  The existence of two phases within one and the same hexagonal lattice of MgB2
compound, differing in Mg and B (in the homogeneity region) and especially in
impurity oxygen content, as well as in microstructure, is demonstrated by
various techniques. The regions corresponding to these two phases of MgB2 have
the sizes of 100-500 {\mu}m, and they fill the whole bulk of specimens,
alternating with each other. It is suggested that the two-phase state of MgB2
compound is caused by specific features of its formation mechanism (as a result
of synthesis at 800-1000{\deg}C), including the stages of Mg melting,
dissolution of solid boron in it up to the composition of MgB2 and further
crystallization of the MgB2 compound from the melt with the formation of
dendrite-like structure with corresponding redistribution of main components
and impurities.
