X-ray powder diffraction with synchrotron radiation was used to study the formation of cubic boron nitride (cBN) in the MgB2-BN system at pressures to 6.8 GPa and temperatures to 2000 K. For the formation of cBN, when it crystallizes from a BN solution in the melt of the system under study, the threshold pressure (4.5 +/- 0.1 GPa) and low-temperature boundary (T (K) = 1633 -9.2p (GPa)) have been established. We have calculated pressure and temperature dependences of the rate of cBN nucleation and have found that the position of the high-temperature boundary of the p,T region of the cBN crystallization is specified by the nucleation of the cubic phase. The existence of the threshold pressure of cBN crystallization is dictated by the strong pressure dependence of the nucleation rate. It is just this dependence that is responsible for the fact that the region of the cBN spontaneous crystallization is well off the hBN reversible arrow cBN equilibrium line by several hundred degrees.