A mixed borate and carbonate chloride Ba-2(BO3)(1-x)(CO3)(x)Cl1+x was obtained by spontaneous crystallization from a high-temperature melt in open air. It crystallizes in the trigonal crystal system with space group of P3 (m) over bar1 and lattice constants of a = 54708(8) angstrom and c = 10.640(2) angstrom. The structure can be viewed as an intergrowth of trigonal Ba2Mg(BO3)(2) (001) slab and (111) slab of the cubic fluorite BaCl2. During Fourier analysis of the single-crystal X-ray diffraction data, additional electron density was found locating at 1b (0, 0, 1/2) site and attributed to chlorine surplus, which was confirmed by chemical titration. Charge balance of the compound was found, unexpectedly in an acidic borate containing high-temperature melt, by partial CO32- group substituting the BO33- group. The existence of CO32- anion in the crystal was detected by thermogravimetry-mass spectrum analysis and Raman spectrum. The transmittance spectrum shows that the crystal is transparent from ultraviolet to infrared with short-wavelength absorption edge at about 220 nm.