Journal of Conference Abstracts

The Origin of the Incommensurate Phase Transition in Melilites in the System Ca₂MgSi₂O₇-Ca₂FeSi₂O₇

J. D. C. McConnell (desmondm@earth.ox.ac.uk)

Department of Earth Sciences, Parks Road, Oxford OX1 3PR.

The existence of incommensurate phase transitions in the melilite system åkermanite-Fe-åkermanite, Ca₂MgSi₂O₇-Ca₂FeSi₂O₇, has been known for some time and extensive x-ray structural and Mossbauer studies have been carried out on these materials. In this communication incommensurate theory is used, in conjunction with existing structural data, to establish the precise nature of the incommensurate structure in these materials. The high temperature space group of melitite is P4­2₁m and structural studies indicate that the large Ca ion, and other ions, are locally displaced from their sites on the mirror plane in the structure. Incommensurate structural theory implies that there are always two structures present within an incommensurate modulation and that these coexist in quadrature along the modulation wavelength. In the case of the incommensurate structure in melilite the loss of the mirror plane can be shown by group theory to be compatible with the presence of two independent component structures with space groups P2₁2₁2 and P4­ both of which are distinguished by the lack of the mirror plane present in P4­2₁m. The results of existing Mossbauer spectroscopic studies on iron bearing melilites are shown to be entirely compatible with the existence of these two spatially independent structures and can be identified with the linearly independent distortions in space groups P2₁2₁2 and P4­. Incommensurate structures in the melilite solid solution are not related to alternative ordering schemes within the solid solution and are therefore not comparable with the incommensurate structures observed in mullite and the plagioclase feldspars.

In-Situ Measurement of Strain Partitioning During Rock Deformation by Neutron Diffraction Imaging

Philip G. Meredith¹ (p.meredith@ucl.ac.uk), Ian G. Wood¹, Kevin S. Knight² & Stephen A. Boon¹

¹ Department of Geological Sciences, University College London.

² ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon.

Rock deformation in the shallow, seismogenic crust is dominantly brittle and involves cracking on all scales in response to gravitational, thermal and tectonic loading. However, the relationship between applied stress, strain response and cracking is still not fully understood. We have therefore studied this relationship by performing deformation experiments on two mineralogically similar rocks with very different mechanical properties. Solnhofen limestone and Carrara marble are both pure calcite rocks, but Solnhofen limestone is four times as strong in uniaxial compression. We believe the difference can be explained by the way in which the mineral grains are bonded together within the rock microstructure.

However, in order to test this quantitatively, we needed a technique that allowed us to differentiate between the strain within the grains and the strain between the grains (i.e. crack strain) in the rock. Because of the penetrating nature of the radiation, neutron diffraction offers an ideal technique for measuring this strain partitioning deep within the interior of the samples, specifically away from any of the free surfaces. The experiments were therefore performed using a specially-constructed deformation apparatus located in-situ in the beam-line at the ISIS spallation neutron source at the Rutherford Appleton Laboratory. Our results show that most of the deformation in the semi-brittle Carrara marble is accomplished by cracking, with very little grain strain, whilst most of the strain in the elastic-brittle Solnhofen limestone occurs within the mineral grains, with very little cracking occurring until just prior to failure.