[en] Magnesium boron nitride is a well known and often applied catalyst for the transformation from hexagonal boron nitride (hBN) to its cubic polymorph cBN. Recently published results of structural investigations contributed to a better understanding of the catalytic properties of this compound and its high pressure-high temperature phase diagram. Its normal pressure crystal structure was analyzed by x-ray powder methods and structural refinements by Hiraguchi et al. while several phase transitions to high pressure-high temperature polymorphs were observed in situ using a multi-anvil high pressure device. The Mg₃BN₃ starting material for these experiments was prepared by chemical procedures. But as already was pointed out Mg₃BN₃ also arises as an intermediate phase in the Mg₃N₂-BN system at high pressures and temperatures. therefore, in the following, the specific formation conditions in this system shall be investigated and, especially, relation to the phase behavior of magnesium nitride will be examined