XPS study of light blue (I) and dark blue (II) lazutires from Baikal region deposits and the dark blue one decolorized by annealing in the air (III) shows the sulfur compounds SO_4 and S_2 in (I), SO_4, S_X and S in (II), and SO_4, SO_3 in (III). Therefore, decolorization of lazurite by annealing in the air is due to reaction of the sulfur disproportionation in the mineral: S~(6+) + S~(2-) = 2 S~(4+). Polysulfide ion and the corresponding radical S_3~- center of the blue coloring -- act as reducing agents upon the sulfate ion. It is supposed that the blue lazurite contains H_2S or/and HS~--bearing clusters taking part in formation of the S_3~--centers by means of reducing the SO_4~(2-) anions inside the crystal. The nature of unusual grain size color effect under the lazurite annealing is discussed. It is concluded that this effect is probably provided by a certain correlation between sizes of SO_3-shell and SO_4-core, due to which the loss of S_3~- in the shell is compensated by its creation in the core.
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