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In this paper, we systematically drew a series of inverse-microemulsion quasi-ternary system phase diagrams of OP-10+C₈H₁₇OH+C₆H₁₂+brine (CaCl₂/BaCl₂) by adjusting the ratio of CaCl₂ and BaCl₂. On this basis, microemulsions have been prepared with seven different molar ratios of Ca²⁺/Ba²⁺, and calcium carbonate and barium carbonate coprecipitation products were obtained by reaction with an equimolar amount of sodium carbonate. The influence of barium ion to morphology and composition of nanometer calcium carbonate were studied. These samples were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The SEM photographs indicated that when the content of Ca²⁺ was higher, some incomplete large cube of coprecipitation particles were formed in solution, but with the content of Ba²⁺ increased gradually, they formed a large number of small spherical particles, with the further increase of Ba²⁺ concentration, the particles mainly had structures of irregular polyhedron eventually. The measurement results of FTIR and XRD indicated that CaCO₃ coprecipitation products gradually changed from calcite to the vaterite, eventually turned into being aragonite with the further increase of Ba²⁺ concentration.