‬Biotechnology
‫ ڤ‬Structural Biology
‫ ڤ‬Synchrotron/Neutron-beams in Crystallography
‫ ڤ‬Technical Advances & Instrumentation
‫ ڤ‬Intermolecular Interactions
‫ڤ‬X Disordered structures / Modulated Structures / Quasicrystals
‫ ڤ‬Supermolecular / Crystal Engineering
‫ ڤ‬Materials Science
‫ ڤ‬Small Molecule Crystallography
X Mineral Structures /Mineralogy
‫ ڤ‬Industrial Applications of Crystallography
‫ ڤ‬Powder Diffraction studies
‫ ڤ‬Other (please specify):...........................................................................................
...................................................................................................................................
ON CALCITE TWINNING AND SUPERSTRUCTURES IN SEA URCHIN TESTS
Ann-Kristin Larsson1 and Andrew G. Christy2
1
Applied Mathematics, RSPhysSE, 2Department of Earth and Marine Sciences,
Australian National University, Canberra, ACT 0200, Australia
Despite the tremendous interest in both mineral and biotic calcite, (spacegroup R–3c, a ~
4.967 Å, c ~ 16.963 Å at 6 mol% MgCO3 [1]) details about its microstructure remain to
be unravelled. In addition to the Bragg reflections of the calcite lattice (a type
reflections), type b (dolomite-type ordering), type c (apparent modulation wave vector q
= (1/2)[110]* or q = (1/2)[104]*) and type d (apparent q = [001]) reflections have been
reported from
ED studies. Macroscopic twinning is also very common, mainly on the
planes (10–8), (104), (10–2) and (001). Many Mg-calcite samples also show a pervasive
modulation in TEM images, which appears to be related to the c type reflections.
Our interest in biomineralisation and biomimetic mineralisation led to the detailed
electron diffraction (ED) investigation of the magnesian calcite nanostructure of
echinoid (sea urchin) test plates. Fig. 1a shows a typical EDP along the [001] zone axis.
Weak diffuse scattering along [110]* is visible, often including a diffuse (or sometimes
sharp) peak corresponding to the c-type reflections. On moving the sample under a
small beam, reflections with an apparent modulation wave vector q = 1/4 [300] appeared.
However, these do not imply a new superstructure
of calcite, but originate from small
domains of calcite oriented along the [1–10] zone axis. Such domains are orientationally
related to the calcite matrix by the (104) twin law, since fortuitously the ratio a*/c*  4.
The domains were (in accordance with reports of domains with c-type reflections) found
to be about 100 nm in size,
and scarcely but regularly distributed through the matrix
(along with occasional (10–8) twins).
No spot splitting has been observed associated with these small twins. Full coherence
implies some strain of the structure, which may influence the distribution of Mg and
also the mechanical properties of the material. The deceptively superstructure-like
appearance of EDP’s from juxtaposed twin+matrix emphasises the need to be cautious
in identifying superstructures in calcite as it shows metrical pseudosymmetry and the
possibility of rich coincidence site lattice behaviour.
a)
b)
c)
006
110
Figure 1. a) shows an EDP’s along <001> from the bulk of the crystal. b) shows an example of additional
scattering from scarce, small (100 nm) twin domains. The extra reflections can be described with an
–
apparent modulation wave vector q = 1/4 [300] but it originates from the superimposed [110] zone axis
EDP scetched in c). (Dynamical scattering acounts for the rest of the scattering.)
Reference
Paquette, J., and Reeder, R.J. (1990), American Mineralogist, 75, 1151-1158.