Reaction-induced strain localization: Torsion experiments on dolomite

Abstract

We investigated the mechanical behaviour and microstructural evolution of a dolomite marble from Mt. Frerone (Adamello, N-Italy) during decarbonation to calcite and periclase in torsion experiments. Tests were performed in a Paterson gas-medium apparatus on cylindrical samples of 10 mm diameter and 10 mm length. Experiments were conducted at 800 °C, 300 MPa confining pressure under both vented and non-vented conditions, up to a maximum bulk shear strain of about γ = 1.8, at different strain rates (3 × 10 − 5 s − 1 up to 3 × 10 − 4 s − 1 ). Under hydrostatic conditions the nominal equilibrium P(CO 2) should be around 100 MPa, but in the vented experiments the CO 2 was free to escape, causing the breakdown of dolomite. During the decomposition, deformation was systematically localized at the ends of the specimens, near the porous spacers into a fine-grained mixture of calcite and periclase. Due to the low permeability of the marble, pore fluid could not escape from the central part of the sample building up CO 2 pressure which suppressed the decarbonation reaction. The fluid pressure embrittled the material and caused the development of en-echelon tension fractures, inclined opposite to the sense of shear. We conclude that decarbonation produced a weak polyphase matrix composed of submicron sized reaction products. Such a small grain size induced strain localization which was probably promoted by a switch from grain-size insensitive to grain-size sensitive deformation mechanism.