An experimental investigation of the mechanical behaviour of synthetic calcite-dolomite composites

The role of dolomite on the strength and evolution of calcite-dolomite fold and thrust belts is largely unknown. Field investigations indicate that, under upper- to mid-crustal conditions, strain in natural systems is localized in calcite, resulting in a ductile response, while dolomite deforms in a brittle manner. The effect of dolomite on limestone rheology, and the potential for strain localization in composites have not yet been fully quantified. I conducted 11 constant displacement rate (3x10⁻⁴ and 10⁻⁴ s⁻¹), high confining pressure (300 MPa), and high temperature (750°C and 800°C) torsion experiments to address the role of dolomite on the strength of calcite-dolomite composites. Starting materials were formed by hot isostatic pressing mixtures of dolomite and calcite powders (given as Dm%: Dm25, Dm35, Dm51, and Dm75) and were deformed up to a maximum shear strain of ~5. Mechanical data show a considerable increase in yield strength with increasing dolomite content. Microstructural analysis shows that dolomite grains ~50 μm are characterized by well-defined grain boundaries and cleavage-controlled fracture. Electron backscatter diffraction (EBSD) shows no crystallographic preferred orientation (CPO) development in dolomite, but optical microscopy confirms brittle deformation of dolomite grains by Mode I cracks, shear fractures, and subsequent grain size reduction. Calcite grains are internally strain-free, equiaxed to tabular in shape, and characterized by triple-junction grain boundaries. EBSD confirms a distinct CPO of calcite c-axes perpendicular to the direction of maximum stretching. The microstructure of calcite aggregates suggests grain boundary sliding, accommodated by diffusion and dislocation glide, which accommodates high shear strains without significant change in grains shape and size. Dolomite is essentially undeformed in run products with less than 35% dolomite; calcite accommodates most of the displacement in these experiments. In contrast, for dolomite contents greater than 51%, dolomite accommodates displacement by brittle processes. My experiments provide insights into the processes controlling rheology within bimodal calcite-dolomite systems, suggesting that a minimum dolomite-content exists (between 35% and 51%) above which dolomite significantly influences composite strength.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat