The control of impurities at each stage of the purification process of Si is the primary topical problem in Si technology. The technique of determining the impurity composition should be multi-element and with low limit-detection of impurities. From this point of view, X-ray analysis of the phase composition of Si, based on the identification of X-ray diffraction lines is very attractive. Obtaining of Si directly from MG-Si is essential to reveal the physical possibilities of the directional crystallization. In this work, there are considered the options on a set of detectable impurities and the limits of their detection in such type of Si by X-ray diffraction method. It has been shown, that applicability of X-ray analysis of the phase composition of Si, based on the identification of X-ray diffraction lines, depends on the stage of Si purification.