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a segment of irregularly shaped intrusive rock that differs from the rest of the rock mass by the quantitative proportions of its constituent minerals or by structure. Gradual transitions are usually observed between schlieren and the rock within which they are enclosed. Schlieren are usually 2–10 cm in size, sometimes reaching as much as 1 m and more. Some schlieren are formed when fragments of foreign rock (xenoliths) are invaded and reworked by a magmatic melt. In many cases, such schlieren show continuous transitions from xenoliths that differ greatly from the actual magmatic rock. Sometimes schlieren are accumulations of minerals, frequently ore minerals, that have separated out from the magmetic melt in the early stages of crystallization.