Mixed-valence iridates are the new playground for studying frustrated magnets and achieving exotic quantum spin liquid ground states. Magnetic interactions in such systems remain largely unknown and obscured by the combination of several competing energy scales that render the microscopic description highly challenging. By resolving the magnetic structure of Ba3LuIr2O9, we envisage to obtain experimental insight into magnetic interactions and gain the knowledge that can be extended to sibling iridates, including Ba3InIr2O9, where signatures of the spin-liquid behavior have been observed.