The structure of the mixed alkaline-earth orthoborate Ba2Mg(BO3)2, a buetschliite derivative, has been established with single-crystal X-ray diffraction methods. Crystal data : a = 5.343(2), c = 16.520(3) angstrom, V = 408.4(2) angstrom3, Z = 3, space group = R3BARm. A Eu2+-doped powder exhibits a luminescence peak at 608 nm and a Stokes shift of approximately 11,000 cm-1.