I. C. Stretton

Dislocation creep of magnesiowüstite (Mg0.8Fe0.2O)

Abstract Magnesiowustite is likely the second most abundant and probably weakest lower-mantle phase. Its deformation properties will therefore influence the rheological and dynamic behaviour of this region. In this study magnesiowustite aggregates with a composition of Mg0.8Fe0.2O have been deformed under dry conditions in axial compression at a confining pressure of 300 MPa and temperatures from 1200 to 1400 K (0.41

Accurate quantification of the modal mineralogy of rocks when image analysis is difficult

Abstract The volume proportions of the mineral phases in two strongly deformed olivine-orthopyroxene rocks have been quantified by whole-pattern stripping of fixed geometry X-ray powder diffraction data. The results were compared with the phase proportions as determined by Rietveld refinement of time-of-flight neutron powder diffraction data, and were shown to be in excellent agreement. The X-ray technique not only provides a very rapid and cost-effective method of determining phase proportions, but it also circumvents several of the problems associated with obtaining this information by image analysis. Moreover, the technique is particularly advantageous in strongly textured rocks or in rocks that contain significant residual strains. As such it offers a powerful technique for analysing the mineralogical composition of fine-grained and/or deformed experimental run products, which makes it of considerable potential for monitoring in situ the progress of mineral reactions during laboratory experiments.

Temperature related structural variation of the hydrous components in gypsum

Single crystal neutron diffraction studies have been carried out on a natural crystal of gypsum (CaSO4 · 2H20) at 50, 115 and 200 K. The structural refinements converged to Rw(F) = 0.072, 0.068 and 0.065 respectively and complement well previous refinements at 294 K. Large changes in the intermolecular hydrogen bonds is observed as a function of temperature. Significant variation between the two intramolecular O(w)-H bonds is evident at all temperatures. The distortion of the water molecules and the hydrogen bond angles display temperature independence. The thermal ellipsoids of the protons determined in our single crystal neutron diffraction experiment are strongly perpendicular to the O(w)-H···O(1) hydrogen bonds, with their orientations temperature independent and with no evidence of directionality of the protons along the hydrogen bond below 294 K.

Using neutron diffraction measurements to characterize the mechanical properties of polymineralic rocks

Abstract Conventional experiments designed to investigate the mechanical properties of polycrystalline geological materials are generally restricted to measurements of whole-rock properties. However, when comparing the measurements with theoretical models, it is frequently essential to understand how the deformation is accommodated at the grain-scale. This is particularly true for polymineralic rocks because in this case most theories express the whole-rock properties as some function of the properties of their constituent minerals, and hence the contribution which each phase makes to those properties must be measured if the theories are to be fully assessed. The penetrating nature of neutrons offers a method of addressing this problem. By performing deformation experiments in the neutron beam-line and collecting neutron diffraction patterns at different applied loads, the lattice parameters of all the mineral phases present may be determined as a function of load. The elastic strain experienced by each phase is then easily determined. Moreover, the strain in different lattice directions is also obtained. From this information a wide range of problems relevant for the characterization of the elastic and plastic deformation behaviour of polymineralic geological materials can be explored. An experimental technique for carrying out such experiments is described, and its validity is demonstrated by showing that the results obtained from deforming an elastically isotropic olivine + magnesiowustite sample agree, to within very tight bounds, with the behaviour predicted by theory for elastically isotropic composites.

Strain partitioning during the elastic deformation of an olivine + magnesiowustite aggregate

In order to measure directly the extent of elastic strain partitioning between the phases in a two phase material, room temperature uniaxial compression experiments have been performed on an olivine + magnesiowustite sample within the neutron beam at the ISIS neutron spallation source, Rutherford Appleton Laboratory, U.K. Neutron diffraction data were collected at fourteen different applied loads. At each load the olivine and magnesiowustite lattice parameters in directions parallel and normal to the direction of applied load, were extracted from the diffraction data, and the axial and radial elastic strains experienced by each phase were calculated from these parameters. The observed strain partitioning is in close agreement with that predicted by the widely used Hashin-Shtrikman analysis of the elastic properties of composites, confirming both the validity of that analysis, and the potential which the experimental technique has for providing insight into the controls on the mechanical properties of polymineralic materials.

Using neutron diffraction to investigate the elastic properties of anisotropic rocks: Results from an olivine + orthopyroxene mylonite

[1] The penetrating power of neutrons means that neutron diffraction may be employed to determine the lattice parameters of mineral phases within samples over 1 cm in size. We have exploited this fact to obtain high-resolution measurements of the elastic strains experienced by olivine and orthopyroxene during room temperature, uniaxial deformation experiments performed in a neutron beam-line, on a mylonite from the Oman ophiolite. Specimens were loaded in three orthogonal directions with respect to the macroscopic fabric of the rock. In each case the average olivine and orthopyroxene strains were the same, even though the elastic stiffnesses of the two phases were different. In comparison with the strains expected from the single-crystal elastic stiffness tensors, the average strains in the different lattice directions for each phase were found to be greater in the stiffer directions, and lesser in the more compliant directions. The overall effect of this tendency toward strain homogenization was for the measured elastic anisotropy of the rock to be significantly lesser than that given by Voigt/Reuss averaging of the single-crystal elastic properties according to the lattice preferred orientations of each phase. The technique used to derive these conclusions potentially provides an important experimental method for the quantitative examination of nonfracture-related controls on the elastic anisotropy of geological materials.

Deformation of Minerals, Applications in Geophysics

Experimental deformation of minerals and rocks provides critical data to constrain processes within the interior of the Earth. While it is not possible to give a complete description of research on mechanical properties of Earth materials in the context of this paper, we will overview those areas of research that are somewhat unique to experimental geophysics. The most notable difference is the requirement of a high-pressure environment for investigation of mechanical properties. Within this context, we will present current research pertinent to our understanding of the deformation behaviour of Earth materials: 1) our current understanding of the effects of chemical environment, especially water activity, on the mechanical properties of mantle minerals and rocks; 2) the importance of strain localisation, fabric development and deformation of Earth materials to high strains in understanding the dynamics of the lithosphere; and 3) new innovations in investigating the mechanical behaviour of the major minerals of the deep mantle, which are not stable under the conditions normally attainable in conventional deformation apparatuses.

Low Temperature Plastic Behaviour of Icosahedral AlCuFe Quasicrystals

The mechanism by which dislocations move in the icosahedral quasicrystalline structure, i.e., glide or climb, is still an open question. In order to check whether pure dislocation glide occurs in this quasi-periodic structure, low temperature deformation tests have been performed under confining pressure conditions. These experimental techniques, which superimpose a shear stress on an isostatic component, enable the brittle-to-ductile transition temperature to be shifted to temperatures at which diffusion processes can be assumed to be negligible. Such techniques have been applied to deform plastically AlCuFe poly-quasicrystals at low and intermediate temperatures, using both gas and solid-confining media. Mechanical data as well as microstructural observations associated with this low temperature deformation range are reported. The first results provide new insights into the deformation mechanisms that control plasticity in the icosahedral quasicrystalline phase.