FLUCTUATIONS IN THE STRUCTURE OF CALCITE. IP Swainson (Atomic Energy Of Canada Limited, Chalk River, ON, K0J 1PO, CANADA), MT Dove (Dept of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK), MJ Harris and ME Hagen (ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK).

Calcite, CaCO₃(I), undergoes a tricritical, order-disorder transition at approximately 1260 K. We present evidence from neutron powder diffraction measurements that this transition occurs by a continuous growth of librational amplitude of the CO₃ ions in the (0001) plane. Once the in-plane librational amplitude reaches a root mean of 30^o, the crystal transforms into the disordered phase, with symmetry change R3cR3m. This transition is associated with an instability at the Z-point of the Brillouin zone. However, considerable diffuse scattering is seen in X-ray and neutron scattering studies at the F-point of the Brillouin zone. Inelastic neutron scattering measurements have shown that a pronounced dip in the transverse acoustic (TA) branch exists at this point and that an anomalous continuous column of scattering exists down from v=v_ta to v=0. The TA mode is of symmetry F^-₂. This is also the symmetry of the soft mode for the high-pressure CaCO₃(I)-CaCO₃(II) transition. The TA mode softens continuously, and the column grows in intensity as the T_c of the Z-point order-disorder transition is approached. This demonstrates that the Z-point disordering and F-point fluctuations are strongly coupled. Such a column of inelastic scattering has been seen in very few other systems. In all cases it appears to be associated with competing ordering schemes. It appears, therefore, that the diffuse scattering seen at ambient pressures at the F-point of CaCO₃(I) is an incipient ordering associated with CaCO₃(II).