The effect of ultrafine dry grinding for up to 1920 min in a planetary ball mill at 300 rpm on size distribution, particle agglomeration and bulk structural changes in a dolomite-rich (CaMg(CO3)(2))rock was studied. The size and shape of the ground particles were characterized by laser scattering and scanning electron microscopy (SEM) respectively. The uniformity index of the size distribution evaluated using the Rosin-Rammler equation showed that the apparent grinding limit was reached after 480 min. Differential- and thermo-gravimetric (DTA-TG) analyses showed that the decomposition temperature of magnesite (MgCO3) was affected to a far greater extent by the input energy of grinding than that of calcite (CaCO3). The activation energy of the total decomposition decreased for the aliquots ground above 480 min. This behavior was explained by the storage of extensive lattice distortions in dolomite crystal structure evaluated by X ray line profile analysis of major diffracting peaks. The infrared (IR) bands related to the bending (877 cm(-1)) and stretching (1418 cm(-1)) vibrations of the anionic CO32- groups increased with the particle size decreasing as well as being sensitive to particle coarsening caused by agglomeration. Coupled to morphological and Rosin-Rammler size distribution analyses, IR data confirmed extensive agglomeration above 480 min. The transformation between calcite and aragonite polymorphs was also noticed in those aliquots ground for periods >= 480 min. The bulk structural changes documented here were useful to show that the inversion from the breakage to the agglomeration regime observed in dolostone powder system is directly associated with the change in the energy relaxation mechanism inside the dolomite grains. (C) 2018 Elsevier B.V. All rights reserved.