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 CO2 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.