Evidence of Chemical Phase Separation in K$_{0.65}$Fe$_{1.74}$Se$_{2}$*

K$_{x}$Fe$_{2-y}$Se$_{2}$ has been widely investigated and many samples show a co--existence of superconductivity and antiferromagnetic properties. Recently the it was shown that this system shows a clear phase separation, however the nature of the two phases remained unclear. In the present work we report on a chemical phase separation in crystalline superconducting K$_{0.65}$Fe$_{1.74}$Se$_{2}$, investigated by means of magnetization experiments, scanning electron microscopy, electron backscatter diffraction, and energy--dispersive X--ray spectrometry. It is shown that the crystal consists of platelets oriented in $<$100$>$ with an approximated volume fraction of about 30\% in the surrounding $<$001$>$ oriented matrix. The platelets (the matrix) are depleted in K (Fe) and enriched in Fe (K). Chemical phase separation is demonstrated by a stable, antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ matrix, and K$_{x}$Fe$_{y}$Se$_{2}$ platelets inducing superconductivity. This time driven chemical spinoidal phase separation may therefore be responsible for several alternative properties measured in K$_{x}$Fe$_{2-y}$Se$_{2}$ samples as superconductivity and antiferromagnetism.