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in your question what is the meaning of mirror?
oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates. Silicon's compounds are also found in water, in the atmosphere, in many plants, and even in certain animals.
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Thanks halcyon and JFraser
You can't. If you use just Si, then metallic Si can be polished to a pretty good mirror finish. But SiO2 is transparent. If you are allowed to add a tiny amount of some metal, e.g. Ag or Al, then you can make glass, as @JFraser suggests, which can be made with a very flat and smooth surface, and then you can coat this glass with a thin layer of metal to get a mirror.